Class 8 Science Chapter 7 Particular Nature of Matter (New Course)

Key Concept: Effect of Temperature on Particle Movement

c) The particles move faster and more vigorously.
[Solution Description]
When a liquid is heated, the kinetic energy of its particles increases, causing them to move faster and more vigorously. This is because heat provides additional energy to the particles, overcoming some of the interparticle forces temporarily.

Key Concept: Behavior of Liquid Particles

b) Interparticle forces keep them bound within the liquid.
[Solution Description]
Liquid particles do not escape like gas particles because the interparticle forces in liquids are stronger than those in gases. These forces keep the particles close together, preventing them from breaking free and spreading out indefinitely while still allowing limited movement.

Key Concept: Particle Movement in Liquids

c) The purple colour spreads slowly throughout the water
[Solution Description]
The purple colour of potassium permanganate slowly spreads throughout the water because the particles of water are in constant motion. This movement allows the potassium permanganate particles to disperse even without stirring.

Key Concept: Particle movement in liquids, interparticle forces

c) Particles can move past each other within confined boundaries
[Solution Description]
The particles in a liquid have weaker interparticle forces compared to solids, allowing them to move past one another and take the shape of the container. However, they are still close enough to maintain a fixed volume because the forces are strong enough to prevent significant expansion or compression.

Key Concept: Compressibility, interparticle spacing

c) Interparticle spaces in liquids are minimal compared to gases
[Solution Description]
In liquids, particles are already closely packed with limited space between them due to relatively strong interparticle forces. Unlike gases, where particles are far apart, applying pressure doesn’t significantly reduce the already small interparticle spaces in liquids.

Key Concept: Heating liquids, phase transition

c) Movement becomes more vigorous until particles escape as gas
[Solution Description]
Heating provides energy to the liquid particles, increasing their kinetic energy. At the boiling point, particles gain enough energy to overcome interparticle forces completely, escaping as gas. Before that, increased heat leads to more vigorous movement without breaking free entirely.

Key Concept: Particle movement in liquids, Interparticle forces

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
In liquids, particles have weaker interparticle forces compared to solids, allowing them to move around within the container but not escape it. This is because the forces are strong enough to hold the particles close together but weak enough to let them slide past one another. The assertion correctly states that particles cannot escape the container boundary due to these forces, and the reason accurately explains why this happens by describing the nature of interparticle forces in liquids. Thus, both the assertion and reason are true, and the reason correctly explains the assertion.

Key Concept: Movement in Gases

c) Because the interparticle forces are negligible
[Solution Description]
Gas particles move freely and rapidly in all directions because the interparticle forces in gases are negligible. This allows gases to expand and fill any available space.

Key Concept: Movement in Solids vs. Liquids

c) Particles in solids vibrate in fixed positions, while particles in liquids slide past each other
[Solution Description]
In solids, particles vibrate in fixed positions and do not move freely, giving solids a definite shape and volume. In liquids, particles can slide past each other within a limited space, allowing liquids to flow and take the shape of their container.

Key Concept: Particle Movement in Liquids

c) Interparticle forces allow sliding movement but prevent indefinite expansion.
[Solution Description]
Liquids take the shape of their container because their particles can move past each other within a limited space, allowing them to flow and conform to the container’s shape. However, unlike gases, the interparticle forces in liquids are strong enough to keep the particles from spreading out indefinitely, maintaining a fixed volume.

Key Concept: Movement of Particles in Liquids

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that particles in a liquid can move past one another but remain confined within the liquid’s volume, which is true according to the syllabus. The reason explains that this behavior is due to interparticle forces being strong enough to keep the particles close yet allow sliding motion. Both the assertion and reason are correct, and the reason accurately explains the assertion based on the given syllabus content.

Key Concept: Movement of Particles in Liquids

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that liquids have a definite volume but no fixed shape, which is correct based on the syllabus. The reason explains that this happens because particles in a liquid can move freely within a limited space, allowing the liquid to take the shape of its container. Both statements are true, and the reason correctly explains the assertion.

Key Concept: Interparticle Forces

c) Gas
[Solution Description]
Gases have the maximum interparticle spacing due to negligible interparticle forces of attraction, which allows the particles to move freely.

Key Concept: Compressibility and Interparticle Spacing

c) Gas
[Solution Description]
The compressibility of a substance depends on the interparticle spacing. Gases have the maximum interparticle spacing, allowing them to be compressed easily compared to liquids and solids.

Key Concept: Thermal Energy, Interparticle Spacing

b) Increases spacing, changes to liquid then gas.
[Solution Description]
Increasing thermal energy provides particles with more kinetic energy, causing them to vibrate more and overcome interparticle forces. This increases spacing, first transitioning to liquid (more spacing than solid) and then to gas (maximum spacing). The state changes from solid $\rightarrow$ liquid $\rightarrow$ gas.

Key Concept: Interparticle Spacing in States of Matter

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that gases can be easily compressed due to their maximum interparticle spacing, which is true. The reason explains that the large spacing allows particles to be pushed closer under pressure, which correctly justifies the assertion. Both statements are factually correct and logically connected.

Key Concept: Interparticle Spacing, States of Matter

c) Solid $\rightarrow$ Liquid $\rightarrow$ Gas
[Solution Description]
As interparticle spacing increases, the forces of attraction between particles weaken. Initially, the substance is a solid with minimum spacing. When spacing increases slightly, it transitions to a liquid with more spacing, allowing particles to move but maintaining definite volume. Further increase leads to a gas with maximum spacing, no fixed shape or volume. The correct sequence is solid $\rightarrow$ liquid $\rightarrow$ gas.

Key Concept: Interparticle Attraction, State Properties

c) It is a solid with fixed shape and volume.
[Solution Description]
Strong interparticle attractions and minimal empty space are characteristics of solids. Solids have fixed shape and volume due to closely packed particles. Liquids have weaker attractions and more spacing, while gases have negligible attractions and maximum spacing.

Key Concept: Interparticle Spacing in States of Matter

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The Assertion states that the interparticle spacing is minimum in solids, which is correct because in solids, particles are tightly packed with the least space between them. The Reason explains this by stating that strong interparticle forces of attraction cause close packing. Both statements are true, and the Reason correctly explains the Assertion because strong forces lead to minimal spacing.

