Key Concept: Combustion, Fuel Types

c) Petrol

[Solution Description]

The combustion of a fuel that produces carbon dioxide and water vapor suggests it is a hydrocarbon fuel burning in the presence of sufficient oxygen. Among the options, petrol is a hydrocarbon fuel that burns with a flame and produces these products.

Key Concept: Flame Production

b) Because candle wax vaporizes to produce a flame.

[Solution Description]

A candle burns with a flame because it vaporizes the wax, which then reacts with oxygen to produce a flame. On the other hand, coal burns without a flame because it directly reacts with oxygen in a process called glowing combustion, producing heat and light but no visible flame.

The correct answer is: Because candle wax vaporizes to produce a flame.

Key Concept: Combustion

d) Assertion is false, but Reason is true.

[Solution Description]

Combustion is defined as a chemical process in which a substance reacts with oxygen to give off heat. Sometimes, light is also given off during combustion, either as a flame or as a glow. The assertion states that combustion always produces heat and light, but this is incorrect because light is not always produced; it depends on the type of combustion. The reason correctly explains that during combustion, a substance reacts with oxygen to give off heat and sometimes light. Therefore, the assertion is false, but the reason is true.

Key Concept: Products of Combustion

c) Magnesium oxide and heat and light

[Solution Description]

When magnesium ribbon burns, it reacts with oxygen to form magnesium oxide and produces heat and light. These are the primary products of this combustion reaction.

Key Concept: Combustion, Types of Fuels

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.

[Solution Description]

The assertion states that CNG is a better fuel than petrol due to its lower harmful emissions. This is true because CNG primarily consists of methane, which burns more cleanly compared to petrol. The reason provided is that CNG burns more efficiently and completely than petrol, leading to reduced emissions of carbon monoxide and hydrocarbons. This is also true and directly explains why CNG is environmentally preferable. Therefore, both the assertion and reason are correct, and the reason is the correct explanation of the assertion.

Key Concept: Combustion Process

c) Carbon dioxide

[Solution Description]

When charcoal burns in air, it undergoes combustion to produce carbon dioxide ($CO_2$) along with heat and light. The chemical reaction can be represented as:

$C + O_2 \rightarrow CO_2$

Thus, the primary product formed is carbon dioxide.

Key Concept: Flame Characteristics

c) A candle produces vapors that ignite into a flame, whereas coal does not.

[Solution Description]

A candle produces a flame during combustion because the heat from the burning wick vaporizes the wax, which then ignites and burns as a flame. Coal, on the other hand, does not vaporize easily and instead burns through smoldering or glowing, which does not produce a visible flame.

Key Concept: Combustion and Flame

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.

[Solution Description]

The assertion states that a candle burns with a flame, which is true. When a candle burns, the heat from the flame melts the wax near the wick. This liquid wax is then drawn up the wick by capillary action, where it vaporizes due to the heat. The vaporized wax then undergoes combustion, reacting with oxygen in the air to produce heat, light, carbon dioxide, and water vapor. This combustion process produces a visible flame. Therefore, the reason correctly explains the assertion as it describes the process of how a candle burns with a flame.

Key Concept: Structure of Flame

c) Deposition of unburnt carbon particles

[Solution Description]

When a glass plate is introduced into the luminous zone of a candle flame, a blackish ring forms on it. This ring indicates the deposition of unburnt carbon particles present in the luminous zone of the flame. Therefore, the correct answer is deposition of unburnt carbon particles.

Key Concept: Combustion and Flame

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.

[Solution Description]

The assertion states that charcoal burns without producing a flame, which is true because charcoal is a solid fuel that does not vaporise during combustion. The reason explains that substances which vaporise during burning produce flames, and since charcoal does not vaporise, it does not produce a flame. Therefore, both the assertion and reason are true, and the reason correctly explains the assertion.

Key Concept: Combustion Definition

b) A chemical process where a substance reacts with oxygen to give off heat

[Solution Description]

Combustion is a chemical process where a substance reacts with oxygen to produce heat and light. The substance that undergoes combustion is called a combustible material or fuel.

Key Concept: Combustion, Types of Fuels

b) Coal and natural gas produce carbon dioxide, while hydrogen produces only water

[Solution Description]

Coal combustion produces carbon dioxide and water. Natural gas (methane) combustion also produces carbon dioxide and water. Hydrogen combustion produces only water. Therefore, all three fuels produce water during combustion, but coal and natural gas produce carbon dioxide as well.

Key Concept: Combustion, Spontaneous Combustion

d) Spontaneous combustion of coal dust

[Solution Description]

Spontaneous combustion occurs when a substance ignites without an external flame or spark due to heat generated by oxidation. In coal mines, coal dust can undergo spontaneous combustion when it reacts with oxygen in the air, generating enough heat to ignite. This is a common cause of fires in coal mines.

