Key Concept: Physical vs. Chemical Change

a) Burning wood
[Solution Description]
Cutting vegetables involves only a change in size and shape (physical change), while burning wood and banana ripening involve the formation of new substances (chemical changes). Making small balls of dough also represents a physical change as only the shape is altered.

Key Concept: Physical Change

c) Melting ice cubes
[Solution Description]
A physical change is a change where the substance does not transform into a new substance but may change its state, shape, or size. Melting ice cubes is an example of a physical change because water changes from solid to liquid without forming any new substance.

Key Concept: Chemical Change

d) Chemical change
[Solution Description]
The ripening of a banana involves changes in color, texture, and smell due to biochemical reactions, leading to the formation of new substances. This is a chemical change.

Key Concept: Sensory Observation of Changes

c) Hearing - listening to the popping sounds
[Solution Description]
Popping corn produces sound when the moisture inside turns to steam and bursts the kernel. This auditory change is best observed through hearing, unlike changes like color or texture which need sight or touch.

Key Concept: Physical and Chemical Changes, Reversibility

d) Burning a piece of paper
[Solution Description]
Physical changes are reversible and do not involve new substances, while chemical changes are irreversible and result in new substances. Sublimation of dry ice is a physical change. Dissolving sugar is also a physical change because the sugar can be recovered. Rusting and burning involve new substances ($Fe_2O_3$ and ashes, respectively) and are irreversible.
Step-by-step reasoning:
1. Sublimation: Dry ice changes directly from solid to gas, but no new substance forms (physical).
2. Rusting: Iron reacts with oxygen to form $Fe_2O_3$, a new substance (chemical).
3. Dissolving sugar: Sugar molecules disperse but remain chemically unchanged (physical).
4. Burning paper: Paper turns into ash, forming new compounds (chemical).
The irreversible changes with new substances are rusting and burning.

Key Concept: Physical vs Chemical Change

c) A permanent change forming new chemical substance
[Solution Description]
Rusting of iron is a chemical change because it forms a new substance (iron oxide). The process involves reaction with oxygen ($Fe + O_2 \rightarrow Fe_2O_3$) and cannot be reversed by simple physical means.

Key Concept: Physical and Chemical Changes

b) Both Assertion and Reason are true, but Reason is NOT the correct explanation of Assertion.
[Solution Description]
The assertion states that the browning and smell of a banana indicate a chemical change. This is true because these observations result from biochemical reactions (oxidation, enzymatic browning) altering the banana's composition. The reason states that changes detectable by multiple senses (sight, smell) often indicate chemical changes. This is also true, as chemical changes typically produce new substances with different properties affecting multiple senses. However, while the reason supports the assertion, it is not universally applicable (some physical changes like melting ice can be detected by multiple senses too). Thus, both are true, but the reason does not fully explain the assertion.

Key Concept: Observation using senses

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that the sense of smell can detect the ripening of bananas. This is correct because as bananas ripen, they emit a noticeable odor. The reason explains why this is possible, as the browning process produces a strong smell. Therefore, both the assertion and reason are true, and the reason correctly explains the assertion.

Key Concept: Observation using senses

d) Sight and smell
[Solution Description]

Key Concept: Physical Changes, State Change

b) A physical change occurs as the identity of water remains unchanged.
[Solution Description]
Heating water converts it into steam (a gas), which is a physical change because no new substance is formed—only the state of water changes. Cooling the steam reverses the process, turning it back into liquid water. This confirms that only physical properties (state) are altered.

Key Concept: Reversible Changes

a) Boiling water
[Solution Description]
Reversible changes can be undone to return the substance to its original state. Option (a) shows boiling water, which can be reversed by condensing steam back to water. Option (b) shows cutting wood, which cannot be reversed as the pieces cannot seamlessly rejoin. Option (c) shows souring milk, a chemical change that cannot be reversed. Option (d) shows baking a cake, another irreversible chemical change. The correct answer is \textbf{a)}.

Key Concept: Physical Changes

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that crushing chalk into powder is a physical change. This is true because, as per the syllabus, physical changes involve alterations in shape or size without forming new substances. The reason explains that no new substance is formed and the original chalk can theoretically be obtained back by compacting the powder, which is also correct. Moreover, the reason correctly justifies why it is a physical change (no new substance is formed). Hence, both Assertion and Reason are true, and Reason is the correct explanation of Assertion.