Key Concept: Interparticle Spacing in Solids

a) Solid
[Solution Description]
According to the particulate nature of matter, solids have the minimum interparticle spacing due to strong intermolecular forces that hold particles closely packed together.

Key Concept: Interparticle Spacing, States of Matter

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that interparticle spacing in solids is minimal due to strong forces of attraction restricting particle movement. This is true because solids have tightly packed particles with minimal spacing. The reason claims that stronger interparticle forces lead to smaller spacing, which is also correct. Moreover, the reason directly explains why solids have minimal spacing, as stated in the assertion. Thus, both are true, and the reason correctly explains the assertion.

Key Concept: Compressibility

c) Gas particles have large interparticle spacing
[Solution Description]
The large interparticle spacing in gases allows them to be compressed, as seen when pushing the plunger of a syringe reduces the volume of air inside.

Key Concept: Interparticle Spacing

c) Strong interparticle forces hold particles in fixed positions
[Solution Description]
Solids have strong interparticle forces of attraction that hold particles in fixed positions with minimum spacing, allowing only vibrational motion. This results in a definite shape and volume.

Key Concept: Properties of Liquids

b) Liquid
[Solution Description]
Liquids exhibit moderate interparticle spacing, making them less tightly packed than solids but more than gases. This allows liquid particles to move past one another while maintaining a definite volume.

Key Concept: Constituent particles of matter

b) The size of chalk particles reduces but they retain their original composition.
[Solution Description]
Grinding chalk is a physical change where the size of chalk particles reduces but their chemical composition remains unchanged. Each speck of powdered chalk is still made up of the same substance and can be broken down further until the smallest indivisible constituent particles are reached.

Key Concept: Dissolution of Sugar

c) Its constituent particles
[Solution Description]
When sugar dissolves in water, it breaks into its constituent particles, which disperse uniformly in the water. This is illustrated in Activity 7.2, where the sweetness of the water confirms the presence of sugar particles even though they are not visible.

Key Concept: Composition of Matter

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
– The assertion states that chalk can be broken down into smaller particles through physical processes like grinding. This is true because grinding is a physical process that reduces the size of chalk particles without changing their chemical composition.
– The reason states that the smallest unit obtained after breaking chalk is its constituent particle, which retains the properties of chalk. This is also true because constituent particles are the fundamental units of matter and retain the properties of the substance they constitute.
– Additionally, the reason correctly explains the assertion because it clarifies that the breakdown of chalk into smaller particles does not alter its fundamental properties, as it remains the same substance in smaller forms.

Key Concept: Constituent Particles, Physical Change

c) The constituent particles remain the same but their size reduces
[Solution Description]
Grinding chalk is a physical change, meaning the substance remains the same. The chalk breaks down into smaller particles, but the fundamental constituent particles remain unchanged. These particles are the smallest units that retain the properties of chalk and cannot be broken further by physical means.

Key Concept: Constituent Particles

c) Constituent particles
[Solution Description]
The smallest units that make up matter are called constituent particles. These are the basic building blocks of all substances and cannot be broken down further by physical means. Examples include atoms or molecules, as discussed in the chapter.

Key Concept: Dissolution, Interparticle Spaces

c) Sugar breaks into tiny particles that fill interparticle spaces in water
[Solution Description]
When sugar dissolves in water, it breaks into its constituent particles, which occupy the interparticle spaces between water molecules. These tiny sugar particles spread uniformly, making the solution taste sweet despite being invisible. This demonstrates that matter is made of extremely small particles.

Key Concept: Interparticle spaces in solutions

c) Sugar dissolves completely, occupying spaces between water particles.
[Solution Description]
Sugar dissolves in water by breaking into its constituent particles, which occupy interparticle spaces between water molecules. These particles are too small to be seen but their presence is detected by taste. No new substance is formed, as dissolution is a physical change.

Key Concept: Constituent Particles, Physical Change

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that sugar breaks into its constituent particles when dissolved in water while retaining its chemical properties. This is true because dissolution is a physical process where the substance does not chemically alter. The reason correctly explains the assertion by stating that dissolving sugar is a physical change as the constituent particles (sugar molecules) remain the same.
Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.

Key Concept: Atoms and Molecules, Parmanu Theory

b) Matter is composed of tiny, indivisible particles called Parmanu
[Solution Description]
Acharya Kanad proposed that matter is composed of tiny, indivisible particles called Parmanu (atoms). These particles are eternal and cannot be broken down further, forming the basic building blocks of all substances. This aligns with modern atomic theory, where atoms combine to form molecules.

Key Concept: Physical Change

b) Physical change
[Solution Description]
Breaking a piece of chalk into smaller pieces is a physical change because the substance remains the same, only its size changes. No new substance is formed during this process, as mentioned in the chapter.

Key Concept: Assertion-Reason

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that matter is composed of tiny, indivisible constituent particles. This is true as seen in Activity 7.1 where chalk is ground into fine powder and in Activity 7.2 where sugar dissolves into water, both demonstrating the particulate nature of matter. The reason explains that chalk and sugar can be broken down into smaller particles by physical processes, which is also true as confirmed by the activities. Moreover, the reason correctly explains why matter is composed of constituent particles, as these processes show that even seemingly continuous substances are made of smaller units. Therefore, both the assertion and reason are true, and the reason is the correct explanation of the assertion.

Key Concept: Ancient concept of Parmanu

d) They are indivisible and eternal particles forming all matter.
[Solution Description]
Acharya Kanad proposed that Parmanu are tiny, indivisible, and eternal particles that constitute all matter. This concept aligns with modern atomic theory, where atoms are considered the fundamental building blocks of matter.

Key Concept: Interparticle Spacing in Liquids vs Solids

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
According to the particulate nature of matter, the interparticle attractions are strongest in solids, slightly weaker in liquids, and weakest in gases. This means that in solids, particles are closely packed with minimal spacing due to strong forces of attraction. In liquids, the forces are weaker, allowing particles to move more freely within a limited space, resulting in slightly larger interparticle spacing than in solids but still less than in gases. Therefore, the assertion is true. The reason correctly explains why liquids have slightly more interparticle spacing than solids—due to their weaker interparticle forces of attraction.

Key Concept: Interparticle Spacing in Liquids

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that the interparticle spacing in liquids is greater than in solids but less than in gases. This is true because liquids have weaker intermolecular forces compared to solids, allowing particles to be spaced slightly apart but not as much as in gases where particles are far apart. The reason given is also correct: the weaker interparticle forces in liquids allow particles to move more freely within a confined space, which directly explains why the interparticle spacing is greater than in solids. Hence, both the assertion and reason are true, and the reason correctly explains the assertion.

