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 and Flame

d) Assertion is false, but Reason is true.

[Solution Description]

The assertion states that combustion always produces a flame, which is incorrect. Combustion does not always produce a flame; for example, coal burns without a flame. The reason states that a flame is produced when a substance undergoes combustion and releases light and heat, which is true. However, this does not explain the assertion because combustion can occur without a flame. Therefore, the assertion is false, but the reason is true.

Key Concept: Combustion, Fuel Types, Chemical Process

c) A substance that reacts with oxygen to produce heat and light

[Solution Description]

A fuel is a substance that undergoes combustion. During this process, the fuel reacts with oxygen to produce heat and often light. The fuel can be in solid, liquid, or gaseous form. For example, wood, gasoline, and natural gas are all fuels that can undergo combustion. Therefore, the correct description of a fuel is a substance that reacts with oxygen to produce heat and light.

Key Concept: Definition of Combustion

c) Combustion

[Solution Description]

The process in which a substance reacts with oxygen to produce heat is known as combustion. This is a fundamental concept in chemistry and is essential for understanding how fuels work.

Key Concept: Types of Fuels

b) CNG

[Solution Description]

Gaseous fuels used in automobiles include Compressed Natural Gas (CNG) and Liquefied Petroleum Gas (LPG). Among these, CNG is more commonly used in automobiles due to its lower emissions and cost-effectiveness compared to petrol and diesel.

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: Types of Fuels

b) Petrol

[Solution Description]

Automobiles typically use fuels like petrol, diesel, and compressed natural gas (CNG). Among these options, petrol is the most common fuel used in automobiles.

Key Concept: Combustion, Flame, Types of Fuel

c) Candle burns with a flame while coal burns without a flame

[Solution Description]

A candle burns with a flame because it vaporizes upon heating, and the vapors combust in the presence of oxygen, producing a flame. Coal, on the other hand, does not vaporize but instead undergoes combustion directly in the solid state, producing heat and light without a flame. Therefore, the key difference lies in the presence of a flame during the combustion process.

Key Concept: Fuel Types

c) Cowdung

[Solution Description] As per the syllabus, common fuels used in homes include cowdung, wood, and coal. Among the options provided, cowdung is explicitly mentioned in the syllabus.

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 Process

c) Magnesium

[Solution Description]

Combustion is a chemical process where a substance reacts with oxygen to produce heat and light. Substances that can undergo this process are called combustible or fuels. Magnesium and charcoal are examples of such substances.

Key Concept: Combustible Substance

d) A substance that reacts with oxygen to produce heat and light

[Solution Description]

A combustible substance, also known as a fuel, is a material that can react with oxygen in a combustion process to produce heat and light. It can exist in solid, liquid, or gaseous states.

Key Concept: Products of Combustion

c) Magnesium oxide, heat, and light

[Solution Description]

When magnesium ribbon burns, it reacts with oxygen to form magnesium oxide, along with the release of heat and light. Thus, the main products are magnesium oxide, heat, and light.

Key Concept: Combustion, Fire Extinguishers

c) Carbon dioxide [Solution Description]Water conducts electricity and can cause harm if used on electrical fires. Carbon dioxide $CO_2$ is the best extinguisher for electrical fires as it does not conduct electricity and cuts off the oxygen supply, effectively controlling the fire.

Key Concept: Combustion Process

b) Heat

[Solution Description]

Combustion is a chemical process where a substance reacts with oxygen to release heat. Sometimes, light is also given off during combustion, either as a flame or as a glow. The primary product of combustion is heat energy, which is used for various purposes like cooking, heating, and generating electricity.

Key Concept: Combustion Process

b) Oxygen

[Solution Description]

Combustion is a chemical process where a substance reacts with oxygen to give off heat. Therefore, the presence of oxygen is essential for combustion to occur.

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: Combustion, Combustible Substances

b) Magnesium oxide, heat, and light

[Solution Description]

When magnesium ribbon is burned in the presence of oxygen, it undergoes combustion to form magnesium oxide. This reaction also produces heat and light. The chemical equation for this reaction is:

$2Mg + O_2 \rightarrow 2MgO$

Thus, the products formed are magnesium oxide, heat, and light.