Key Concept: Physical change, reversibility

b) No new substance is formed, and the rubber remains chemically unchanged.
[Solution Description]
In this case, the rubber material of the balloon remains chemically the same before and after inflation and pricking. The change involves only the shape and size of the balloon, not its chemical composition. Although the balloon cannot be returned to its original state due to irreversible damage (hole created by the pin), no new substance is formed. Thus, the process fits the definition of a physical change.

Key Concept: Examples of Physical Changes

c) Ripening of fruits
[Solution Description]
Physical changes do not form new substances. Ripening of fruits involves biochemical reactions that alter the fruit's composition, making it a chemical change. The other options are physical changes involving changes in state or shape without forming new substances.

Key Concept: Physical Changes

c) Assertion is true, but Reason is false.
[Solution Description]
The assertion (A) states that crushing chalk into powder is a physical change, which is true according to the syllabus as it involves only a change in shape and size without forming a new substance. However, the reason (R) claims that the original chalk can be obtained back by reversing the process, which is false because crushing chalk into powder is an irreversible physical change. Thus, Assertion is true, but Reason is false.

Key Concept: Reversibility of Physical Change

d) Unfolding a folded paper
[Solution Description]
Unfolding a folded paper returns it to its original form without forming a new substance. This is a reversible physical change. Burning or tearing paper is irreversible.

Key Concept: Physical Changes - Reversibility

d) It alters only the appearance without changing the substance.
[Solution Description]
Physical changes involve alterations in physical properties like shape or size without forming new substances. Folding paper is reversible and does not produce any new material, which classifies it as a physical change.

Key Concept: Physical Changes, Substance Properties

c) Burning the paper completely in a flame
[Solution Description]
Physical changes only alter appearance without changing the substance. We need to identify a process that changes the paper's chemical composition rather than just its physical form. Burning paper produces new substances (ash, smoke) through combustion, while folding, cutting, and wetting (without dissolving) only change physical properties.

Key Concept: Inflating and Bursting Balloons

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
Inflating a balloon involves only a change in shape and size, which are physical properties. The air inside the balloon remains chemically unchanged. Therefore, the assertion (A) is true as inflating a balloon is a physical change. The reason (R) is also true because no new substances are formed during this process. Moreover, the reason correctly explains why it is a physical change by stating that no new substance is formed. Thus, both Assertion and Reason are true, and Reason is the correct explanation of Assertion.

Key Concept: Energy transfer, Elasticity

c) Rapid expansion of air generates shockwaves.
[Solution Description]
The sudden release of compressed air from the balloon converts stored elastic potential energy into kinetic energy, creating a sound wave due to rapid air displacement.

Key Concept: Physical Change

b) Physical change
[Solution Description]
Crushing chalk changes its appearance and size, but the substance remains the same chemically. This is a physical change where no new substance is formed. The chalk pieces become smaller in size but retain their original properties.

Key Concept: Physical Change: Crushing Chalk

d) Assertion is false, but Reason is true.
[Solution Description]
Crushing chalk breaks it into smaller pieces but does not alter its chemical structure. The assertion claims that the chemical composition changes during crushing, which is incorrect. The reason correctly identifies crushing chalk as a physical change since no new substance is formed. Therefore, the assertion is false, and the reason is true.

Key Concept: \textbf{Evaporation}

b) Water changes from liquid to gas state
[Solution Description]
Evaporation is a process where water changes from liquid to gas (water vapor). It is a physical change because no new substance is formed, only the state changes.

Key Concept: Melting of Ice

b) Physical change with no new substance formed
[Solution Description]
Melting of ice is a physical change because only the state of water changes from solid to liquid. No new substance is formed; the chemical composition remains H$_2$O. The appearance changes but the substance remains the same.

Key Concept: Examples of Physical Changes

d) The balloon changes shape but remains the same material
[Solution Description]
Inflating and deflating a balloon is a physical change because the rubber sheet of the balloon remains the same material. Only its shape and size change, and it can be restored to its original state (uninflated form).

Key Concept: Physical Change, Reversibility

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
Crushing a piece of chalk into powder is a physical change because no new substance is formed; only the size and shape of the chalk change. However, it is not possible to get back the original piece of chalk from the powder, making the change irreversible. This aligns with the Reason given, which states that irreversible physical changes prevent the original substance from being recovered in its initial form. Therefore, both the Assertion and Reason are true, and the Reason correctly explains the Assertion.

Key Concept: Chemical Changes, Combustion, Rusting

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that both rusting and combustion are chemical changes due to the formation of new substances. This is correct as rusting forms iron oxide (rust), and combustion of magnesium forms magnesium oxide. The reason correctly identifies that oxygen from the air reacts in both cases (with iron to form rust and with magnesium to form magnesium oxide). Moreover, the reason accurately explains why these are chemical changes since the reaction with oxygen leads to the formation of new substances. Therefore, both the assertion and reason are true, and the reason is the correct explanation for the assertion.