Key Concept: Interparticle spacing in liquids

c) The interparticle spacing in liquids is slightly greater than in solids.
[Solution Description]
The interparticle spacing in liquids is slightly greater than in solids because the particles in liquids have more freedom to move but are still held together by weaker interparticle forces compared to solids. This results in a fixed volume but no fixed shape for liquids.

Key Concept: Interparticle Spacing in Liquids vs Solids

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that the interparticle spacing in liquids is slightly more than that in solids, which is correct based on the syllabus. The reason given is that the interparticle forces of attraction are weaker in liquids compared to solids, which is also true and directly explains why liquids have slightly more interparticle spacing than solids (as weaker forces allow particles more freedom to move apart). Therefore, both the assertion and reason are true, and the reason correctly explains the assertion.

Key Concept: Shape and volume properties of liquids

c) Liquids have a fixed volume but no fixed shape, unlike solids which have both.
[Solution Description]
Liquids have a fixed volume but no fixed shape because their interparticle spacing allows particles to move freely while remaining close together due to intermolecular forces. Solids have both fixed shape and volume due to minimal interparticle spacing and strong forces, whereas gases have neither fixed shape nor fixed volume because their particles are far apart with negligible forces. Thus, liquids occupy an intermediate state in terms of interparticle spacing and forces.

Key Concept: Interparticle spacing in liquids

c) Interparticle spacing increases slightly
[Solution Description]
When a solid melts into liquid, the interparticle forces weaken slightly compared to solids. This allows particles to move more freely but still remain close together. The interparticle spacing increases slightly but not as much as in gases. From the syllabus: “The interparticle attraction in liquids is slightly weaker than in solids… providing them with a little more interparticle spacing.”

Key Concept: Interparticle spacing and properties of liquids

b) Interparticle spacing in liquids is slightly larger than in solids, and the forces are weaker but still significant.
[Solution Description]
The observation shows that liquids have no fixed shape but maintain a definite volume. This happens because the interparticle spacing in liquids is slightly larger than in solids, allowing particles to move freely within the liquid’s volume. However, the interparticle forces are still strong enough to keep the particles close together, preventing them from spreading out indefinitely like gases. In solids, particles are tightly packed with minimal spacing, giving them a fixed shape and volume. Hence, liquids have more interparticle spacing than solids but less than gases.

Key Concept: Properties of liquids

b) No fixed shape
[Solution Description]
From Activity 7.4 in the syllabus: “You will notice that the water takes the shape of the container… we can say that the liquids do not have a fixed shape…” This shows that liquids have no fixed shape but have definite volume.

Key Concept: Movement of particles and interparticle forces

d) The forces are weaker than in solids but strong enough to restore the original arrangement.
[Solution Description]
Moving a finger through water displaces the liquid temporarily due to the ability of liquid particles to slide past each other. This happens because the intermolecular forces in liquids are weaker than in solids, allowing movement. However, these forces are still strong enough to pull the particles back into place once the external force (the finger) is removed. The spacing between particles in liquids is large enough to permit this fluidity but not so large as to prevent restoration of their arrangement.

Key Concept: Comparison of interparticle forces

d) Interparticle forces are stronger in solids than in liquids.
[Solution Description]
Interparticle forces in solids are stronger than in liquids, restricting particle movement and giving solids a fixed shape. In liquids, these forces are slightly weaker, allowing limited movement and resulting in no fixed shape but a fixed volume.

Key Concept: Shape and volume of liquids

c) The water takes the shape of the container but retains its volume.
[Solution Description]
Liquids do not have a fixed shape but take the shape of their container due to the ability of particles to move freely within the liquid. However, the volume remains constant as the interparticle forces keep them close together.

Key Concept: Interparticle forces comparison

c) Stronger than gases but weaker than solids
[Solution Description]
From the syllabus snapshots: “The interparticle attractions are the strongest in solids, a little weaker in liquids, and the weakest in gases.” Therefore, liquids have intermediate strength of interparticle forces.

Key Concept: Diffusion in Gases vs. Liquids

b) Gas particles have higher energy and move faster than liquid particles.
[Solution Description]
Gas particles move freely and rapidly in all directions due to high kinetic energy and large interparticle spaces, enabling fast diffusion. In liquids, particles are closer together and move slower, so diffusion takes more time. Thus, perfume vapors disperse quickly in air but mix slowly in water.

Key Concept: Particle Movement in Different States

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
According to the syllabus, gases have negligible interparticle attraction, which enables their particles to move freely in all available space. This freedom of movement is directly attributed to the weak interparticle forces, as stated in the reason. Therefore, both the assertion and reason are true, and the reason correctly explains why particles in gases move freely.

Key Concept: Thermal Energy, Interparticle Forces

b) Interparticle forces weaken and thermal energy increases during both transitions.
[Solution Description]
During melting, thermal energy overcomes some interparticle forces, allowing particles to move past each other but still remain close. During vaporization, thermal energy completely overcomes interparticle forces, enabling free movement. Thus, the correct answer must reflect the progressive weakening of interparticle forces and increasing particle mobility with rising thermal energy.

Key Concept: Particle Movement in States of Matter

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that in solids, particles can only vibrate about their fixed positions, which is true because the interparticle forces in solids are very strong due to minimum interparticle spacing. The reason correctly explains the assertion by stating that the strong interparticle forces restrict the movement of particles to just vibrations. Therefore, both the assertion and reason are true, and the reason correctly explains the assertion.

Key Concept: Particle Movement in Solids

b) Solid particles cannot diffuse, while liquid particles help dissolve the sugar.
[Solution Description]
In solids, particles vibrate in fixed positions and cannot move freely, which is why the sugar cube remains intact on the table. However, in water (a liquid), the liquid particles can move past each other, allowing them to interact with and dissolve the sugar particles over time. Diffusion occurs as water molecules collide with sugar particles, breaking them apart and dispersing them evenly.