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

c) Magnesium[Solution Description]Magnesium and charcoal are examples of combustible substances that react with oxygen to produce heat and light during combustion.

Key Concept: Combustion and Air

c) Oxygen

[Solution Description]

Combustion is the process of burning, which requires oxygen (found in air) to take place. Without oxygen, combustion cannot occur as it is a necessary component for the chemical reaction that produces heat and light.

Key Concept: Role of Air in Combustion

d) The glass cuts off the air supply

[Solution Description]

When a candle flame is covered with a glass, the air (oxygen) supply is cut off. Combustion cannot occur without oxygen, so the flame will eventually go off as the available oxygen inside the glass is used up. This demonstrates that air (oxygen) is essential for sustaining combustion.

Key Concept: Combustion, Ignition Temperature

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

[Solution Description]

The assertion states that a paper cup filled with water does not burn when heated because water absorbs the heat supplied. This is true because water has a high specific heat capacity and can absorb significant amounts of heat before reaching its boiling point. The reason given is that the ignition temperature of paper is not reached due to the heat transfer to water by conduction. This is also true because the heat supplied to the paper cup is transferred to the water, preventing the paper from reaching its ignition temperature. Moreover, the reason correctly explains the assertion, as the heat transfer to water directly prevents the paper from burning. Therefore, both the assertion and reason are true, and the reason is the correct explanation of the assertion.

Key Concept: Ignition Temperature, Heat Transfer

c) The heat is absorbed by the water, preventing the paper from reaching its ignition temperature.

[Solution Description]

The heat supplied to the paper cup is transferred to the water inside by conduction. This process prevents the temperature of the paper from reaching its ignition temperature. Since the ignition temperature is not reached, the paper does not catch fire.

Key Concept: Ignition Temperature

c) The room temperature is lower than the ignition temperature of the matchstick.

[Solution Description]

A matchstick does not catch fire on its own at room temperature because the temperature of the room is lower than the ignition temperature of the matchstick. For a substance to catch fire, it must reach its ignition temperature. The matchstick requires friction from rubbing against the side of the matchbox to generate enough heat to reach its ignition temperature and ignite.

Key Concept: Heat Transfer

c) Heat is transferred to water, preventing the paper from reaching its ignition temperature.

[Solution Description]

When a paper cup with water is heated, the heat supplied to the paper cup is transferred to the water by conduction. This transfer of heat prevents the paper from reaching its ignition temperature. As long as the paper does not reach its ignition temperature, it will not burn. The presence of water thus protects the paper cup from burning.

Key Concept: Inflammable Substances

b) Low ignition temperature

[Solution Description]

Liquified Petroleum Gas (LPG) is an inflammable substance because it has a very low ignition temperature, which means it can easily catch fire when exposed to a flame or spark.

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: Fire Control Mechanism

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

[Solution Description]

Water is heavier than oil, so when poured on an oil fire, it sinks below the oil, leaving the oil to continue burning on top. This makes water ineffective for extinguishing oil fires.

Key Concept: Types of Fire Extinguishers

c) Carbon dioxide

[Solution Description]

For electrical fires, water is not suitable as it conducts electricity and can cause harm. Carbon dioxide ($CO_2$) is the best extinguisher for electrical fires because it cuts off the oxygen supply and does not harm electrical equipment.

Key Concept: Fire Extinguishers

d) Assertion is false, but Reason is true.

[Solution Description]

The assertion states that water is the most suitable fire extinguisher for all types of fires. However, this is not true as water is only effective for fires involving materials like wood and paper. For electrical fires or fires involving oil and petrol, water can be dangerous or ineffective. The reason states that water cools the combustible material and cuts off the supply of air, which is true but only for specific types of fires. Therefore, the assertion is false, while the reason is true.

Key Concept: Fire brigade role, Emergency response

b) Call the fire service immediately

[Solution Description] The first action after ensuring personal safety is to call the fire service immediately. The fire brigade is equipped to handle fires effectively and can prevent further damage or danger.

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

b) It cuts off the supply of oxygen and cools the material.

[Solution Description] Water helps extinguish fire by cooling the combustible material to a temperature below its ignition point, which prevents the fire from spreading. Additionally, water vapours surround the combustible material, cutting off the supply of air (oxygen), which is essential for combustion.