Key Concept: Chemical change, Lime water reaction

c) Calcium carbonate and water
[Solution Description]
When carbon dioxide reacts with lime water (calcium hydroxide), calcium carbonate (a white precipitate) and water are formed. The chemical equation is:
$\text{CO}_2 + \text{Ca(OH)}_2 \rightarrow \text{CaCO}_3 + \text{H}_2\text{O}$
Carbon dioxide gas combines with lime water to produce calcium carbonate (the white precipitate) and water.

Key Concept: Chemical Changes

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that blowing air into lime water turns it milky due to the formation of a new substance. This is true as per the syllabus description. The reason explains that carbon dioxide reacts with lime water to form calcium carbonate (which is insoluble and makes the solution milky) and water, confirming a chemical change. Hence, both the assertion and reason are true, and the reason correctly explains the assertion.

Key Concept: Rusting

c) Rusting of iron
[Solution Description]
Rusting involves the reaction of iron with oxygen and moisture to form iron oxide (rust), which is a new substance. This is an irreversible chemical change. Other options like melting ice or breaking glass are physical changes where no new substance is formed.

Key Concept: Observation of Chemical Change

b) Lime water turns milky
[Solution Description]
Blowing air (exhaled air contains carbon dioxide) into lime water causes a chemical reaction. The carbon dioxide reacts with calcium hydroxide (lime water) to form calcium carbonate and water. Calcium carbonate is insoluble, making the solution appear milky or cloudy.

Key Concept: Chemical Changes

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description] When air is blown into lime water, the carbon dioxide ($CO_2$) present in the exhaled air reacts with calcium hydroxide ($Ca(OH)_2$) in the lime water. This reaction forms calcium carbonate ($CaCO_3$), which is insoluble in water and appears as a milky white precipitate. Along with this, a small amount of water ($H_2O$) is also formed. The chemical equation for this reaction is: $Ca(OH)_2 + CO_2 \rightarrow CaCO_3 + H_2O$. Therefore, both the Assertion and Reason are true, and the Reason correctly explains why the Assertion occurs.

Key Concept: Test for Gas

c) By passing it through lime water
[Solution Description]
The presence of carbon dioxide can be confirmed by passing the gas through freshly prepared lime water. If the lime water turns milky, it indicates the formation of calcium carbonate, proving that the gas is carbon dioxide.

Key Concept: Chemical vs. physical change, control experiment

c) It highlights that vinegar’s acidity drives the chemical change.
[Solution Description]
The absence of bubbles with water shows no gas (new substance) forms, ruling out a chemical change. With vinegar, CO$_2$ formation confirms a chemical reaction. This controlled experiment isolates reactivity differences between vinegar and water.

Key Concept: Chemical change, Lime water test, Carbon dioxide properties

d) Carbon monoxide (\$\text{CO}\$)
[Solution Description]
Exhaled air contains carbon dioxide ($\text{CO}_2$), which turns lime water milky by forming calcium carbonate. Sodium hydroxide absorbs acidic gases like sulfur dioxide ($\text{SO}_2$) but not carbon dioxide. Sulfur trioxide ($\text{SO}_3$) reacts with sodium hydroxide to form sodium sulfate, leaving no gas to react with lime water. Nitrogen dioxide ($\text{NO}_2$) is acidic but does not form a precipitate with lime water. Only carbon monoxide ($\text{CO}$), when oxidized to carbon dioxide ($\text{CO}_2$) by passing through sodium hydroxide, can turn lime water milky. However, this requires additional steps not mentioned here. Thus, none of the given options directly fit except sulfur dioxide ($\text{SO}_2$), which can be absorbed by sodium hydroxide and not react with lime water. Hence, the correct answer is none would turn lime water milky under these conditions.

Key Concept: Chemical Change - Lime Water Test

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that passing carbon dioxide through lime water turns it milky because of the formation of calcium carbonate. This is true as per the syllabus, where the reaction $\text{Ca(OH)}_2 + \text{CO}_2 \rightarrow \text{CaCO}_3 + \text{H}_2\text{O}$ occurs. The reason explains that this milky appearance confirms the formation of a new substance (calcium carbonate), which is correct and directly supports the assertion as a chemical change involves the formation of new substances.

