Key Concept: Combustion

d) Assertion is false, but Reason is true.

[Solution Description]

Combustion is a chemical process where a substance reacts with oxygen to give off heat. However, not all combustion processes produce a flame. For example, coal burns without a flame, whereas a candle burns with a flame. The assertion that combustion always produces a flame is false. The reason states that a flame is produced when a substance burns in the presence of oxygen, which is true but does not explain why combustion always produces a flame, as it does not always do so.

Key Concept: Flame Production

c) Candle vaporizes wax

[Solution Description]

A candle burns with a flame because it vaporizes the wax, which then combusts in the presence of oxygen, producing a visible flame. On the other hand, coal burns without a flame because it undergoes solid-phase combustion, releasing heat and light but not forming a visible flame.

Key Concept: Combustion, Combustible Substances

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

[Solution Description]

Combustion is a chemical process where a substance reacts with oxygen to produce heat and light. The assertion states that combustion always requires the presence of oxygen, which is true as oxygen is essential for the combustion process. The reason explains that oxygen acts as a supporting agent by facilitating the chemical reaction, 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: 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, Types of Fuels

a) Charcoal

[Solution Description]

Combustion is a chemical process where a substance reacts with oxygen to produce heat and light. Some fuels burn with a flame, while others burn without a visible flame. Charcoal is a solid fuel that burns without producing 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: 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 and Flame

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

[Solution Description]

The assertion states that the burning of a candle produces a flame because the wax vapourises during combustion. This is true as the wax melts and rises through the wick, where it vapourises and burns, producing a flame. The reason explains that substances which vapourise during burning produce flames, while those that do not vapourise do not produce flames. This is also true and directly explains why the candle produces a flame. Therefore, both the assertion and reason are true, and the reason correctly explains the assertion.

Key Concept: Structure of Flame, Combustion Process

b) Luminous zone

[Solution Description] The structure of a candle flame includes different zones: the dark zone, the luminous zone, and the non-luminous zone. The luminous zone is where partial combustion occurs, leading to the deposition of unburnt carbon particles on a glass plate. The correct answer identifies this zone.

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 Output

c) Magnesium oxide, heat, and light

[Solution Description]

When magnesium burns in air, it reacts with oxygen to form magnesium oxide. This reaction also produces heat and light as outputs. Therefore, the primary outputs are magnesium oxide, heat, and light.

Key Concept: Types of Fuels

d) Fuels can be in any physical state.

[Solution Description]

Fuels can exist in different states: solid, liquid, or gas. The syllabus mentions that the fuel undergoing combustion may be solid, liquid, or gas. Examples include coal (solid), petrol (liquid), and natural gas (gas). The statement that fuels can be in any physical state is correct.

Key Concept: Combustible Substance

c) Charcoal[Solution Description]A combustible substance is one that undergoes combustion. Magnesium and charcoal are examples of combustible substances as they burn in the presence of oxygen to produce heat and light.

Key Concept: Combustible Substances, Combustion Requirements

d) All of the above

[Solution Description]

Combustion requires three essential elements: a combustible substance (fuel), oxygen, and heat to reach ignition temperature. Without any one of these, combustion cannot occur. Therefore, all three conditions must be met.

Key Concept: Combustion, 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 per the syllabus, which mentions that charcoal burns in air. The reason states that any substance that reacts with oxygen to give off heat and light during combustion is classified as a combustible substance. This is also true as per the definition of combustion provided in the syllabus. Furthermore, the reason correctly explains why charcoal is a combustible substance, as it fits the definition provided in the reason. Therefore, both the assertion and reason are true, and the reason is the correct explanation of the assertion.

Key Concept: Combustion, Role of Air

b) The flame flickers and produces smoke

[Solution Description]

When the glass chimney is resting on wooden blocks, air can enter from below, allowing the candle to burn freely. However, when the blocks are removed and the chimney rests directly on the table, air cannot enter from below. This causes the flame to flicker and produce smoke due to insufficient oxygen supply.

Key Concept: Conditions necessary for combustion

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

[Solution Description]

The assertion states that a substance will only burn if it reaches its ignition temperature, which is true because combustion requires the substance to reach a temperature where it can react with oxygen to produce heat and light. The reason states that ignition temperature is the lowest temperature at which a combustible substance catches fire, which is also true and directly explains why the substance needs to reach this temperature to burn. Therefore, both the assertion and reason are true, and the reason is the correct explanation of the assertion.