Key Concept: Combustion Process

d) A chemical process in which a substance reacts with oxygen to give off heat.

[Solution Description]

Combustion is a chemical process in which a substance reacts with oxygen to produce heat and often light. This definition aligns with the syllabus, where it is explicitly mentioned that combustion involves the reaction of a substance with oxygen, releasing energy in the form of heat and sometimes light.

Key Concept: Combustible Substances, Combustion Products

b) Charcoal[Solution Description]The production of only carbon dioxide and water during combustion indicates that the fuel is likely a pure hydrocarbon. Charcoal, being primarily carbon, would produce carbon dioxide upon complete combustion in the presence of oxygen, fitting the description provided.

Key Concept: Combustible Substances

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.

[Solution Description]

The assertion states that charcoal is a combustible substance because it burns in air to produce carbon dioxide, heat, and light. This is true as charcoal undergoes combustion when exposed to oxygen, producing the mentioned products. The reason explains that combustion is a chemical process in which a substance reacts with oxygen to give off heat and light. This is also true and directly explains why charcoal burns. Therefore, both the assertion and reason are true, and the reason correctly explains the assertion.

Key Concept: Combustion, Combustible and Non-combustible Substances

c) Iron nails

[Solution Description]

Non-combustible substances do not undergo combustion when exposed to a flame. Materials like iron nails, stone pieces, and glass do not burn when exposed to fire. Therefore, they are classified as non-combustible.

Key Concept: Conditions necessary for combustion

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.

[Solution Description]

Combustion is a chemical process in which a substance reacts with oxygen to produce heat and light. Air contains oxygen, which is essential for combustion to occur. Therefore, the presence of air (specifically oxygen) is a necessary condition for combustion. The assertion correctly states that air is necessary for combustion, and the reason provides the correct explanation by linking the necessity of air to the presence of oxygen, which is required for the chemical process of combustion.

Key Concept: Combustion, Types of Fuels

c) Coal (Solid Fuel)

[Solution Description]

The ignition temperature is the minimum temperature required for a fuel to start burning. Solid fuels generally have higher ignition temperatures compared to liquid or gaseous fuels because their molecules are more tightly packed, requiring more energy to break the bonds and initiate combustion. Therefore, among the given options, solid fuels like coal would require the highest temperature to initiate combustion.

Key Concept: Combustible Substances

b) Magnesium

[Solution Description]

Magnesium is a combustible substance that burns in air to form magnesium oxide, producing heat and light. This is a characteristic reaction of magnesium when it undergoes combustion.

Key Concept: Combustion, Oxygen

b) The flame flickers and produces smoke

[Solution Description]

When the chimney is placed on the table without the wooden blocks, air cannot enter from below. This restricts the supply of oxygen to the flame. As a result, the flame flickers and produces smoke due to incomplete combustion. The correct answer is (b).

Key Concept: Combustion and Oxygen

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.

[Solution Description]

The assertion states that combustion cannot occur without oxygen, which is true because oxygen is a key component required for the combustion process. The reason explains that oxygen supports the burning of substances, which is also true and directly explains why combustion requires oxygen. Therefore, both the assertion and reason are true, and the reason correctly explains the assertion.

Key Concept: Ignition Temperature and Inflammable Substances

d) Petrol

[Solution Description]

Inflammable substances like petrol, alcohol, and LPG have very low ignition temperatures, making them easily catch fire with a flame. Among the given options, petrol has the lowest ignition temperature compared to wood, paper, and kerosene oil.

Key Concept: Ignition Temperature

c) The temperature at room temperature is below its ignition temperature.

[Solution Description]

A matchstick does not catch fire at room temperature because the temperature of the matchstick is below its ignition temperature. The ignition temperature is the minimum temperature required for a substance to catch fire. When rubbed against the side of a matchbox, friction generates enough heat to reach the ignition temperature, causing the matchstick to burn.

Key Concept: Combustion, Ignition temperature

c) Rubbing generates heat that raises the temperature of the matchstick to its ignition temperature.

[Solution Description]

The matchstick does not catch fire on its own at room temperature because the temperature is below its ignition temperature. When the matchstick is rubbed against the side of the matchbox, friction generates enough heat to raise the temperature of the matchstick to its ignition temperature, causing it to burn.

Key Concept: Ignition temperature, Heat transfer

c) The water absorbs the heat, preventing the paper from reaching its ignition temperature.

[Solution Description]

The heat supplied to the paper cup is transferred to the water by conduction. Since the water absorbs the heat, the temperature of the paper does not reach its ignition temperature. Therefore, the paper cup does not burn.