Key Concept: Combustion

b) Magnesium oxide
[Solution Description]
Combustion of magnesium ribbon results in the formation of magnesium oxide, along with heat and light. The reaction can be written as: $Magnesium + Oxygen \rightarrow Magnesium\ oxide + Heat + Light$. Thus, magnesium oxide is the main product.

Key Concept: Rusting

c) Formation of a new substance with different properties
[Solution Description]
Rusting involves the formation of a new substance, iron oxide (rust), which has different properties from iron. This transformation cannot be reversed by simple physical means, making it a chemical change.

Key Concept: Rusting, Chemical Change

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
Assertion (A) states that painting iron surfaces prevents rusting by blocking the supply of water and oxygen. This is true because paint acts as a barrier, preventing moisture and air from coming into contact with the iron surface, thereby inhibiting rust formation. Reason (R) states that rusting is a chemical change that requires both water and oxygen to form iron oxide. This is also true since rusting is an oxidation reaction where iron reacts with oxygen in the presence of water to form hydrated iron(III) oxide (rust). The reason correctly explains why blocking water and oxygen (as mentioned in the assertion) prevents rusting. Therefore, both the assertion and reason are true, and the reason is the correct explanation of the assertion.

Key Concept: Rusting, Environmental Factors

c) The portion at the water-air interface due to simultaneous oxygen and water availability
[Solution Description]
Rusting requires both oxygen and water. The portion at the water-air interface has continuous access to dissolved oxygen in water and constant exposure to moisture, making it the most prone to rusting. Fully submerged parts have limited oxygen, and parts in air alone may not have sufficient moisture for rapid rusting compared to the interfacial region.

Key Concept: Rusting, Chemical Change, Iron Oxide Formation

b) Both Assertion and Reason are true, but Reason is NOT the correct explanation of Assertion.
[Solution Description]
The assertion states that rusting of iron is a chemical change due to the formation of a new substance, iron oxide. This is correct as per the syllabus, which describes rusting as a chemical change involving the irreversible formation of iron oxide. The reason states that forming rust requires oxygen and water, which is also true according to the syllabus. However, while both statements are true, the reason does not directly explain why rusting is a chemical change; it only provides the conditions required for rusting. Therefore, the reason is not the correct explanation of the assertion.

Key Concept: Prevention of Rusting

b) Galvanization
[Solution Description]
Galvanization is the process of applying a protective zinc coating to steel or iron to prevent rusting. Zinc acts as a sacrificial anode, corroding before the underlying metal does.

Key Concept: Fire Safety

c) Wrap in a blanket
[Solution Description]
To extinguish fire on clothes, the best method is to wrap the person in a blanket (not synthetic) to cut off the air supply, which is essential for sustaining combustion. This removes one of the three requirements for combustion: oxygen.

Key Concept: Combustion, Ignition Temperature, Requirements for Combustion

c) Assertion is true, but Reason is false.
[Solution Description]
The assertion states that wood will not catch fire if its temperature is below the ignition temperature. This is correct because reaching the ignition temperature is essential for combustion to start. The reason claims that reaching the ignition temperature alone is sufficient for combustion, ignoring the other two requirements: oxygen and heat. This makes the reason false because without oxygen or heat, combustion cannot occur.

Key Concept: Combustion

d) Magnesium oxide
[Solution Description]
When magnesium ribbon burns, it reacts with oxygen to form magnesium oxide along with the release of heat and light. The chemical reaction can be written as: $Magnesium + Oxygen \rightarrow Magnesium\ oxide + Heat + Light$. Thus, the correct product is magnesium oxide.

Key Concept: Chemical Change in Combustion

c) Magnesium oxide, heat, and light.
[Solution Description]
Burning magnesium ribbon reacts with oxygen in the air to form magnesium oxide. The reaction also produces heat and light. The chemical equation for this process is:
$Magnesium + Oxygen \rightarrow Magnesium\ oxide + Heat + Light$

Key Concept: Fire Triangle, Combustible Substances, Safety Note

c) Removing the cotton cloth from the heat source.
[Solution Description]

Key Concept: Combustion Definition

b) A chemical reaction involving oxygen, producing heat and/or light
[Solution Description]
Combustion is a chemical reaction where a substance reacts with oxygen, producing heat and/or light. The syllabus defines it as such, emphasizing substances like wood, paper, cotton, and kerosene as examples.

Key Concept: Combustion and Oxygen Requirement

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that a candle stops burning when covered with a glass tumbler because the oxygen supply is cut off. This is true as combustion requires oxygen, which is part of the fire triangle. The reason states that combustion requires oxygen to sustain the chemical reaction that produces heat and light. This is also true and directly explains why the candle stops burning when oxygen is removed. Therefore, both the assertion and reason are true, and the reason correctly explains the assertion.