Key Concept: Fire Control

b) It cools the combustible material below ignition temperature

[Solution Description]

Water is used to cool the combustible material below its ignition temperature, which stops the fire from spreading. Additionally, water vapours cut off the supply of air, further helping to extinguish the fire.

Key Concept: Fire Extinguishing Principles, Water Vapour Effect

c) It cuts off the supply of air (oxygen)

[Solution Description]

Water vapour plays a crucial role in extinguishing fire by surrounding the combustible material and cutting off the supply of air (oxygen). Without oxygen, the fire cannot sustain itself as it is one of the three essential requirements for combustion (fuel, oxygen, and heat). Therefore, the primary role of water vapour is to cut off the supply of air.

Key Concept: Role of Water in Extinguishing Fire

b) It cuts off the supply of air and cools the fuel

[Solution Description] Pouring water on a fire helps in two ways: first, it cools the combustible material, bringing its temperature below the ignition temperature, which prevents the fire from spreading. Second, the water vapor produced surrounds the combustible material, cutting off the supply of air (oxygen), which is essential for combustion.

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: Fire extinguishers for oil fires

d) It sinks below the oil

[Solution Description]

Water is not suitable for extinguishing oil fires because it is heavier than oil and sinks below the oil, allowing the oil to keep burning on top.

Key Concept: CO$_2$ as a fire extinguisher, Chemical reactions

c) They release $CO_2$ which cuts off the oxygen supply

[Solution Description]

Dry powder chemicals like sodium bicarbonate or potassium bicarbonate, when released near a fire, give off carbon dioxide ($CO_2$). This $CO_2$ forms a blanket around the fire, cutting off the supply of oxygen, which is essential for combustion. Additionally, the expansion of $CO_2$ cools down the fuel, further helping to control the

Key Concept: Carbon Dioxide Extinguisher

c) $CO_2$ forms a blanket around the fire and cuts off oxygen

[Solution Description]

Carbon dioxide ($CO_2$) is heavier than oxygen and forms a blanket around the fire, cutting off the contact between the fuel and oxygen. It also cools down the fuel by expanding and lowering the temperature.

Key Concept: Fire Extinguishing Mechanism

b) $CO_2$ expands and cools down, forming a blanket around the fire

[Solution Description]

When $CO_2$ is released from a high-pressure cylinder, it expands enormously in volume and cools down. This helps in forming a blanket around the fire and reducing the temperature of the fuel, thus extinguishing the fire.

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: Chemical Fire Extinguishers

c) It gives off $CO_2$ which helps in controlling the fire

[Solution Description]

Sodium bicarbonate releases carbon dioxide ($CO_2$) when heated, which helps in extinguishing the fire by cutting off the oxygen supply.

Key Concept: Storing CO₂

c) As a liquid under high pressure.

[Solution Description]

Carbon dioxide ($CO_2$) is stored as a liquid under high pressure in cylinders for use in fire extinguishers.

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: Storage of $CO_2$, Role of pressure

b) It expands enormously in volume and cools down.

[Solution Description] When $CO_2$ is stored in cylinders at high pressure, it exists as a liquid. Upon release, the liquid $CO_2$ rapidly expands into a gas, occupying a much larger volume. This expansion causes the temperature to drop significantly, cooling the surrounding area. This cooling effect, combined with the blanket of $CO_2$ that covers the fire, helps in extinguishing the flames.

Key Concept: Dry chemical powders, Chemical reaction

b) $NaHCO_3$ decomposes to release $CO_2$.

[Solution Description] When sodium bicarbonate ($NaHCO_3$) is heated, it undergoes thermal decomposition. The reaction can be represented as:

$2NaHCO_3 \rightarrow Na_2CO_3 + H_2O + CO_2$

In this reaction, sodium bicarbonate decomposes to form sodium carbonate ($Na_2CO_3$), water ($H_2O$), and carbon dioxide ($CO_2$). The $CO_2$ released helps in extinguishing the fire by cutting off the oxygen supply.

Key Concept: Rapid Combustion

b) Rapid combustion

[Solution Description]

The gas burns quickly and releases heat and light when the stove is turned on. This is an example of rapid combustion, which occurs when a substance burns rapidly with the release of energy.