Key Concept: Particle Movement in Solids, Liquids, Gases

c) Liquid particles can move past each other but not as freely as gas particles.
[Solution Description]
The spreading of potassium permanganate in water demonstrates the movement of liquid particles. In solids, particles are tightly packed and only vibrate, preventing diffusion. In gases, particles move rapidly and randomly, causing rapid diffusion. In liquids, particles can move past each other but are restricted by intermolecular forces, resulting in slower diffusion compared to gases. Therefore, the correct choice should describe intermediate particle movement between solids and gases.

Key Concept: Change in Particle Motion During Melting

c) Vibrational motion changes to free movement past neighboring particles.
[Solution Description]
In ice (solid), particles vibrate in fixed positions. Upon melting, the particles gain enough energy to overcome rigid bonds and start moving past one another, though they remain close. This transition explains why liquids flow but retain a fixed volume.

Key Concept: Particle Movement in States of Matter

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The Assertion states that particles in solids vibrate about fixed positions and cannot move freely. This is true because the interparticle forces in solids are very strong, restricting their movement. The Reason correctly explains the Assertion by stating that strong interparticle forces leave minimal space between particles, which prevents free movement. Therefore, both the Assertion and Reason are true, and the Reason provides the correct explanation for the Assertion.

Key Concept: Particle Arrangement in Solids and Liquids

d) Closely packed in solids and move past each other in liquids.
[Solution Description]
In solids, particles are closely packed and vibrate about fixed positions due to strong interparticle forces. In liquids, particles are also closely packed but can move past each other due to weaker interparticle forces. Thus, the correct difference is: particles are closely packed in solids and move past each other in liquids.

Key Concept: Interparticle Forces in States of Matter

c) Gas
[Solution Description]
Gases have negligible interparticle forces compared to solids and liquids, allowing particles to move freely and occupy any available space. This results in no fixed shape or volume for gases.

Key Concept: Diffusion, States of Matter

a) Gas particles move randomly, and lighter particles diffuse faster than heavier ones.
[Solution Description]
The formation of a white ring closer to the hydrochloric acid end indicates that ammonia particles diffuse faster due to their lower molecular mass. This highlights that gaseous particles move freely and their speed depends on mass. Solids cannot exhibit such diffusion, and liquids show slower diffusion due to restricted movement. The correct option should link the observation to unrestricted gas particle motion and mass-dependent speed.

Key Concept: Properties of Solids

c) Particles vibrate but remain in fixed positions.
[Solution Description]
Solids have strong interparticle attractions and minimal interparticle spaces, restricting particle movement to vibrations about fixed positions. Hence, solids maintain a fixed shape and volume.

Key Concept: Motion of Gas Particles

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that gas particles move freely in all directions and occupy the entire space, which is correct because gases have no fixed shape or volume. The reason given is that interparticle forces in gases are negligible, which allows the particles to move freely. This is also true and correctly explains why the assertion is valid. Hence, both assertion and reason are true, and the reason explains the assertion accurately.

Key Concept: Diffusion in gases, Kinetic theory

c) Random motion and collisions of gas particles
[Solution Description]
Diffusion occurs due to the random motion of gas particles from higher concentration to lower concentration. Bromine vapor particles collide with air molecules and spread until uniform distribution is achieved, demonstrating the kinetic energy-driven motion of gases.

Key Concept: Gas particle behavior, Fluid properties

c) Gas particles move randomly and occupy all available space
[Solution Description]
Gases have negligible interparticle attractions and maximum interparticle space. Their particles move freely in all directions, colliding with each other and the container walls, leading to uniform distribution. Unlike solids or liquids, they do not retain fixed positions or volumes.

Key Concept: Particle movement in gases

c) As gas particles move freely in all directions and fill the available space
[Solution Description]
The smoke particles represent gas particles. In gases, particles have negligible interparticle attractions and move freely in all directions. This free movement causes them to spread out and occupy the entire available space inside the jar. Therefore, the smoke spreads uniformly.

Key Concept: Particle motion in gases, Interparticle forces

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that smoke diffuses due to negligible attractive forces between gas particles. This is true because gases exhibit minimal interparticle attractions, as explained in the syllabus. The reason states that maximum interparticle spacing allows free movement, which is also correct and directly explains why gases diffuse rapidly (as observed in Activity 7.5). Therefore, both statements are true, and the reason correctly explains the assertion.

Key Concept: Particle properties in gases

c) They move randomly in all directions with high speed
[Solution Description]
Gas particles have high kinetic energy due to negligible interparticle forces. They move randomly in all directions and collide with each other and the container walls. Their movement is unrestricted, allowing them to fill any available space.

Key Concept: Interparticle forces in gases

d) Due to high kinetic energy overcoming weak forces
[Solution Description]
The particles in gases are far apart and have high kinetic energy, which overcomes the weak intermolecular forces, resulting in negligible attractions between particles.

Key Concept: Shape and volume of gases

d) No fixed shape or volume
[Solution Description]
Gases do not have a fixed shape or volume because their particles move freely and are far apart, allowing them to occupy the entire available space.

Key Concept: Particle motion in gases

c) Move freely in all directions
[Solution Description]
In gases, particles move freely in all directions due to negligible interparticle attractions. They have no fixed shape or volume and occupy the entire available space.

Key Concept: Movement of particles in gases

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The Assertion states that gases do not have a fixed shape or volume because their particles move freely and occupy the entire available space. This is true as per the syllabus, which mentions that gases have maximum interparticle spacing and negligible interparticle forces, allowing particles to move freely. The Reason states that the interparticle attractions in gases are negligible, making the particles completely free to move. This is also true, as the syllabus confirms that interparticle attraction in gases is minimal. Moreover, the Reason correctly explains the Assertion because negligible interparticle forces lead to free movement of particles, resulting in no fixed shape or volume for gases.

Key Concept: Particle motion in gases, Interparticle attractions

c) Movement becomes more random and faster due to increased kinetic energy
[Solution Description]
Heating a gas increases the kinetic energy of its particles. Since the interparticle attractions in gases are negligible, both the smoke particles and iodine vapor will move faster in all directions. The increased energy causes more collisions and greater dispersion.

Key Concept: Diffusion in gases

c) The gases mix uniformly over time due to particle movement
[Solution Description]
When two gas jars are placed together, the gases mix due to diffusion. Gas particles move freely and randomly, spreading from regions of high concentration to low concentration until uniform distribution is achieved. This demonstrates that gases have no fixed volume or shape.

Key Concept: Physical Change

d) Grinding chalk into powder
[Solution Description]
Grinding chalk into fine powder is a physical change because it does not alter the chemical composition of chalk—only the size of the particles changes (Activity 7.1). No new substance is formed in this process.