Key Concept: Inflammable Substances

b) Petrol[Solution Description]Inflammable substances are those that have very low ignition temperatures and can easily catch fire with a flame. Examples include petrol, alcohol, and Liquified Petroleum Gas (LPG).

Key Concept: Inflammable Substances

c) Petrol

[Solution Description]

Inflammable substances are those that have very low ignition temperatures and can easily catch fire with a flame. Examples include petrol, alcohol, and LPG. Among the given options, petrol is a well-known inflammable substance.

Key Concept: Essential Requirements for Fire

c) Fuel, Air, Heat

[Solution Description]

The three essential requirements for producing fire are fuel, air (to supply oxygen), and heat (to raise the temperature of the fuel beyond its ignition temperature).

Key Concept: Fire Control, Fire Extinguishers

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.

[Solution Description]

The assertion states that carbon dioxide ($CO_2$) is the best extinguisher for fires involving electrical equipment and inflammable materials like petrol. This is true because $CO_2$ is non-conductive and does not harm electrical equipment, and it effectively suffocates the fire by cutting off the oxygen supply. The reason explains that $CO_2$ cuts off the supply of oxygen and cools down the fuel, preventing the fire from spreading. This is also true because $CO_2$ forms a blanket around the fire, displacing oxygen, and its expansion causes cooling, reducing the temperature of the fuel. The reason correctly explains why $CO_2$ is effective in controlling such fires.

Key Concept: Fire Brigade Station

b) To ensure quick access in case of a fire

[Solution Description]

Knowing the location of a fire brigade station is important because it ensures that help can be reached quickly in case of a fire emergency, reducing potential damage and saving lives.

Key Concept: Fire Extinguishing Method

b) It cools the combustible material

[Solution Description]

Water helps in extinguishing a fire by cooling the combustible material, bringing its temperature below the ignition point. Additionally, water vapours surround the material, cutting off the supply of air.

Key Concept: Role of water in extinguishing fire

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.

[Solution Description]

The assertion states that water is used to extinguish fires by cooling the combustible material below its ignition temperature. This is true because water absorbs heat from the burning material, lowering its temperature and stopping the combustion process. The reason states that cooling the material reduces the temperature, preventing the fire from spreading. This is also true and directly explains why water is effective in extinguishing fires. Therefore, both the assertion and reason are true, and the reason correctly explains the assertion.

Key Concept: Role of Water in Extinguishing Fire

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.

[Solution Description]

The assertion states that water is effective in extinguishing fire by cooling the combustible material below its ignition temperature. This is true because when water is poured on fire, it absorbs heat from the burning material, reducing its temperature below the ignition point. The reason states that water vapours cut off the supply of air, which is essential for combustion. This is also true because water vapours form a barrier around the combustible material, preventing oxygen from reaching it. Since both the assertion and reason are true, and the reason correctly explains why water is effective in extinguishing fire, the correct option is (a).

Key Concept: How Do We Control Fire?

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.

[Solution Description]

The assertion states that water is used to extinguish fire by cooling the combustible material below its ignition temperature. This is true because water absorbs heat from the burning material, reducing its temperature and preventing it from reaching the ignition point. The reason explains that cooling the material below its ignition temperature prevents the fire from spreading, which is also true and directly explains the assertion. Therefore, both the assertion and reason are true, and the reason correctly explains the assertion.

Key Concept: Water Cooling, Fire Extinguishing Principles

c) 1.19 kg

[Solution Description]

First, calculate the heat energy to be removed from the burning material: $Q_{\text{material}} = m_{\text{material}} c_{\text{material}} \Delta T_{\text{material}} = 2 \times 1500 \times (500 - 200) = 900000$ J. The final temperature of the water after absorbing this heat must be less than or equal to 200$^\circ$C. Using $Q_{\text{water}} = m_{\text{water}} c_{\text{water}} \Delta T_{\text{water}}$, with $\Delta T_{\text{water}} = 200 - 20 = 180^\circ$C, we solve for $m_{\text{water}}$: $m_{\text{water}} = \frac{Q_{\text{material}}}{c_{\text{water}} \Delta T_{\text{water}}} = \frac{900000}{4200 \times 180} \approx 1.19$ kg. Therefore, approximately 1.19 kg of water is required.

Key Concept: Fire Extinguisher's Role

c) To cut off the supply of air or reduce the temperature of the fuel

[Solution Description] The primary function of a fire extinguisher is to control or extinguish a fire by either cutting off the supply of air (oxygen) or by reducing the temperature of the fuel, or both. In most cases, the fuel cannot be eliminated, so the focus is on removing one or more of the other essential requirements for combustion.

Key Concept: Combustion and Flame - Water Vapour effect

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.