Key Concept: Combustion, Combustible Substances

c) Charcoal[Solution Description]Combustion is a chemical process where a substance reacts with oxygen to produce heat and light. In the given scenario, the substance produces carbon dioxide, which indicates it is a carbon-based fuel. Among the options, charcoal is a well-known carbon-based fuel that burns in the presence of oxygen to produce carbon dioxide, heat, and light.

Key Concept: Combustion, Products of Combustion

c) Excitation of Electrons in Magnesium Atoms

[Solution Description]

When magnesium ribbon burns, it reacts with oxygen to form magnesium oxide. This reaction is highly exothermic, meaning it releases a significant amount of energy in the form of heat and light. The light emitted is due to the excitation of electrons in the magnesium atoms to higher energy levels, which then return to their ground state by releasing photons of light. This phenomenon is characteristic of combustion reactions that involve the production of light.

Key Concept: Combustion, Oxygen

d) Assertion is false, but Reason is true.

[Solution Description] The Assertion states that a candle will continue to burn indefinitely in a closed container if there is sufficient wax. This is incorrect because, even if there is enough wax, combustion requires oxygen, which is finite in a closed container. Once the oxygen is depleted, the flame will extinguish, regardless of the amount of wax. The Reason claims that oxygen is continuously replenished in a closed container during combustion, which is false. Oxygen is consumed during combustion and is not replenished in a closed system. Therefore, the Assertion is false, but the Reason is also false.

Key Concept: Combustion Process

b) Combustion gives out heat and light

[Solution Description]

During combustion, heat and light are given out as energy. This is a fundamental characteristic of the combustion process. Additionally, oxygen (in air) is essential for combustion to occur, as it acts as a reactant in the chemical reaction that takes place during burning.

Key Concept: Inflammable Substances

c) Petrol[Solution Description]Inflammable substances have very low ignition temperatures and can easily catch fire. Examples include petrol, alcohol, and LPG.

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: Ignition Temperature

b) The lowest temperature at which a substance catches fire

[Solution Description]

The ignition temperature is the lowest temperature at which a substance catches fire and starts burning.

Key Concept: Inflammable Substances

b) Petrol

[Solution Description]

Inflammable substances like petrol, alcohol, and LPG have very low ignition temperatures, which means they can easily catch fire with a flame. Among the given options, petrol is known to have one of the lowest ignition temperatures, making it highly inflammable.

Key Concept: Inflammable substances, Multi-concept challenge

b) Availability of oxygen

[Solution Description]

The rate of combustion of LPG depends on several factors, including the availability of oxygen, temperature, and pressure. Among these, the availability of oxygen is crucial because combustion cannot occur without it. Temperature and pressure also play significant roles, but oxygen availability is the primary factor.

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: 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: 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 Brigade Role

b) Call the fire service

[Solution Description]

The first step in case of a fire is to call the fire service immediately. This ensures that professional help arrives as quickly as possible to control and extinguish the fire.

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: Heat transfer, Role of water in extinguishing fire

b) Water absorbs heat and prevents the paper from reaching its ignition temperature.

[Solution Description]

The heat supplied to the paper cup is transferred to the water by conduction. As long as water is present, it absorbs the heat, preventing the temperature of the paper from reaching its ignition temperature. This ensures that the paper cup does not burn.

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: Control of Fire

b) It cools the combustible material below ignition temperature

[Solution Description]

Water is used to extinguish fires because it cools the combustible material below its ignition temperature, preventing the fire from spreading. Additionally, water vapours cut off the supply of air, which is essential for combustion.

Key Concept: Control of Fire

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

[Solution Description]

The Assertion states that pouring water on a fire helps in extinguishing it by cutting off the supply of oxygen. This is true because water cools the combustible material, bringing its temperature below the ignition temperature, and also produces water vapor that surrounds the combustible material, cutting off the supply of air (which contains oxygen). The Reason states that water vapor surrounds the combustible material and prevents air from reaching it. This is also true and directly explains why the Assertion is correct. Therefore, both the Assertion and Reason are true, and the Reason is the correct explanation of the Assertion.

Key Concept: Fire Extinguishing Mechanism

b) Cooling the material and cutting off the oxygen supply[Solution Description]When water is poured on a fire, it cools the combustible material and generates water vapor. The water vapor surrounds the material, cutting off the oxygen supply, which is necessary for the fire to sustain. This dual action of cooling and oxygen deprivation extinguishes the fire.

Key Concept: Carbon Dioxide as Fire Extinguisher

c) $CO_2$ forms a blanket around the fire, cutting off oxygen

[Solution Description]

Carbon dioxide cuts off the contact between fuel and oxygen by forming a blanket around the fire and does not harm electrical equipment.