Key Concept: Interparticle spaces

b) The water level first rises, then falls after dissolution.
[Solution Description]
When sugar is added to water, the initial water level increases because sugar occupies space. After dissolving, the total volume decreases slightly because sugar particles occupy the interparticle spaces between water molecules. This indicates the presence of gaps between water particles.

Key Concept: Constituent particles, Interparticle spaces

d) Assertion is false, but Reason is true.
[Solution Description]
Assertion (A) states that the volume of the solution remains unchanged when sugar dissolves in water because sugar particles occupy interparticle spaces. However, the syllabus mentions that while sugar particles do occupy interparticle spaces, the volume may decrease slightly, making the assertion false. Reason (R) correctly explains that the sweet taste proves the presence of sugar particles, which aligns with the observation from Activity 7.2. Therefore, the reason is true, but the assertion is incorrect.

Key Concept: Constituent particles

d) They occupy the spaces between particles of another substance when dissolved.
[Solution Description]
Constituent particles are the smallest units that make up a substance and cannot be broken down further by physical means. For example, grinding chalk reduces it to tiny grains, but these grains are still composed of even smaller particles called constituent particles. Similarly, sugar dissolves into its constituent particles in water, which cannot be observed directly but can be detected by taste.

Key Concept: Interparticle attractions, states of matter

d) The strength of attractions between its constituent particles.
[Solution Description] The strength of interparticle attractions decides the state of matter. Stronger forces hold particles tightly (solids), weaker forces allow movement (liquids), and very weak forces lead to free movement (gases).

Key Concept: States of matter

d) Strongest in solids and weakest in gases.
[Solution Description]
Interparticle forces are strongest in solids, keeping particles closely packed with minimal movement. In liquids, these forces are weaker, allowing particles to move past each other. In gases, interparticle forces are negligible, making particles highly mobile and far apart.

Key Concept: Constituent particles, physical change

c) The constituent particles remain the same but become smaller in size.
[Solution Description] Breaking chalk into smaller pieces and grinding it into fine powder is a physical change. The substance remains the same, only the size of the particles changes. The smallest units obtained are the constituent particles of chalk, which cannot be broken further by physical means.

Key Concept: Constituent Particles

c) Constituent particles
[Solution Description]
Constituent particles are the smallest units of matter that retain the properties of a substance. According to Activity 7.1, even when chalk is broken into fine powder, each grain retains the properties of chalk, indicating that constituent particles are the fundamental building blocks.

Key Concept: Dissolution, interparticle spaces

c) The sugar particles break apart and fill the gaps between water particles.
[Solution Description] Dissolving sugar in water breaks it into its constituent particles, which occupy the interparticle spaces between water molecules. These tiny sugar particles disperse uniformly, making the solution sweet despite being invisible.

Key Concept: Constituent particles of substances

b) Both Assertion and Reason are true, but Reason is NOT the correct explanation of Assertion.
[Solution Description]
The assertion states that chalk can be broken down into smaller constituent particles through grinding. This is true as demonstrated in Activity 7.1 where chalk was ground into fine powder and observed under a magnifying glass. The reason mentions Acharya Kanad’s concept of Parmanu, which are tiny indivisible particles. While both statements are true, the reason does not directly explain why chalk can be broken down into smaller particles—it only provides a philosophical context. Therefore, the correct answer is b).

Key Concept: Interparticle Spaces

c) They occupy interparticle spaces between water molecules
[Solution Description]
As mentioned in Activity 7.2, dissolving sugar in water causes its constituent particles to separate and occupy the interparticle spaces between water molecules. These particles are too small to be seen but can still be sensed by taste.

Key Concept: Constituent particles and physical changes

d) Assertion is false, but Reason is true.
[Solution Description]
The assertion states that grinding chalk into fine powder results in a chemical change. This is incorrect because grinding is a physical process that only reduces the size of the particles without changing their chemical nature.
The reason correctly explains that grinding chalk breaks it into smaller constituent particles while retaining its original chemical composition.
Therefore, the Assertion is false, but the Reason is true.

Key Concept: Interparticle spacing, compressibility

b) Both Assertion and Reason are true, but Reason is NOT the correct explanation of Assertion.
[Solution Description]
The Assertion states that solids cannot be compressed easily due to closely packed particles, which is true as solids have minimal interparticle spacing and high density. The Reason states that strong interparticle forces restrict particle motion to vibrations, which is also true but does not directly explain why solids are incompressible. While both statements are correct, the Reason does not provide a causal explanation for the Assertion. The correct answer is b).

Key Concept: Movement of particles in solids

c) Only vibrations or oscillations
[Solution Description]
Particles in solids vibrate or oscillate about their fixed positions due to strong interparticle forces of attraction. They do not move freely.

Key Concept: Particle Behavior in Solids

c) Particles vibrate but cannot move past each other due to strong forces.
[Solution Description]
Solids maintain a fixed shape and volume because the particles are held together by strong interparticle forces of attraction, have minimum interparticle spacing, and cannot move freely. This restricts them from flowing or changing shape easily.

Key Concept: Effect of heating on solids

c) Thermal energy overcomes the attractive forces, turning it into a liquid
[Solution Description]
Heating provides thermal energy that increases particle vibrations, overcoming the interparticle forces at the melting point, allowing the solid to change into a liquid state.

Key Concept: Interparticle Spacing in Solids

c) Particles are closely packed with strong forces of attraction and minimum spacing.
[Solution Description]
According to the syllabus, solids have closely packed particles with minimum interparticle spacing and maximum interparticle forces of attraction. The particles cannot move past each other; they only vibrate about their fixed positions. This means that while there is some space between particles, it is minimal compared to liquids and gases.

Key Concept: Interparticle Spacing, States of Matter

c) Step-like increase with flat regions for solid and liquid phases followed by a sharp rise for gas.
[Solution Description]
The interparticle spacing changes as follows:
1. **Solid State**: Minimum spacing due to tightly packed particles.
2. **Melting (Transition to Liquid)**: Spacing increases slightly but remains moderate.
3. **Vaporization (Transition to Gas)**: Spacing increases dramatically as particles move far apart.
Therefore, the correct graph should show:
– Low spacing initially (solid phase).
– Gradual increase during melting.
– Sharp rise upon vaporization.