[Solution Description]

The assertion states that water helps in extinguishing fire by cutting off the supply of oxygen, which is true. The reason states that water vapours surround the combustible material, preventing air from reaching it, which is also true. Moreover, the reason correctly explains how water cuts off the supply of oxygen by surrounding the combustible material with water vapors. Therefore, both the assertion and reason are true, and the reason is the correct explanation of the assertion.

Key Concept: Fire extinguishers for electrical equipment

c) Carbon dioxide

[Solution Description]

Carbon dioxide ($CO_2$) is the most effective gas for extinguishing fires involving electrical equipment because it cuts off the contact between fuel and oxygen, and does not harm electrical equipment.

Key Concept: Limitations of Water as Fire Extinguisher

b) Water conducts electricity and can cause harm

[Solution Description]

Water conducts electricity, which can harm people trying to extinguish the fire. Therefore, it is not suitable for fires involving electrical equipment.

Key Concept: Fire Extinguishers, Electrical Fires

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.

[Solution Description]

Water is a good conductor of electricity, which makes it unsafe for use on electrical fires as it can cause electric shock or short circuits. On the other hand, carbon dioxide ($CO_2$) is heavier than oxygen and forms a blanket around the fire, cutting off the oxygen supply. Additionally, $CO_2$ is non-conductive, making it safe for use on electrical fires. Therefore, both the assertion and reason are true, and the reason correctly explains why water is not suitable for electrical fires.

Key Concept: Fire Extinguishers

a) Water conducts electricity and can harm people

[Solution Description]

Water conducts electricity, which can harm the person trying to douse the fire and may also cause the fire to spread. Additionally, it does not cut off the oxygen supply to the fire effectively.

Key Concept: Fire Extinguishers

c) Water sinks below oil, allowing it to keep burning

[Solution Description]

Water is heavier than oil and petrol, so it sinks below the oil, leaving the oil to continue burning on the surface. This makes water ineffective for extinguishing such fires.

Key Concept: Fire Extinguishers

a) Water sinks below the oil, allowing it to keep burning

[Solution Description]

Water is heavier than oil, so it sinks below the oil, allowing the oil to continue burning on the surface. This makes water ineffective for extinguishing oil or petrol fires.

Key Concept: CO₂ as the best extinguisher for electrical and petrol fires

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.

[Solution Description]

The assertion states that carbon dioxide is the best extinguisher for fires involving electrical equipment and inflammable materials like petrol. This is true because $CO_2$ does not harm electrical equipment and is effective in cutting off oxygen supply to the fire. The reason given is that $CO_2$ cuts off the contact between fuel and oxygen, which is also true and directly explains why $CO_2$ is effective in controlling such fires. Therefore, both the assertion and reason are true, and the reason correctly explains the assertion.

Key Concept: CO₂ as Fire Extinguisher

c) It forms a blanket around the fire and does not harm electrical equipment.

[Solution Description]

Carbon dioxide ($CO_2$) is heavier than oxygen and forms a blanket around the fire, cutting off the contact between fuel and oxygen. Additionally, $CO_2$ does not harm electrical equipment, making it suitable for electrical fires.

Key Concept: Role of Sodium Bicarbonate

c) Sodium bicarbonate $NaHCO_3$

[Solution Description]

Sodium bicarbonate $NaHCO_3$ is a chemical compound that releases carbon dioxide $CO_2$ when heated. This property makes it useful as a dry powder fire extinguisher, especially for fires involving electrical equipment.

Key Concept: Storage of CO$_2$

b) As a liquid under high pressure

[Solution Description]

Carbon dioxide (\$CO_2\$) is stored in cylinders as a liquid under high pressure. When released, it expands into a gas, which is then used to extinguish fires.

Key Concept: Rapid Combustion

c) Rapid combustion

[Solution Description]

When a gas stove is turned on, the gas burns quickly and produces heat and light. This is an example of rapid combustion, where the burning happens at a fast rate with the application of an external source like a matchstick or lighter.

Key Concept: Types of Combustion

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.

[Solution Description]

The assertion states that phosphorus burns in air at room temperature without any external heat source, which is true as phosphorus is highly reactive and can ignite spontaneously in the presence of air. The reason states that the combustion of phosphorus is an example of spontaneous combustion, which is also true because spontaneous combustion refers to the process where a material bursts into flames without any apparent cause. Therefore, both the assertion and reason are true, and the reason correctly explains the assertion.

Key Concept: Rapid Combustion

a) Rapid Combustion

[Solution Description]

When a gas stove is lit, the gas burns rapidly, producing heat and light. This is known as rapid combustion.