Key Concept: Types of Combustion, Combustible Substances

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

[Solution Description]

The Assertion states that spontaneous combustion occurs only in substances that are highly combustible at room temperature. This is true because spontaneous combustion requires the substance to be able to ignite without an external heat source, which implies it must be highly combustible. The Reason mentions that phosphorus is an example of a substance that undergoes spontaneous combustion because it reacts with oxygen in the air at room temperature. This is also true as phosphorus is known to ignite spontaneously in air due to its high reactivity with oxygen. However, the Reason does not fully explain why spontaneous combustion occurs in highly combustible substances; it merely provides an example. Therefore, both Assertion and Reason are true, but Reason is NOT the correct explanation of 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: Spontaneous Combustion, Explosion

b) Spontaneous combustion and explosion

[Solution Description]

The ignition of coal dust in a coal mine without any apparent cause is an example of spontaneous combustion, as it occurs due to the material's inherent properties. The explosion of a cracker, which produces heat, light, and sound, is an example of explosion, where a sudden reaction liberates a large amount of gas. Thus, the two types of combustion described are spontaneous combustion and explosion.

Key Concept: Spontaneous Combustion

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

[Solution Description]

The assertion states that phosphorus catches fire at room temperature without any external heat source, which is true as phosphorus undergoes spontaneous combustion at room temperature. The reason explains that spontaneous combustion occurs when a material bursts into flames without any apparent cause, which is also true. Moreover, the reason correctly explains why phosphorus catches fire at room temperature, making it a valid explanation for the assertion.

Key Concept: Spontaneous Combustion

c) Phosphorus catching fire at room temperature

[Solution Description]

Spontaneous combustion occurs when a material bursts into flames without any apparent external cause. Phosphorus catching fire at room temperature is a classic example of spontaneous combustion, as it does not require any external heat source to ignite.

Key Concept: Explosion, Heat, Light, Sound

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

[Solution Description]

The assertion states that when a firecracker explodes, it releases heat, light, and sound due to a rapid chemical reaction. This is true as the explosion involves a combustion reaction that releases energy in the form of heat, light, and sound. The reason states that the explosion occurs because the reaction produces a large amount of gas that is suddenly liberated. This is also true and explains why the firecracker explodes with such force. The reason correctly explains the assertion.

Key Concept: Explosion, Heat, Light, Sound

c) 1500 J

[Solution Description]

First, calculate the percentage of energy converted to sound:

100% - (40% + 30%) = 30%.

Now, calculate the energy released as sound:

30\% of 5000 J = $0.3 \times 5000$ J = 1500 J.

Key Concept: Temperature of Flame Zones

b) The non-luminous zone has a high temperature

[Solution Description]

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

Key Concept: Flame Structure

d) Charcoal

[Solution Description]

Charcoal does not vaporize during burning and thus does not produce a flame. Materials like candle, kerosene, and magnesium vaporize during burning and produce flames.

Key Concept: Flame Formation

c) Charcoal

[Solution Description]

Materials that vaporise during burning produce flames. Charcoal does not vaporise when burned, hence it does not produce a flame. Materials like candle wax, kerosene, and camphor vaporise and produce flames. Therefore, the correct answer is charcoal.

Key Concept: Flame Colours, Combustion Efficiency

c) LPG

[Solution Description]

A blue flame indicates complete combustion, which occurs when the fuel burns efficiently with sufficient oxygen. Among the given materials, LPG (Liquefied Petroleum Gas) is known to produce a blue flame because it burns completely in the presence of oxygen. In contrast, materials like charcoal and camphor produce yellow or orange flames due to incomplete combustion.

Key Concept: Flame Characteristics

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

[Solution Description] The assertion states that charcoal does not produce a flame when burnt, which is true as charcoal does not vapourise during burning. The reason given is that substances that vapourise during burning produce flames, which is also true. Since the reason correctly explains why charcoal does not produce a flame, 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, Luminous and Non-Luminous Zones

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 indeed the hottest part of the flame because it is where complete combustion takes place. In this zone, the fuel burns completely in the presence of sufficient oxygen, releasing more energy and thus producing higher temperatures. The luminous zone, on the other hand, has incomplete combustion, which results in lower temperatures and the formation of soot particles. Therefore, the Assertion is true, and the Reason correctly explains why the non-luminous zone has the highest temperature.