Key Concept: Combustion, Fire Triangle

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The Assertion states that wrapping a person in a woolen blanket extinguishes fire, which is correct as it cuts off the oxygen supply, an essential component for combustion (as per the Fire Triangle). The Reason correctly explains the scientific basis behind this action by stating that woolen blankets cut off oxygen, thereby fulfilling one of the conditions of the Fire Triangle. Hence, both are true, and the Reason correctly explains the Assertion.

Key Concept: Physical Change

a) Physical change
[Solution Description]
Crushing a piece of chalk into powder only changes its size and shape. No new substance is formed, so it is a physical change.

Key Concept: Physical and Chemical Changes

c) Burning a candle
[Solution Description]
Burning a candle involves both physical change (melting of wax) and chemical change (burning of wax producing new substances). This is given in the syllabus as an example of a process with both types of changes.

Key Concept: Physical and Chemical Changes in a Candle

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The Assertion states that burning a candle involves both physical and chemical changes. This is true because:
1. Physical Change: Wax melts due to heat, changing from solid to liquid (reversible).
2. Chemical Change: The vaporized wax burns, producing CO$_2$ and water (irreversible).
The Reason correctly explains the Assertion by specifying melting as a physical change and combustion as a chemical change. Thus, both are true, and Reason justifies the Assertion.

Key Concept: Physical and Chemical Changes

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that the melting of wax is a physical change, which is correct because it involves a change in state from solid to liquid without forming any new substance. The reason provided is also correct as it explains why melting of wax is a physical change—no new substance is formed. Moreover, the reason correctly explains the assertion. Therefore, both the assertion and reason are true, and the reason is the correct explanation of the assertion.

Key Concept: Chemical Change in Candle Burning

c) Burning of wax vapor
[Solution Description]
The burning of wax vapor is a chemical change because it forms new substances (carbon dioxide and water). Melting and evaporation of wax are physical changes.

Key Concept: Simultaneous Physical and Chemical Changes

d) Digestion of food
[Solution Description]
The burning of a candle involves both physical (melting, evaporation) and chemical (combustion) changes. Similarly, digestion of food involves physical (chewing, mixing) and chemical (enzyme action breaking down food) changes.

Key Concept: Multi-concept, Non-straightforward path

c) P and Q
[Solution Description]
Folding paper is reversible (physical), baking cake is irreversible (chemical). Stretching rubber is reversible (physical). Thus, P-Q is the correct pair with one reversible and one irreversible change.

Key Concept: Reversible and Irreversible Changes

d) Assertion is false, but Reason is true.
[Solution Description]
The Assertion claims that chopping vegetables is a reversible change, but according to Table 5.2 in the syllabus, chopping vegetables is an irreversible change (S.No. 2). Therefore, the Assertion is false. The Reason states that chopping alters the physical structure of vegetables, preventing restoration to the original form, which is true as per the syllabus's explanation (irreversible changes). Thus, Assertion is false, but Reason is true.

Key Concept: Physical Change

d) Reversible physical change
[Solution Description]
Folding paper and then unfolding it is a physical change because no new substance is formed, and the paper retains its original properties after unfolding. Physical changes are generally reversible, unlike chemical changes which are irreversible. The correct answer is a reversible physical change.

Key Concept: Reversible Changes, Physical and Chemical Changes

b) Both Assertion and Reason are true, but Reason is NOT the correct explanation of Assertion.
[Solution Description]
The assertion states that stitching cloth into a shirt is an irreversible change. This is correct because once the cloth is stitched into a shirt, it cannot revert to its original unstitched state without undoing the stitches, which is an external intervention and not a natural reversal.
The reason claims that any change involving cutting or joining materials with external intervention is irreversible since the original structure is destroyed. While this is true in many cases, the reason does not fully explain why stitching is irreversible, as stitching specifically alters the fabric's structure in a way that cannot naturally reverse itself. Hence, both are true but the reason does not correctly explain the assertion.

Key Concept: Key characteristics of reversible changes

c) It turns back into ice
[Solution Description]
When melted ice (water) is frozen again, it turns back into ice. This demonstrates that the change is reversible because the original form (ice) is regained.

Key Concept: Reversible Changes - Melting Ice

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that melting of ice is reversible, which is correct because liquid water can be frozen back to ice. The reason explains why physical changes like melting are generally reversible—they don't change the chemical composition. The reason directly supports the assertion as it provides the underlying principle for reversibility in physical changes.