Key Concept: Explosion

c) Explosion

[Solution Description]

When a cracker is ignited, it undergoes a sudden reaction with the evolution of heat, light, and sound. This type of combustion is called an explosion.

Key Concept: Spontaneous Combustion

c) High temperature causing dry grass to catch fire[Solution Description]During extreme summer heat, dry grass can catch fire due to the high temperature. This fire can then spread to trees and eventually engulf the entire forest. Human carelessness is another cause of forest fires, but they are not considered spontaneous.

Key Concept: Spontaneous Combustion

c) Phosphorus

[Solution Description]

The syllabus mentions that substances like phosphorus burn in air at room temperature. This property makes phosphorus a prime example of a material that undergoes spontaneous combustion.

Key Concept: Explosion

b) Slow release of energy

[Solution Description]

An explosion in a firecracker is characterized by the rapid release of heat, light, and sound, along with the liberation of a large amount of gas. It does not involve the slow release of energy or the formation of liquid products.

Key Concept: Explosion

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.

[Solution Description]

Firecrackers undergo a rapid combustion reaction when ignited, leading to the release of heat, light, and sound. This is due to the sudden formation of a large volume of gas, which is characteristic of an explosion. The assertion correctly states that firecrackers release heat, light, and sound during combustion. The reason explains that this occurs because of the rapid combustion and sudden liberation of a large amount of gas, which is accurate. Therefore, both the assertion and the reason are true, and the reason correctly explains the assertion.

Key Concept: Flame zones, Temperature, Deposition

b) Presence of unburnt carbon particles

[Solution Description] The luminous zone of the candle flame contains unburnt carbon particles. When a clean glass plate is introduced into this zone, these carbon particles deposit on the plate, forming a circular blackish ring. This indicates the presence of unburnt carbon particles in the luminous zone of the flame. The non-luminous zone, on the other hand, has a high temperature but does not deposit carbon particles.

Key Concept: Flame Structure, Combustion

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.

[Solution Description] The non-luminous zone of a candle flame is the outermost zone where complete combustion takes place. In this zone, the fuel burns completely in the presence of sufficient oxygen, releasing maximum energy in the form of heat and light. This causes the temperature in this zone to be the highest. Therefore, both the Assertion and Reason are true, and the Reason correctly explains the Assertion.

Key Concept: Flame Structure, Combustion

c) Assertion is true, but Reason is false.

[Solution Description]

The luminous zone of a candle flame is the region where incomplete combustion occurs, leading to the presence of unburnt carbon particles, which give it a yellowish glow. However, the non-luminous zone of the flame is where complete combustion takes place, resulting in a higher temperature. This is evidenced by the fact that when a copper wire is held in the non-luminous zone, it gets red hot, indicating high heat. The reason provided incorrectly states that the non-luminous zone contains unburnt carbon particles, which is false. Unburnt carbon particles are found in the luminous zone, not the non-luminous zone. Therefore, the assertion is true, but the reason is false.

Key Concept: Flame Structure, Combustion

b) Deposition of a blackish ring

[Solution Description]

When a glass plate is introduced into the luminous zone of a candle flame, it comes into contact with unburnt carbon particles. These particles deposit on the glass plate, forming a blackish ring. This deposition indicates incomplete combustion, as the carbon particles did not fully burn in the luminous zone.

Key Concept: Combustion and Flame, Characteristics of substances that give flames

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.

[Solution Description]

The assertion states that the non-luminous zone of a candle flame has the highest temperature. This is true because the non-luminous zone is where complete combustion takes place, producing more heat. The reason given is that the non-luminous zone is where complete combustion occurs due to sufficient oxygen supply. This is also true and correctly explains why the non-luminous zone has the highest temperature. Therefore, both the assertion and reason are true, and the reason is the correct explanation of the assertion.

Key Concept: Substances that Vapourise

d) Charcoal

[Solution Description]

According to the syllabus, substances that vapourise during burning produce flames. Charcoal does not vapourise during burning and hence does not produce a flame. Therefore, the correct answer is charcoal.

Key Concept: Structure of a Flame

c) Non-luminous zone[Solution Description]When a thin long copper wire is held just inside the non-luminous zone of a flame, the portion just outside the flame gets red hot. This indicates that the non-luminous zone of the flame has a high temperature.

Key Concept: Combustion and Flame

b) Kerosene oil

[Solution Description]

Substances that vaporise during burning produce flames. For example, kerosene oil and molten wax vaporise and form flames, whereas charcoal does not vaporise and hence does not produce a flame.

Key Concept: Substances that produce flames

c) Charcoal

[Solution Description]

Substances that vaporize during burning produce flames. For example, kerosene oil and molten wax vaporize and form flames. Charcoal, however, does not vaporize during burning and thus does not produce a flame.