Key Concept: Comparison of States of Matter

b) Solids have the least spacing, followed by liquids, then gases.
[Solution Description]
The interparticle spacing increases from solids to liquids to gases. In solids, particles are closely packed with minimum spacing, in liquids they are slightly more spaced out, and in gases, the spacing is the largest with particles being free to move.

Key Concept: Real-World Application, Compression

c) Ice cube, because solids have negligible interparticle spacing.
[Solution Description]
Compression depends on interparticle spacing:
1. **Air (Gas)**: Large spacing; highly compressible.
2. **Water (Liquid)**: Moderate spacing; resists compression but still possible.
3. **Ice (Solid)**: Negligible spacing; almost incompressible due to tightly packed particles.
Thus, the ice cube will show the least compression.

Key Concept: Interparticle Spacing in Solids

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that solids have a fixed shape and volume because their particles are closely packed. This is true since the particles in solids are held together by strong interparticle forces, which prevent them from moving freely and maintain their fixed positions. The reason explains that the strong interparticle forces restrict particle movement, which is also true and correctly justifies the assertion. Therefore, both the assertion and reason are true, and the reason is the correct explanation of the assertion.

Key Concept: Interparticle Forces, Phase Change

c) Liquid iron has weaker interparticle forces than solid iron but stronger than gaseous iron.
[Solution Description]
Key concepts:
1. **Solids** have maximum interparticle forces and minimum spacing.
2. **Liquids** have weaker forces and greater spacing than solids.
3. **Gases** have negligible forces and maximum spacing.
Since temperature is constant, the state-dependent properties dominate. Thus:
– Solid iron has the strongest forces and smallest spacing.
– Gaseous iron has negligible forces and largest spacing.

Key Concept: Interparticle spacing in solids

c) Minimum compared to liquids and gases
[Solution Description]
In solids, particles are closely packed with minimum interparticle spacing, and they have strong forces of attraction. They only vibrate in fixed positions.

Key Concept: Interparticle Spacing in Solids

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
Assertion (A) states that solids have fixed shapes due to their closely packed particles with minimal interparticle spacing, which is true as per the syllabus. Reason (R) mentions that strong interparticle forces restrict particle movement, which is also true and correctly explains why solids maintain fixed shapes. Therefore, both Assertion and Reason are true, and Reason is the correct explanation of Assertion.

Key Concept: Volume Change Upon Dissolving

d) Sugar particles occupy spaces between water particles
[Solution Description]
The sugar particles fill the intermolecular gaps between water molecules, reducing the overall volume compared to the sum of their separate volumes.
Step-by-Step Explanation:
1. Water molecules have empty spaces between them.
2. Sugar particles fit into these spaces when dissolved, occupying part of the volume initially occupied by water alone.
3. Hence, the total volume after mixing is less than the sum of the initial volumes of water and sugar.
This demonstrates the particulate nature of matter.

Key Concept: Insoluble vs. Soluble Substances, Constituent Particles

c) Water cannot overcome the strong interparticle forces in sand
[Solution Description]
Sand doesn’t dissolve because its particles have strong interparticle bonds that water cannot break, and their size/shape doesn’t allow them to fit between water particles. Sugar dissolves because its molecular structure allows water particles to overcome weaker intermolecular forces and fit into spaces between water molecules.
Key differences:
1. Chemical nature: Sugar forms hydrogen bonds with water; sand (SiO₂) doesn’t
2. Particle interactions: Stronger ionic bonds in sand resist separation
3. Size compatibility: Sugar molecules fit between water molecules

Key Concept: Diffusion in Gases

b) The fragrance spreads uniformly in the entire room
[Solution Description]
Fragrance from the incense stick spreads throughout the room because gas particles move freely and mix with air.
Step-by-Step Explanation:
1. The heat from burning causes the particles of the incense to move rapidly.
2. These particles mix with air particles, which are also in constant motion.
3. As they collide and spread, the fragrance becomes detectable everywhere in the room.
Thus, the movement of gas particles allows the fragrance to diffuse.

Key Concept: Volume Change

c) The total volume decreases slightly due to particle arrangement
[Solution Description]
When sugar dissolves in water, the sugar particles occupy the spaces between water particles, resulting in a volume that is less than the sum of individual volumes before mixing.

Key Concept: Dissolution

b) Because sugar particles are attracted to water and fill the spaces between water particles
[Solution Description]
Sugar dissolves in water because its particles disperse and occupy the spaces between the water particles. However, sand does not dissolve because its particles do not interact similarly with water.

Key Concept: Particle nature of matter, dissolution, particle motion

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
Initially, when sugar is added to water, the water level rises due to the addition of sugar particles. However, upon dissolving, the sugar particles occupy the empty spaces between water molecules, leading to a slight decrease in the overall volume. This phenomenon supports the assertion that the volume of the solution decreases compared to the sum of individual volumes. The reason correctly explains this observation by stating that sugar particles fill the intermolecular spaces of water. Hence, both the assertion and the reason are true, and the reason justifies the assertion.

Key Concept: Constituent Particles of Matter, Space Between Particles

c) Constituent particles of sugar occupy space between water particles
[Solution Description]
The initial rise in water level from 50 mL to 52 mL shows that sugar occupies space. After dissolving, the final volume reduces to 51.5 mL because sugar particles fit into the spaces between water particles, demonstrating that there is space between particles of matter. This reduction in volume confirms that constituent particles occupy intermolecular spaces.
Key steps:
1. Initial volume without sugar: 50 mL
2. Volume after adding sugar: 52 mL (shows sugar occupies space)
3. Volume after dissolving sugar: 51.5 mL (proves sugar particles occupy space between water particles)

Key Concept: Particle Composition

c) The particles retain their original properties
[Solution Description]
The chalk can be broken down into smaller and smaller pieces, but each tiny piece still retains the properties of chalk. This shows that matter is made up of tiny particles that are the basic building blocks.