Key Concept: Reversibility, Thermodynamics

c) It partially condenses back into liquid water.
[Solution Description]
Cooling vapor below its dew point at constant volume causes condensation, reversing the boiling process. The phase change is not permanent as it can return to vapor upon heating.

Key Concept: Energy Input, Entropy

c) Entropy increases due to greater molecular freedom.
[Solution Description]
Entropy ($S$) measures disorder. During boiling, the system absorbs heat ($Q$) to break intermolecular bonds, increasing molecular randomness. Thus, $\Delta S = Q/T > 0$.

Key Concept: Chemical and Physical Changes, Irreversibility

b) A chemical change due to new substance formation making it irreversible
[Solution Description]
Rusting of iron is a chemical change where iron reacts with oxygen and moisture to form iron oxide (rust). This process cannot be reversed because the original iron is chemically transformed into a new compound (iron oxide), which has different properties. The formation of a new substance makes this change irreversible.

Key Concept: Human-Induced Irreversible Changes

d) Irreversible chemical change
[Solution Description]
Cooking an egg involves denaturation of proteins, which permanently alters its structure—raw eggs cannot be obtained again once cooked. This aligns with irreversible changes like making popcorn or chopping vegetables mentioned in the syllabus. Reversible changes (e.g., melting wax) do not apply here.

Key Concept: Physical vs chemical changes, Multi-concept challenge

c) One reversible physical change, one irreversible chemical change, and one irreversible physical change
[Solution Description]
Chopping onions is a reversible physical change (if done carefully). Frying causes an irreversible chemical change due to Maillard reactions and caramelization. Blending results in an irreversible physical change as the onion cells are thoroughly broken down.

Key Concept: Reversible vs Irreversible Changes

c) Chopping onions
[Solution Description]
Chopping vegetables is a change that cannot be reversed since the original form of the vegetable cannot be restored after being chopped. This is mentioned in Table 5.2 from the syllabus as an irreversible change.

Key Concept: Reversible and Irreversible Changes

b) No, it is an irreversible change
[Solution Description]
Making popcorn is an irreversible change because once the corn kernel pops, it cannot be converted back to its original form due to the physical and chemical changes that occur during heating.

Key Concept: Multi-Concept: Reversible Changes, Physical vs. Chemical Changes

c) The first is reversible, while the second is irreversible.
[Solution Description]
Stretching a rubber band is a reversible physical change because the rubber band returns to its original shape when released. Toasting bread is an irreversible chemical change (like making popcorn) due to the Maillard reaction, which alters the bread's composition permanently. These examples contrast reversible and irreversible changes as per the syllabus.

Key Concept: Desirable vs Undesirable Changes

b) Both Assertion and Reason are true, but Reason is NOT the correct explanation of Assertion.
[Solution Description]
The assertion states that rusting of iron is undesirable as it weakens metallic structures, which is true according to the syllabus. The reason mentions that undesirable changes can be converted into desirable ones, like using rusted iron for art, which is also true. However, the reason does not directly explain why rusting is undesirable; instead, it provides an alternative use case. Thus, both statements are true, but the reason does not correctly explain the assertion.

Key Concept: Environmental Impact of Changes

a) Increased fuel consumption in vehicles
[Solution Description]
Some changes due to human activities have long-term negative impacts on the environment. Increased consumption of fuels in cars, trains, and airplanes leads to higher carbon dioxide levels in the atmosphere, contributing to climate change. Drying of paint releases pollutants into the air. While composting is beneficial, burning plastics releases harmful chemicals. Among the options, increased fuel consumption leading to higher CO\_2 levels has a significant negative impact.

Key Concept: Desirable and Undesirable Changes

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that the decomposition of food waste into compost is a desirable change, which is true as composting helps recycle organic waste into nutrient-rich material beneficial for plants. The reason provided is that it improves soil fertility by recycling nutrients, which is also true and correctly explains why composting is a desirable change. Therefore, both the assertion and reason are true, and the reason is the correct explanation of the assertion.

Key Concept: Environmental Impact

c) Undesirable change with long-term environmental impact
[Solution Description]
Burning fuels releases carbon dioxide, contributing to environmental pollution and climate change. This is considered an undesirable change as it has long-term negative impacts on the environment. The correct answer should identify this as an undesirable change.

Key Concept: Undesirable Changes in Cooking

c) Burnt toast forms harmful chemicals
[Solution Description]
Overcooking or burning food leads to undesirable changes, such as loss of nutrients or formation of harmful compounds. Burning toast produces acrylamide, a potentially harmful substance.