Key Concept: Zones of a Flame

c) Luminous zone

[Solution Description]

The luminous zone of the candle flame is characterized by the presence of unburnt carbon particles. These particles emit light, making the zone appear bright. In the activity mentioned in the syllabus, when a clean glass plate is introduced into the luminous zone, a blackish ring forms due to the deposition of these unburnt carbon particles.

Key Concept: Innermost zone of a flame

d) Deposition of unburnt carbon particles

[Solution Description]

When a glass plate is held in the luminous zone of a flame, a blackish ring forms due to the deposition of unburnt carbon particles. This indicates the presence of unburnt wax vapors in the innermost zone of the flame.

Key Concept: Innermost zone of a flame

d) Black

[Solution Description]

The innermost zone of a candle flame contains unburnt wax vapors, which appear black in color. This zone is characterized by incomplete combustion.

Key Concept: Structure of a Flame

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.

[Solution Description]

The middle zone of a candle flame, also known as the luminous zone, is yellow due to the presence of unburnt carbon particles. This occurs because partial combustion takes place in this zone, leading to the deposition of carbon particles which emit yellow light when heated. Therefore, both the Assertion and Reason are true, and the Reason correctly explains the Assertion.

Key Concept: Deposition of Carbon Particles

b) A circular blackish ring forms

[Solution Description]

When a glass plate is introduced into the luminous zone of a candle flame, it gets exposed to the unburnt carbon particles present in this zone. These carbon particles deposit on the glass plate, forming a circular blackish ring. This deposition indicates the presence of unburnt carbon particles in the luminous zone.

Key Concept: Outer zone of flame, Temperature

c) The non-luminous zone is the hottest part of the flame.

[Solution Description]

When the copper wire becomes red hot just outside the flame, it indicates that the non-luminous zone is the hottest part of the flame. This is because the heat from this zone is sufficient to make the wire red hot. The non-luminous zone is where complete combustion occurs, resulting in the highest temperature.

Key Concept: Application of Outer Zone

c) It is the hottest part

[Solution Description] Goldsmiths use the outermost zone of the flame because it is the hottest part of the flame. The high temperature in this zone allows for the melting of metals like gold and silver efficiently.

Key Concept: Structure of a Flame

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.

[Solution Description]

The assertion states that goldsmiths use the outermost zone of a flame for melting gold and silver because it is the hottest part of the flame. This is true as the outermost zone, known as the non-luminous zone, is indeed the hottest part of the flame due to complete combustion. The reason given is that the outermost zone has complete combustion and reaches the highest temperature, which is also true and correctly explains why this zone is used by goldsmiths. Therefore, both the assertion and reason are true, and the reason is the correct explanation of the assertion.

Key Concept: Structure of a Flame, Goldsmith’s use of the outermost zone for melting gold and silver

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.

[Solution Description]

The outermost zone of a flame is indeed the hottest part because it undergoes complete combustion due to the presence of sufficient oxygen. This high temperature makes it suitable for melting metals like gold and silver. Therefore, the assertion and reason are both true, and the reason correctly explains why goldsmiths use the outermost zone for melting precious metals.

Key Concept: Fuel Characteristics, Types of Fuels

d) Natural gas

[Solution Description] The ideal fuel for this scenario should produce a large amount of heat and leave behind minimal undesirable substances. Natural gas fits these criteria as it burns cleanly with high heat output and leaves almost no residue.

Key Concept: Characteristics of Fuel

b) It burns easily in air at a moderate rate

[Solution Description]

A fuel is considered good based on its availability, cost, ease of burning, amount of heat produced, and the absence of undesirable residues. The primary reason among these options is that it burns easily in air at a moderate rate.

Key Concept: Fuel Characteristics, Types of Fuels

d) All of the above

[Solution Description]

To determine the best fuel for the power plant, the company must consider several characteristics of a good fuel: availability, cost, combustion rate, heat production, and byproducts. Coal is a solid fuel that is generally cheaper and widely available but produces more undesirable substances like ash and smoke. Natural gas, a gaseous fuel, burns cleaner with fewer byproducts and produces more heat per unit volume but may be more expensive and less available in some regions.

Key Concept: Characteristics of a Good Fuel

b) It leaves behind undesirable substances

[Solution Description]

A good fuel is one that is readily available, cheap, burns easily in air at a moderate rate, produces a large amount of heat, and does not leave behind any undesirable substances. The option that contradicts these characteristics is the correct answer.

Key Concept: Characteristics of a good fuel

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.