Key Concept: Movement of Particles, Dissolution of Sugar

b) Sugar will dissolve faster in Beaker A due to greater particle movement
[Solution Description]
Sugar dissolves faster in Beaker A (hot water) because higher temperature increases the kinetic energy of water particles, making them move faster. This faster movement allows water particles to interact with sugar particles more frequently and break them apart quicker.
Key observations:
1. Faster dissolution in hot water due to increased particle movement
2. No difference in final solubility, only rate differs
3. Temperature affects particle kinetics but not chemical composition

Key Concept: Particle Behavior in Matter

c) Spread uniformly throughout the water
[Solution Description]
The sugar particles occupy the spaces between water particles due to their small size and constant motion. This results in the sugar spreading uniformly throughout the water.
Step-by-Step Explanation:
1. Sugar particles are tiny and can fit between the gaps of water molecules.
2. As stirring is applied, these particles move and distribute equally.
3. The sweetness indicates that sugar particles are still present, just not visibly separate.
Therefore, sugar particles spread evenly in the solvent.

Key Concept: Particle Nature of Matter

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that grinding chalk produces smaller particles that are still chalk, which is true as per Activity 7.1. The reason states that grinding chalk is a physical change where the substance remains the same, which is also true and correctly explains why the assertion is valid. Therefore, both the assertion and reason are true, and the reason is the correct explanation of the assertion.

Key Concept: Constituent particles of matter

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that grinding chalk into fine powder is a physical change involving only a reduction in particle size without altering the chemical nature of chalk. This is true because no new substance is formed during grinding. The reason claims that the tiny grains obtained are still chalk particles and represent the smallest constituent units of chalk, which is also correct since they retain chalk’s properties. Furthermore, the reason correctly explains why the assertion holds true, as it demonstrates how grinding affects the physical state without changing the identity of the substance.

Key Concept: Interparticle Attraction in Solids

c) Particles vibrate about fixed positions due to strong interparticle forces
[Solution Description]
In solids, particles are tightly packed with strong interparticle forces of attraction. These forces hold the particles in fixed positions, allowing them to only vibrate but not move freely. This results in a definite shape and volume for solids.

Key Concept: Interparticle Attraction, Thermal Energy, State Transition

c) At the melting point, interparticle forces weaken but remain significant
[Solution Description]
When a solid is heated, thermal energy increases the vibrational motion of particles. At the melting point, the thermal energy becomes sufficient to overcome the strong interparticle forces holding the particles in fixed positions. This allows the particles to move more freely, transitioning the solid into a liquid. The interparticle forces weaken but do not disappear entirely.

Key Concept: Movement of Particles

a) Solid
[Solution Description]
Solids have the least freedom of movement as particles can only vibrate in fixed positions due to strong interparticle forces. Liquids allow some movement, while gases have maximum freedom.

Key Concept: Interparticle attraction, States of matter

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The Assertion states that the melting point of a substance increases with stronger interparticle forces. This is true because solids with stronger interparticle attractions need more thermal energy to break these forces and transition into liquids.
The Reason explains that stronger interparticle forces require higher thermal energy to overcome, which correctly justifies why the melting point is higher. Therefore, both Assertion and Reason are true, and the Reason is the correct explanation of the Assertion.

Key Concept: Interparticle Attraction and States of Matter

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that solids have a fixed shape due to strong interparticle forces holding particles in fixed positions, which is true. The reason given is that interparticle forces in solids are stronger than in liquids and gases, restricting particle movement, which is also true. Moreover, the reason correctly explains why solids have a fixed shape. Therefore, both the assertion and reason are true, and the reason is the correct explanation of the assertion.

Key Concept: Interparticle Spacing in Liquids

c) Interparticle spacing increases slightly but remains moderate
[Solution Description]
During melting, thermal energy overcomes some of the interparticle forces in solids, increasing the spacing between particles. However, the spacing is still moderate compared to gases, as liquids retain a definite volume but lose their fixed shape due to restricted movement of particles.

Key Concept: Interparticle Attraction

a) Solid
[Solution Description]
The interparticle forces of attraction are strongest in solids due to tightly packed particles with minimal spacing. Liquids have weaker forces, and gases have negligible forces.

Key Concept: Interparticle Spacing

a) Increases
[Solution Description]
When a solid melts into a liquid, the interparticle spacing increases slightly because particles gain energy to move past each other, though they stay close due to weaker but still significant attractions.

Key Concept: Interparticle Spacing, Compressibility, States of Matter

d) Gases have maximum interparticle spacing with negligible forces, allowing compression
[Solution Description]
Gases have negligible interparticle forces and maximum interparticle spacing, allowing particles to move freely. When external pressure is applied, the large empty spaces between particles can be reduced, making gases highly compressible. In contrast, liquids and solids have much smaller interparticle spacing, with stronger forces resisting compression.

Key Concept: Particle Movement in Gases

b) Gas particles move freely, whereas liquid particles have restricted movement
[Solution Description]
In gases, interparticle attraction is negligible, allowing particles to move freely in all directions. In contrast, liquids have weaker but still significant interparticle forces that restrict particle movement to a limited space. Therefore, gases have no fixed shape or volume, unlike liquids which have a fixed volume.

Key Concept: Melting Point, Interparticle Forces, Energy Balance

c) Substance X has stronger interparticle forces, requiring more thermal energy to melt
[Solution Description]
A higher melting point indicates stronger interparticle forces in Substance X compared to Substance Y. More thermal energy is required to overcome these stronger forces and transition Substance X from solid to liquid. Thus, the melting point is directly related to the strength of interparticle forces.

Key Concept: Interparticle Attraction in States of Matter

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that solids have a fixed shape due to tight packing and strong interparticle forces, which is true according to the syllabus. The reason claims that interparticle attraction is strongest in solids, which is also true as per the syllabus. Moreover, the reason correctly explains why solids have a fixed shape (assertion), since strong interparticle forces restrict particle movement, resulting in a fixed shape.

Key Concept: Melting Point

b) Particles start moving freely, breaking their fixed positions
[Solution Description]
At the melting point, the thermal energy overcomes the attractive forces between the particles, allowing the solid to change into a liquid. The particles gain enough energy to break free from their fixed positions.

Key Concept: Particle Arrangement, Forces of Attraction

c) 40 amu
[Solution Description]
To find the mass of one particle:
1. First, calculate the volume of the unit cell:
$V = (\text{edge length})^3 = (2 \times 10^{-8} \text{cm})^3 = 8 \times 10^{-24} \text{cm}^3$
2. Next, calculate the mass of the unit cell using the given density:
$\text{Mass} = \text{Density} \times \text{Volume} = 8 \text{g/cm}^3 \times 8 \times 10^{-24} \text{cm}^3 = 6.4 \times 10^{-23} \text{g}$
3. Assuming a simple cubic lattice (1 particle per unit cell), the mass of one particle equals the mass of the unit cell. Convert it to amu:
$\text{Mass in amu} = \frac{6.4 \times 10^{-23} \text{g}}{1.66 \times 10^{-24} \text{g/amu}} \approx 38.55 \text{amu}$
The approximate mass of one particle is $\sim 40 \text{amu}$.