Key Concept: Limitations of water as a fire extinguisher

b) It conducts electricity

[Solution Description]

Water is not suitable for extinguishing fires involving electrical equipment because it conducts electricity, which can harm those trying to douse the fire.

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, CO$_2$ as Extinguisher

c) Water conducts electricity

[Solution Description]

Water conducts electricity and can cause harm to those trying to douse an electrical fire. In contrast, $CO_2$ does not conduct electricity and acts as a blanket by cutting off the oxygen supply to the fire. Additionally, $CO_2$ cools down the fuel, making it an effective extinguisher for electrical fires without damaging the equipment.

Key Concept: CO$_2$ extinguisher, electrical fires

b) CO$_2$ forms a blanket around the fire and cuts off oxygen

[Solution Description]

CO$_2$ is non-conductive and does not harm electrical equipment. It also cuts off the oxygen supply to the fire by forming a blanket around it, effectively controlling the fire without damaging the equipment.

Key Concept: Fire Extinguishers for Oil Fires

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

[Solution Description]

The assertion states that water is not effective in extinguishing oil fires, which is true. Water is heavier than oil and sinks below it, failing to cut off the oxygen supply to the fire. Therefore, the reason provided correctly explains why water is ineffective in such cases. The Assertion and Reason are both true, and the Reason is the correct explanation of the Assertion.

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 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 ($CO_2$) is effective in extinguishing electrical and petrol fires by cutting off the oxygen supply. This is true because $CO_2$, being heavier than oxygen, settles around the fire and creates a barrier that prevents oxygen from reaching the fuel. The reason explains that $CO_2$ forms a blanket around the fire, which is accurate as it describes the mechanism by which $CO_2$ cuts off the oxygen supply. Therefore, both the assertion and reason are true, and the reason correctly explains the assertion.

Key Concept: Role of dry chemical powders like sodium bicarbonate

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

[Solution Description]

Sodium bicarbonate ($NaHCO_3$) decomposes when heated to release carbon dioxide ($CO_2$). This $CO_2$ acts as a fire extinguisher by covering the fire and cutting off the supply of oxygen, which is necessary for combustion. The assertion correctly states that sodium bicarbonate is effective in extinguishing fires involving electrical equipment due to the release of $CO_2$. The reason accurately explains that $CO_2$ controls the fire by cutting off the oxygen supply. Therefore, both the assertion and reason are true, and the reason is the correct explanation of the assertion.

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: Spontaneous Combustion

c) Phosphorus

[Solution Description]

Phosphorus is a substance that can burn in air at room temperature without any external cause. This type of combustion is known as spontaneous combustion, where the material suddenly bursts into flames.

Key Concept: Spontaneous Combustion

c) Phosphorus burning in air at room temperature

[Solution Description]

Spontaneous combustion occurs when a material bursts into flames without any apparent cause. An example is phosphorus burning in air at room temperature.

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: Rapid Combustion

d) Rapid combustion

[Solution Description]

When the gas stove is turned on, the gas burns rapidly, producing heat and light. This is an example of rapid combustion because it happens quickly with the application of an external source (the gas stove knob). Rapid combustion is characterized by the fast burning of a substance when exposed to a flame or spark.

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. This is true because phosphorus has a very low ignition temperature, which allows it to catch fire spontaneously when exposed to air. The reason provided is that phosphorus undergoes spontaneous combustion due to its low ignition temperature. This is also true and correctly explains why the assertion is valid. Therefore, both the assertion and reason are true, and the reason is the correct explanation of the assertion.

Key Concept: Causes of Forest Fires

c) Carelessness of human beings[Solution Description]According to the syllabus, forest fires are often caused by human carelessness, such as not extinguishing campfires properly before leaving a forest. While natural causes like lightning or extreme heat can also start fires, the majority are due to human actions.

Key Concept: Explosion

c) It explodes

[Solution Description]

When pressure is applied to a cracker, it can cause an explosion due to the sudden release of energy in the form of heat, light, and sound.

Key Concept: Explosion, Energy, Pressure

c) 2000 W/m$^2$

[Solution Description]

Calculate the energy used for pressure waves:

50\% of 8000 J = $0.5 \times 8000$ J = 4000 J.

Intensity is given by the formula Intensity = Energy / (Area $\times$ Time).