[Solution Description]

Natural gas is widely used as a domestic fuel because it fulfills most of the characteristics of a good fuel. It is readily available, relatively cheap, and burns efficiently with a high calorific value. Additionally, natural gas burns cleanly, leaving minimal residue or pollutants, which makes it environmentally friendly. Therefore, the Assertion that natural gas is considered an ideal fuel for domestic use is true. The Reason provided states that natural gas produces a large amount of heat, burns easily at a moderate rate, and does not leave behind any undesirable substances. These are accurate characteristics of natural gas that justify its classification as an ideal fuel for domestic purposes. Thus, both the Assertion and Reason are true, and the Reason correctly explains the Assertion.

Key Concept: Characteristics of a good fuel, Heat Production, Undesirable Substances

b) Fuel Y because it does not leave any residue

[Solution Description]

For domestic heating purposes, a good fuel should produce a large amount of heat and not leave behind any undesirable substances. Fuel X produces a large amount of heat but leaves behind ash, which is an undesirable substance. Fuel Y produces less heat but does not leave any residue, making it more suitable for domestic use as it avoids the inconvenience of cleaning up ash.

Key Concept: Good Fuel Characteristics

b) Both Assertion and Reason are true, but Reason is NOT the correct explanation of Assertion.

[Solution Description] The assertion states that charcoal is considered a good fuel because it is readily available. This is true as charcoal is widely used and accessible in many regions. The reason states that a good fuel should be easily accessible and burn at a moderate rate, which is also true according to the syllabus. However, the reason does not fully explain why charcoal is specifically considered a good fuel beyond its availability. While accessibility is a characteristic of a good fuel, it does not directly explain the assertion about charcoal. Therefore, both statements are true, but the reason does not correctly explain the assertion.

Key Concept: Fuel Availability, Ideal Fuel

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.

[Solution Description]

The Assertion states that natural gas is considered an ideal fuel for domestic purposes due to its availability and moderate burning rate. This is true because natural gas is widely available and burns efficiently. The Reason explains that an ideal fuel must meet specific criteria such as being cheap, easily available, and producing a large amount of heat without leaving undesirable substances. This is also true and directly explains why natural gas is considered an ideal fuel. Therefore, both the Assertion and Reason are true, and the Reason correctly explains the Assertion.

Key Concept: Calorific Value

d) hydrogen

[Solution Description]

The calorific values of different fuels are given in Table 4.4. We compare the calorific values of the fuels listed in the options:

- Cow dung cake: 6000-8000 $kJ/kg$

- Wood: 17000-22000 $kJ/kg$

- Coal: 25000-33000 $kJ/kg$

- Hydrogen: 150000 $kJ/kg$

From these values, it is clear that hydrogen has the highest calorific value.

Key Concept: Calorific Value of Fuels

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.

[Solution Description]

The calorific value of a fuel is the amount of heat energy produced on complete combustion of 1 kg of the fuel. From the given data in Table 4.4, hydrogen has a calorific value of 150000 kJ/kg, which is the highest among all listed fuels. Therefore, the assertion is true. The reason correctly states the calorific value of hydrogen and explains why it has the highest calorific value among all fuels. Thus, the reason is also true and correctly explains the assertion.

Key Concept: Fuel Efficiency, Calorific Value

d) CNG

[Solution Description]

The factory needs a fuel with high calorific value and low harmful emissions. From the given options, CNG has a calorific value of $50000 kJ/kg$ and produces minimal harmful emissions. LPG has a slightly higher calorific value ($55000 kJ/kg$) but may produce more harmful emissions compared to CNG. Diesel and coal have lower calorific values and produce significant harmful emissions. Therefore, CNG is the most suitable choice.

Key Concept: Calorific Value, Fuel Efficiency

d) Hydrogen

[Solution Description]

Hydrogen has the highest calorific value ($150000 kJ/kg$) among the given options, making it the best choice for maximizing heat output. Methane and CNG have lower calorific values ($50000 kJ/kg$), and cow dung cake has the lowest ($6000-8000 kJ/kg$). Although hydrogen is more expensive, its high calorific value makes it an efficient choice for maximizing heat energy.

Key Concept: Types of Fuels

b) Wood

[Solution Description]

Solid fuels are those that exist in the solid state at room temperature and pressure. Examples include coal, wood, and charcoal. From the given options, coal is a solid fuel.

Key Concept: Types of Fuels

c) Natural gas

[Solution Description]

Gaseous fuels are those that remain in a gaseous state at room temperature. Based on Table 4.3, natural gas, LPG, and CNG are examples of gaseous fuels. Among the given options, natural gas is the correct answer.

Key Concept: Fuel Types

d) Charcoal

[Solution Description]

Solid fuels include substances like coal, wood, and charcoal. From Table 4.3, the examples of solid fuels are Coal, Wood, and Charcoal. Petrol, on the other hand, is a liquid fuel.