Key Concept: Interparticle Forces in Solids

b) Strong interparticle forces hold particles in fixed positions
[Solution Description]
Solids have a definite shape because their particles are held together by strong interparticle forces, which keep them in fixed positions. These forces prevent the particles from moving freely.

Key Concept: Definite Shape and Volume

c) Particles are tightly packed and vibrate about fixed positions due to strong forces.
[Solution Description]
Solids have a definite shape and volume because their particles are tightly packed and held together by strong interparticle forces. These forces restrict the movement of particles, allowing them only to vibrate in fixed positions. This prevents the solid from changing its shape or volume under normal conditions.

Key Concept: Particle Arrangement, Comparison with Other States

b) 4 times stronger
[Solution Description]
Given:
1. Interparticle distance in solid ($d_s$) = $\frac{1}{2} \times$ distance in liquid ($d_l$). So, $d_s = \frac{d_l}{2}$.
2. Force follows $F \propto \frac{1}{d^2}$. Let $F_s$ and $F_l$ be the forces in solid and liquid states respectively. Then:
$\frac{F_s}{F_l} = \frac{1/d_s^2}{1/d_l^2} = \left(\frac{d_l}{d_s}\right)^2 = \left(\frac{d_l}{d_l/2}\right)^2 = 2^2 = 4$
Therefore, the interparticle forces are 4 times stronger in the solid state.

Key Concept: Melting Point

c) The particles gain enough energy to overcome the attractive forces and move freely.
[Solution Description]
When a solid reaches its melting point, the thermal energy provided overcomes the strong interparticle forces holding the particles in fixed positions. As a result, the particles gain enough energy to break free from their positions and start moving more freely, transitioning the solid into the liquid state.

Key Concept: Interparticle Spacing

c) The strong interparticle forces pull the particles closely together.
[Solution Description]
Solids have minimal empty space between particles because the interparticle forces are very strong, pulling the particles close together. Although some empty space exists due to particle size and arrangement, it is much smaller than in liquids or gases where particles are farther apart due to weaker forces or higher thermal energy.

Key Concept: Solid State

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description] The assertion states that solids have a definite shape and volume due to tightly packed particles with strong interparticle forces. This is true as per the syllabus, which mentions that solids have closely packed particles and strong interparticle forces. The reason states that in solids, particles can only vibrate about fixed positions due to these strong forces. This is also accurate according to the syllabus, where it is explained that particles in solids cannot move freely but can only vibrate. Moreover, the reason correctly explains why solids have a definite shape and volume, as the restricted movement of particles (only vibration) ensures the rigidity and fixed structure of solids. Thus, both the assertion and reason are true, and the reason is the correct explanation of the assertion.

Key Concept: Particle Arrangement in Solids

b) Particles are tightly packed and vibrate about fixed positions
[Solution Description]
In solids, particles are tightly packed and held in fixed positions due to strong interparticle attractive forces. They can only vibrate or oscillate about their fixed positions but cannot move freely.

Key Concept: Interparticle forces in solids

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that solids with strong interparticle forces have high melting points. This is true because stronger forces hold particles closely together, requiring more energy (higher temperature) to break these forces during melting. The reason correctly explains that strong interparticle forces necessitate more thermal energy to overcome them, hence resulting in a higher melting point. Both the assertion and reason are true, and the reason is the correct explanation for the assertion.

Key Concept: Solid state, interparticle forces, thermal energy

d) Assertion is false, but Reason is true.
[Solution Description]
The Assertion states that the melting point of a solid is independent of the strength of interparticle forces. This is false because the melting point is directly related to the strength of interparticle forces; stronger forces require more thermal energy to break, resulting in higher melting points.
The Reason claims that the thermal energy required depends only on temperature and not on interparticle forces. This is also false because the required thermal energy must overcome the interparticle forces, which vary depending on the substance.
Since both the Assertion and Reason are false, none of the options apply perfectly, but the closest logical match is where the Reason is true while the Assertion is false.

Key Concept: Effect of Heat, Melting Point

d) Strong interparticle forces and low thermal energy
[Solution Description]
The melting point depends on two factors:
1. **Strength of interparticle forces**: Stronger forces require more energy to break, leading to higher melting points.
2. **Thermal energy influence**: Higher thermal energy can disrupt bonding more easily, lowering the melting point if all else is equal.
Thus, the highest melting point occurs when interparticle forces are maximized, and thermal energy input is minimized.

Key Concept: Interparticle Forces and States of Matter

d) Assertion is false, but Reason is true.
[Solution Description]
The Assertion states that ice has a lower density than water due to weaker interparticle forces. However, this is incorrect because the lower density of ice arises from its open hexagonal structure, not weaker forces. The Reason correctly explains that ice particles are arranged in a rigid structure with larger spacing, which is responsible for lower density. Thus, Assertion is false, but Reason is true.

Key Concept: Interparticle Attraction

b) It decreases
[Solution Description]
The strength of interparticle forces decreases as the distance between particles increases. This is because interparticle attractions are inversely proportional to interparticle spacing.

Key Concept: Interparticle forces and states of matter

c) Interparticle forces are stronger in solids, and spacing is minimal
[Solution Description]
The compressibility of a substance depends on the interparticle spacing and forces. In solids, particles are tightly packed with strong interparticle forces, making them nearly incompressible. In gases, particles are far apart with weak interparticle forces, allowing compression.

Key Concept: Interparticle spacing in different states

c) Interparticle spacing decreases, but not as much as in solids
[Solution Description]
When a gas is compressed, the interparticle spacing decreases significantly as particles are forced closer together. Upon further compression or cooling, the gas may condense into a liquid, where interparticle spacing is smaller than in the gaseous state but larger than in solids.

Key Concept: Melting point and interparticle forces

c) Iron has stronger interparticle forces than urea
[Solution Description]
The melting point of a solid depends on the strength of interparticle forces. Iron has metallic bonds that are much stronger than the hydrogen bonds in urea, so it requires more energy (higher temperature) to break these bonds and melt.