Key Concept: Basic Concept

c) Because it involves chemical reactions that transform the food
[Solution Description]
Cooking food involves chemical reactions (like protein denaturation, caramelization) that change the composition and properties of the food, making it a chemical change.

Key Concept: Ripening of Fruits

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that the ripening of a banana is a desirable change because it enhances taste and edibility. This is true as ripening softens the fruit, increases sweetness, and makes it palatable. The reason explains that ripening involves chemical changes converting complex molecules (like starch) into simpler ones (like sugars), which improves flavor and texture. Since both statements are factually correct and the reason accurately explains why the assertion is valid, they are logically connected.

Key Concept: Reversibility, chemical change, real-world application

b) Chemical changes are irreversible
[Solution Description]
Ripening is a chemical change involving the breakdown of complex substances into simpler ones. Chemical changes are irreversible under normal conditions because new substances with different properties are formed. Therefore, the banana cannot revert to its original unripe state.

Key Concept: Atmospheric Pollution, Fuel Consumption

c) Natural gas combustion generates significantly less carbon dioxide
[Solution Description]
Natural gas combustion emits less carbon dioxide and fewer pollutants (e.g., sulfur oxides) compared to coal, reducing atmospheric pollution and long-term environmental harm.

Key Concept: Environmental Impact, Rusting of Iron

c) Frequent need for replacement and higher pollution risk
[Solution Description]
High humidity and salt levels accelerate rusting, which weakens the iron railings over time, leading to maintenance costs and potential safety hazards. Additionally, rust particles can contribute to environmental pollution.

Key Concept: Chemical Change

b) Chemical change forming a new substance
[Solution Description]
Rusting is a chemical change because it results in the formation of a new substance, iron oxide (rust). The original iron reacts with oxygen and water to form this new compound, which has different properties from pure iron.

Key Concept: Rusting, Undesirable Nature of Rusting, Exceptions

d) Assertion is false, but Reason is true.
[Solution Description]
The assertion states that rusting is always undesirable because it weakens iron objects. However, this is not entirely true. While rusting is generally undesirable due to the weakening of iron structures, there are exceptions where controlled rusting can be beneficial, such as in weathering steel used in construction, where a protective rust layer forms to prevent further corrosion. Therefore, the assertion is false.
The reason claims that rusting always leads to a non-protective iron oxide layer accelerating corrosion. This is also false, as certain cases like weathering steel demonstrate that rusting can form a protective layer. Hence, both the assertion and reason are incorrect.

Key Concept: Factors affecting food spoilage, Economic impact

b) \$800
[Solution Description]
Spoilage reduction = 40\% of \$2000 = 0.4 × \$2000 = \$800. Thus, new monthly loss = \$2000 - \$800 = \$1200. Savings = Original loss - New loss = \$2000 - \$1200 = \$800.

Key Concept: Chemical Changes in Spoilage

b) Because it produces new substances that cannot revert to the original form
[Solution Description]
The decomposition of food involves chemical changes where organic matter is broken down into simpler substances like compost and gases. These changes are irreversible, unlike physical changes such as cutting or freezing.

Key Concept: Desirable and Undesirable Changes

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that composting of food waste is a desirable change, which is true as composting helps in recycling organic waste into nutrient-rich compost. The reason provided is also true because composting indeed converts waste into useful manure and helps in reducing environmental pollution by minimizing landfill waste. Moreover, the reason correctly explains why composting is a desirable change.

Key Concept: Desirable and Undesirable Changes

c) Decomposition of food waste
[Solution Description]
The decomposition of food waste into compost is a desirable change as it helps in recycling nutrients back into the soil. However, the same decomposition process is undesirable when food spoils during storage. Thus, decomposition is desirable in composting but undesirable in food spoilage.

Key Concept: Erosion

c) Constant rubbing by smaller particles carried by flowing water
[Solution Description]
River pebbles become smooth due to constant physical abrasion caused by flowing water. The water carries smaller particles that rub against the pebbles, wearing them down over time—an example of erosion.

Key Concept: Erosion, Physical changes

b) Continuous abrasion by flowing water
[Solution Description]
Smooth pebbles are formed due to constant erosion by flowing water over time. The physical action of water breaks down and rounds the edges of the rocks, making them smooth. This is an example of physical change caused by natural forces like flowing water.
Therefore, the correct answer is continuous abrasion by flowing water.