Key Concept: Combustion, Flame Zones

c) The vapours of wax are collected and ignited to form a flame.

[Solution Description]

The dark zone of the candle flame contains unburnt wax vapours. When these vapours are collected using a glass tube and exposed to a lighted matchstick, they ignite and produce a flame. This happens because the vapours of wax are combustible and can sustain a flame when provided with heat.

Key Concept: Zones of a Flame

c) Non-luminous zone

[Solution Description]

The non-luminous zone of the flame is the hottest part. When a thin long copper wire is held just inside this zone, the portion of the wire just outside the flame gets red hot, indicating high temperatures. This zone is where complete combustion occurs, resulting in the highest temperature.

Key Concept: Three different zones of a flame

b) A circular blackish ring

[Solution Description]

When a clean glass plate is introduced into the luminous zone of a candle flame, it results in the deposition of unburnt carbon particles present in the luminous zone. This deposition forms a circular blackish ring on the glass plate, indicating the presence of these unburnt carbon particles.

Key Concept: Structure of a Flame

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

[Solution Description]

The innermost zone of a candle flame is indeed composed of unburnt wax vapors, as the temperature in this zone is not sufficient to completely combust the wax. The black appearance of this zone is due to the presence of unburnt carbon particles, which are formed as a result of incomplete combustion. Therefore, both the Assertion and Reason are true, and the Reason correctly explains the Assertion.

Key Concept: Innermost zone of a flame

c) Innermost zone

[Solution Description]

The innermost zone of the candle flame is where unburnt wax vapors are present. This zone is black in color due to the presence of carbon particles.

Key Concept: Partial Combustion, Luminous Zone

c) Due to the presence of unburnt carbon particles

[Solution Description]

The middle zone of a candle flame is called the luminous zone. Here, partial combustion of the fuel occurs, leading to the formation of unburnt carbon particles. These carbon particles get heated and emit a yellow glow, which gives the zone its characteristic yellow color.

Key Concept: Middle zone: Partial combustion

b) Deposition of unburnt carbon particles

[Solution Description]

When a clean glass plate is introduced into the middle zone of a candle flame, a blackish ring forms due to the deposition of unburnt carbon particles. This happens because the middle zone is where partial combustion occurs, and some carbon particles do not burn completely. These unburnt carbon particles deposit on the glass plate, forming the blackish ring.

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: Outer Zone of Flame

c) Outermost zone

[Solution Description] The outermost zone of the flame is the hottest and undergoes complete combustion. This is because it receives the maximum amount of oxygen, allowing for complete burning of the fuel, resulting in a blue flame.

Key Concept: Structure of a Flame

d) It is the hottest part of the flame

[Solution Description]

Goldsmiths use the outermost zone of a flame because it is the hottest part of the flame. When a thin long copper wire is held just inside the non-luminous zone of the flame, the portion outside the flame gets red hot, indicating that this zone has a high temperature. The outermost zone provides the necessary heat for melting metals like gold and silver.

Key Concept: Flame Zones, Temperature, Goldsmith’s Technique

d) It has the highest temperature.

[Solution Description]

The outermost zone of a flame is the non-luminous zone, which is the hottest part of the flame. This zone provides the highest temperature required for melting metals like gold and silver. Goldsmiths use a metallic blow-pipe to direct this high-temperature zone onto the metal, ensuring efficient melting without impurities.

Key Concept: What is a Fuel?

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

[Solution Description]

The assertion states that charcoal is considered a good fuel because it is readily available and burns easily in air at a moderate rate. This is true as charcoal meets these criteria for a good fuel. The reason provides a detailed definition of a good fuel, which includes being readily available, cheap, burning easily in air at a moderate rate, producing a large amount of heat, and not leaving behind any undesirable substances. Since the assertion aligns with the reason and the reason correctly explains why charcoal is considered a good fuel, both statements are true and the reason is the correct explanation of the assertion.