Key Concept: Calorific Value

d) Hydrogen

[Solution Description]

The calorific value of a fuel is the amount of heat energy produced per unit mass when it is completely burned. From Table 4.4, the calorific values are: Cow dung cake (6000-8000 kJ/kg), Wood (17000-22000 kJ/kg), Coal (25000-33000 kJ/kg), and Hydrogen (150000 kJ/kg). Clearly, Hydrogen has the highest calorific value among the given options.

Key Concept: Calorific Value

c) 45000

[Solution Description]

The calorific value of petrol is given as 45000 kJ/kg in the syllabus.

Key Concept: Calorific Value, CNG, LPG

b) 775000 kJ

[Solution Description]

To find the total energy produced, we calculate the energy from CNG and LPG separately and then add them together.

Energy from CNG = Mass of CNG $\times$ Calorific Value of CNG

Energy from CNG = $10 \, \text{kg} \times 50000 \, \text{kJ/kg} = 500000 \, \text{kJ}$

Energy from LPG = Mass of LPG $\times$ Calorific Value of LPG

Energy from LPG = $5 \, \text{kg} \times 55000 \, \text{kJ/kg} = 275000 \, \text{kJ}$

Total energy = Energy from CNG + Energy from LPG

Total energy = $500000 \, \text{kJ} + 275000 \, \text{kJ} = 775000 \, \text{kJ}$

Key Concept: Fuel Efficiency

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.

[Solution Description] The assertion states that LPG is the most efficient fuel for boiling water compared to cow dung and coal. This is supported by the reason, which mentions the calorific values of these fuels. Calorific value indicates the amount of heat energy produced per unit mass of fuel upon complete combustion. The calorific value of LPG (55000kJ/kg) is significantly higher than that of cow dung (6000-8000kJ/kg) and coal (25000-33000kJ/kg). Therefore, LPG produces more heat energy per kilogram, making it more efficient for boiling water. Both the assertion and the reason are true, and the reason correctly explains the assertion.

Key Concept: Unburnt Carbon Particles

b) Asthma

[Solution Description]

Unburnt carbon particles are dangerous pollutants that cause respiratory diseases such as asthma. This is a direct effect of inhaling these fine particles.

Key Concept: Acid Rain

b) Sulphur dioxide and nitrogen oxides

[Solution Description] Oxides of sulphur and nitrogen produced by the burning of coal, diesel, and petrol dissolve in rainwater to form acid rain.

Key Concept: Global Warming, Acid Rain, Respiratory Diseases

b) Unburnt carbon particles, oxides of nitrogen, and carbon dioxide

[Solution Description]

The increase in respiratory diseases is likely due to unburnt carbon particles and carbon monoxide from incomplete combustion of fuels. Damage to historical buildings is a result of acid rain caused by oxides of sulphur and nitrogen dissolving in rainwater. The rise in average temperatures is linked to increased carbon dioxide emissions leading to global warming.

Key Concept: Unburnt carbon particles, Respiratory diseases

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.

[Solution Description]

Unburnt carbon particles are indeed dangerous pollutants that can lead to respiratory problems. When inhaled, these fine particles can reach the alveoli in the lungs, causing inflammation and irritation. This inflammation can exacerbate conditions like asthma, making it difficult for individuals to breathe. Therefore, the assertion is true. The reason provided explains why unburnt carbon particles cause respiratory diseases by detailing how they penetrate the lungs and cause inflammation. Hence, both the assertion and reason are true, and the reason correctly explains the assertion.

Key Concept: Incomplete combustion, Carbon monoxide poisoning

c) 180 mL

[Solution Description]

First, calculate the amount of CO inhaled per breath:

$\text{CO per breath} = \frac{0.05}{100} \times 400 = 0.2 \text{ mL}$

Then, calculate the total number of breaths in one hour:

$\text{Breaths per hour} = 15 \times 60 = 900 \text{ breaths}$

Finally, calculate the total CO inhaled in one hour:

$\text{Total CO inhaled} = 0.2 \times 900 = 180 \text{ mL}$

Therefore, the person inhales 180 mL of carbon monoxide in one hour.

Key Concept: Global Warming

c) Release of carbon dioxide

[Solution Description]

The primary cause of global warming due to fuel combustion is the release of carbon dioxide into the atmosphere. Increased carbon dioxide concentration traps heat, leading to a rise in Earth's temperature.

Key Concept: Prevention

c) It produces fewer harmful products

[Solution Description]

CNG (Compressed Natural Gas) produces harmful products like sulfur dioxide and nitrogen oxides in very small amounts compared to diesel and petrol. This makes CNG a cleaner fuel, reducing the formation of acid rain. Therefore, the correct answer is that it produces fewer harmful products.