Key Concept: Physical and Chemical Weathering

d) The red layer forms due to the oxidation of iron in the rock, producing iron oxide.
[Solution Description]
The red color on the basalt rock's surface is due to chemical weathering. The iron present in basalt reacts with oxygen and water to form iron oxide ($Fe_2O_3$), which has a reddish-brown color. This reaction occurs over long periods when the rock is exposed to atmospheric conditions.

Key Concept: Chemical Weathering

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
Chemical weathering involves reactions between rocks and chemicals like water or oxygen. Here, iron in basalt reacts with oxygen and water vapor to form iron oxide hydrate ($\text{Fe}_2\text{O}_3 \cdot \text{H}_2\text{O}$), turning the rock red. This explains why the assertion about color change due to chemical weathering is true, and the reason correctly describes the chemical reaction causing this change.

Key Concept: Physical Weathering

a) Roots of trees growing into cracks in rocks
[Solution Description]
Physical weathering involves breaking down rocks without changing their chemical composition. Examples include temperature changes, tree roots growing into cracks, and freezing water.

Key Concept: Weathering, Chemical Change

b) 71.4 grams
[Solution Description]
First, calculate the mass of iron in 100 grams of basalt: $100 \times 0.5 = 50$ grams. The chemical reaction is $4\text{Fe} + 3\text{O}_2 \rightarrow 2\text{Fe}_2\text{O}_3$. Molar mass of Fe is 56 g/mol, and $\text{Fe}_2\text{O}_3$ is 160 g/mol. Moles of Fe in 50 grams: $\frac{50}{56} \approx 0.893$ mol. From stoichiometry, 2 moles of $\text{Fe}_2\text{O}_3$ are formed from 4 moles of Fe, so moles of $\text{Fe}_2\text{O}_3$ formed are $\frac{0.893}{2} \approx 0.446$ mol. Mass of $\text{Fe}_2\text{O}_3$: $0.446 \times 160 \approx 71.4$ grams.

Key Concept: Factors Affecting Erosion

b) The speed of the water decreases in these areas
[Solution Description]
Sediments settle when the speed of water decreases because it no longer has enough energy to carry particles. Lakes and oceans act as areas of reduced water flow, allowing eroded materials to deposit and eventually harden into new rocks over long periods.

Key Concept: Physical Change

b) Due to constant erosion by flowing water
[Solution Description]
River rocks become smoother due to constant erosion caused by the friction of flowing water and sediments rubbing against them.

Key Concept: Erosion, Sedimentation

d) Most sediments settle at the bottom of the lake.
[Solution Description]
When the speed of flowing water decreases in a lake, its ability to carry sediments reduces. According to sediment transport principles, particles settle when the water's velocity falls below the settling velocity. Since the speed drops from 2 m/s to 0.5 m/s, the sediment load will predominantly deposit at the lakebed due to reduced kinetic energy. This matches the syllabus concept that sedimentation occurs where flow velocity decreases.

Key Concept: Physical Change, Erosion

d) Color of the sand.
[Solution Description]
Erosion by wind depends on factors like wind speed, particle size, and surface roughness. However, the color of the sand does not influence the physical process of erosion as it is unrelated to the mechanical forces (wind speed) or material properties (particle hardness). Thus, according to the syllabus, erosion is governed by physical interactions, not visual characteristics like color.

Key Concept: Chemical Weathering

c) Iron oxide ($Fe_2O_3$)
[Solution Description]
According to the syllabus, basalt contains iron, which reacts with water or air to form iron oxide ($Fe_2O_3$). This chemical change gives the rock its red colour. Thus, iron oxide is the compound responsible.

Key Concept: Weathering, Chemical Change

d) The chemical composition of the rock has permanently changed.
[Solution Description]
The formation of $Fe_2O_3$ (iron oxide) from iron in basalt rock is a chemical change because it alters the composition of the rock itself. This process involves oxidation, which cannot be reversed under normal environmental conditions. Once iron oxide forms, it remains a part of the rock permanently, making the change irreversible.

Key Concept: General Science

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
- Assertion (A) is correct because water does boil at 100°C under standard pressure.
- Reason (R) is also correct as boiling point varies with pressure.
- Further, Reason correctly explains Assertion since external pressure affects boiling point.

Key Concept: Basic Concepts

b) Both Assertion and Reason are true, but Reason is NOT the correct explanation of Assertion.
[Solution Description]
The assertion (A) states that the sum of the angles in a triangle is 180 degrees, which is a fundamental property of Euclidean geometry. The reason (R) states that a triangle is a three-sided polygon, which is also true. However, (R) does not explain why the sum of the angles is 180 degrees; it merely defines what a triangle is. Therefore, both (A) and (R) are true, but (R) is not the correct explanation of (A).