Key Concept: Characteristics of a Good Fuel

b) It leaves behind undesirable substances

[Solution Description]

A good fuel should be readily available, cheap, burn easily in air at a moderate rate, produce a large amount of heat, and not leave behind any undesirable substances. The option that does not align with these characteristics is the correct answer.

Key Concept: Definition of fuel

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

[Solution Description]

The assertion states that wood is considered a fuel because it burns easily in air and produces heat. This is true because wood is a common example of a fuel used for domestic purposes. The reason states that a good fuel is one that burns easily in air at a moderate rate and produces a large amount of heat. This is also true as per the characteristics of a good fuel mentioned in the syllabus. The reason correctly explains why wood is considered a fuel, as it fulfills the criteria of a good fuel.

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, 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: Characteristics of a good fuel

c) It should be readily available

[Solution Description]

A good fuel should be readily available, cheap, burn easily at a moderate rate, produce a large amount of heat, and not leave behind any undesirable substances.

Key Concept: Fuel properties, Availability

c) Wood

[Solution Description]

The fuel described is readily available, cheap, burns at a moderate rate, does not leave undesirable substances, but produces less heat. Among the common fuels, wood fits this description as it is easily available, cheap, burns moderately, and leaves minimal ash. However, it produces less heat compared to fuels like petrol or kerosene.

Key Concept: Fuel Definition

b) Wood

[Solution Description]

A fuel is a substance that can be burned to produce heat energy for domestic or industrial purposes. Examples include wood, charcoal, petrol, and kerosene.

Key Concept: Fuel Efficiency

c) methane

[Solution Description]

We refer to Table 4.4, which lists the calorific values of different fuels. The fuels with a calorific value of 50000 $kJ/kg$ are methane and CNG. Therefore, the fuel most likely being referred to is either methane or CNG.

Key Concept: Fuel Efficiency

b) Petrol

[Solution Description]

We refer to Table 4.4, which lists the calorific values of different fuels. The fuel with a calorific value of 45000 $kJ/kg$ is petrol, kerosene, and diesel. Therefore, the fuel most likely being referred to is one of these three.

Key Concept: Calorific Value Comparison

d) LPG

[Solution Description]

The calorific values of the fuels are as follows: Cow dung cake (6000-8000 kJ/kg), Wood (17000-22000 kJ/kg), Coal (25000-33000 kJ/kg), and LPG (55000 kJ/kg). Among these, LPG has the highest calorific value of 55000 kJ/kg.

Key Concept: Cleaner Fuel

d) Compressed Natural Gas (CNG)

[Solution Description]

CNG (Compressed Natural Gas) is considered a cleaner fuel because it produces harmful products in very small amounts compared to other fuels like petrol and diesel. Therefore, CNG is a cleaner fuel.

Key Concept: Properties of Good Fuel

b) Highly explosive

[Solution Description]

A good fuel should be readily available, cheap, burn easily at a moderate rate, produce a large amount of heat, and not leave behind undesirable substances. However, being highly explosive is not a desirable property as it makes the fuel unsafe to handle.

Key Concept: Classification of Fuels

a) Coal

[Solution Description]

Solid fuels are those that remain in a solid state at room temperature. Based on Table 4.3, coal, wood, and charcoal are examples of solid fuels. Among the given options, coal is the correct answer.

Key Concept: Solid Fuels

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

[Solution Description]

Wood is indeed a solid fuel, which means it exists in a solid physical state. It is readily available, especially in rural areas, and burns easily in air at a moderate rate, producing a significant amount of heat. The assertion correctly states that wood is a solid fuel due to these properties. The reason also correctly identifies that wood is classified as a solid fuel because of its physical state and its ability to produce a large amount of heat when burned. Therefore, both the assertion and the reason are true, and the reason correctly explains the assertion.

Key Concept: Calorific Value

c) Coal

[Solution Description]

Calorific value is the amount of heat energy released by a fuel when it is burned. From Table 4.4, coal has the highest calorific value (25000-33000 kJ/kg) among the given options.

Key Concept: Fuel Characteristics

a) Readily available

[Solution Description]

A good fuel should be readily available, cheap, burn easily in air at a moderate rate, produce a large amount of heat, and not leave behind any undesirable substances.

Key Concept: Gaseous fuels: CNG, LPG

c) Assertion is true, but Reason is false.

[Solution Description]

The assertion states that CNG is considered a sustainable fuel for automobiles because it produces fewer harmful emissions. This is true as CNG burns more cleanly compared to petrol and diesel, emitting lower levels of pollutants like carbon monoxide and nitrogen oxides.

The reason states that the calorific value of CNG is higher than that of petrol and diesel, making it a more efficient fuel. This is false. According to the syllabus, the calorific value of CNG is 50000 kJ/kg, which is actually lower than that of petrol and diesel, both of which have a calorific value of 45000 kJ/kg.

Therefore, the assertion is true, but the reason is false.

Key Concept: Calorific Value, Fuel Efficiency

b) Coal

[Solution Description]

To determine the most efficient fuel per dollar, we calculate the energy output per dollar for each fuel:

1. For Cow dung cake: \[
\text{Energy per dollar} = \frac{\text{Calorific value}}{\text{Cost per kg}} = \frac{7000 \, \text{kJ/kg}}{\$0.5/\text{kg}} = 14000 \, \text{kJ/\$}.
\]
\\
2. \textbf{For Coal:}
\[
\text{Energy per dollar} = \frac{29000 \, \text{kJ/kg}}{\$1.2/\text{kg}} = 24166.67 \, \text{kJ/\$}.
\]

3. For \[
\text{LPG: Energy per dollar} = \frac{55000 \, \text{kJ/kg}}{\$2.5/\text{kg}} = 22000 \, \text{kJ/\$}.
\]

Comparing these values, Coal provides the highest energy output per dollar.

Key Concept: Burning of Fuels Leads to Harmful Products

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

[Solution Description]

Carbon monoxide is a gas that is produced when fuels like coal do not burn completely. This happens when there is not enough oxygen for combustion to occur fully. In a closed room, the oxygen supply is limited, leading to incomplete combustion of coal and the production of carbon monoxide. Carbon monoxide is highly toxic and can be fatal if inhaled in large quantities. Thus, the Assertion is true as burning coal in a closed room does produce carbon monoxide, and the Reason is also true because carbon monoxide is indeed formed due to incomplete combustion of coal. Moreover, the Reason correctly explains the Assertion.

Key Concept: Combustion and Flame, Environmental Impact of Burning Fuels

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

[Solution Description]

The assertion states that replacing diesel and petrol with CNG reduces the emission of harmful gases like carbon monoxide and sulphur dioxide. This is true because CNG combustion produces fewer pollutants compared to diesel and petrol. The reason states that CNG is a cleaner fuel as it produces harmful products in very small amounts. This is also true and correctly explains why CNG is preferred over diesel and petrol for reducing harmful emissions. Therefore, both the assertion and reason are true, and the reason is the correct explanation of the assertion.

Key Concept: Acid Rain

c) Coal and diesel

[Solution Description]

Burning of coal, diesel, and petrol releases oxides of sulphur and nitrogen. These oxides dissolve in rainwater to form acids, leading to acid rain. Acid rain is harmful to crops, buildings, and soil. Therefore, the fuels that contribute to acid rain are coal, diesel, and petrol.

Key Concept: Unburnt Carbon Particles

a) Asthma

[Solution Description]

Unburnt carbon particles released from fuels like wood, coal, and petroleum are dangerous pollutants. These fine particles can enter the respiratory system and cause diseases such as asthma.

Key Concept: Harmful Effects of Combustion

c) It produces carbon monoxide which is poisonous

[Solution Description]

Burning coal in a closed room leads to incomplete combustion, producing carbon monoxide gas. This poisonous gas can accumulate in the room and cause death to anyone sleeping in it.

Key Concept: Effects of Global Warming

b) Melting of polar glaciers

[Solution Description]

Melting of polar glaciers is a direct consequence of global warming, leading to rising sea levels and potential flooding in coastal areas.

Key Concept: Cleaner Fuel Alternative

c) It produces less harmful products than diesel and petrol

[Solution Description]

CNG is considered a cleaner fuel because it produces very small amounts of harmful products like sulphur dioxide and nitrogen oxides, which are major contributors to acid rain. This makes it environmentally friendlier than diesel and petrol.