Assuming the time taken for the wave to travel is 1 second, the intensity is:

4000 J / (2 m$^2$ $\times$ 1 s) = 2000 W/m$^2$.

Key Concept: Flame Structure

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 due to the presence of sufficient oxygen. This complete combustion results in the highest temperature in this zone. Therefore, both the Assertion and Reason are true, and the Reason correctly explains the Assertion.

Key Concept: Temperature Zones of Flame

c) The wire gets red hot

[Solution Description]

The portion of the copper wire just outside the flame gets red hot when held in the non-luminous zone. This indicates that the non-luminous zone has a high temperature, causing the wire to heat up and become red hot.

Key Concept: Flame Zones, Temperature Distribution

c) The copper wire gets red hot outside the flame

[Solution Description]

When a thin copper wire is held just inside the non-luminous zone of a candle flame, the portion of the wire outside the flame gets red hot. This observation indicates that the non-luminous zone has a high temperature, as it heats the wire to a point where it glows red. This supports the claim that the non-luminous zone is the hottest part of the flame.

Key Concept: Flame Structure

c) The flame contains unburnt carbon particles

[Solution Description]

The deposition of unburnt carbon particles on a glass plate indicates that the luminous zone of the flame contains unburnt carbon particles. This is observed when a glass plate is introduced into the luminous zone of a flame and a blackish ring forms on it.

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: Flame, Combustion, Vaporization

d) Wax and kerosene vaporize during burning, while charcoal does not.

[Solution Description]

Substances that produce flames vaporize during combustion. Wax and kerosene vaporize when heated, forming vapors that burn to produce flames. Charcoal, on the other hand, does not vaporize; it burns directly without forming a flame. This is why wax and kerosene produce flames, but charcoal does not.

Key Concept: Structure of a Flame

b) Deposition of unburnt carbon particles

[Solution Description]

The blackish ring formed on the glass plate indicates the deposition of unburnt carbon particles present in the luminous zone of the flame. This shows that incomplete combustion occurs in the luminous zone.

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 substances which vapourise during burning give flames, which is true as per the syllabus. The reason provides examples of kerosene oil and molten wax, which are substances that vapourise during burning and form flames. Since the reason correctly explains the assertion, both are true and the reason is the correct explanation of the assertion.

Key Concept: Three zones of a flame, dark zone

c) The wax vapors escaping through the tube

[Solution Description]

The dark zone of a candle flame contains wax vapors that have not yet undergone combustion. When a glass tube is introduced into this zone and a lighted matchstick is brought near the other end, the wax vapors escaping through the tube catch fire, producing a flame. This demonstrates that wax vapors are responsible for flame formation.

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: Structure of a Flame, Innermost zone: Unburnt wax vapors

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 the zone where the wax vapors are not yet burnt. This zone appears black because it contains vapors of unburnt wax and carbon particles that have not undergone combustion. The reason provided correctly explains that the black color is due to the deposition of carbon particles, which are a result of incomplete combustion. Thus, both the assertion and the reason are true, and the reason correctly explains the assertion.

Key Concept: Innermost Zone of Flame

c) Deposition of unburnt carbon particles

[Solution Description] The blackish ring formed on the glass plate is due to the deposition of unburnt carbon particles present in the luminous zone of the flame. This zone contains unburnt wax vapors, which deposit as carbon particles when they come in contact with the cooler surface of the glass plate.

Key Concept: Middle zone: Partial combustion

b) A circular blackish ring

[Solution Description]

When a clean glass plate is introduced into the luminous zone of a candle flame, a circular blackish ring is formed on the glass plate. This ring indicates the deposition of unburnt carbon particles present in the luminous zone of the flame.

Key Concept: Middle zone: Partial combustion

b) Partial combustion of carbon particles

[Solution Description]

The yellow color in the middle zone of a candle flame is due to the partial combustion of carbon particles. When the candle burns, some of the carbon particles do not completely burn and instead emit yellow light as they heat up. This incomplete combustion results in the luminous (yellow) zone of the flame.

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: Combustion characteristics

c) Blue

[Solution Description]

The outermost zone of a flame appears blue because it is the region of complete combustion. Complete combustion occurs here due to the availability of sufficient oxygen, resulting in the release of a blue flame and maximum heat.

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: Combustion Process

b) It does not vaporize

[Solution Description]

Charcoal does not produce a flame during combustion because it does not vaporize; it only glows and burns without forming a flame.

Key Concept: Basic Concept

c) It produces a large amount of heat

[Solution Description]

A good fuel should be readily available, cheap, burn easily at a moderate rate, produce a large amount of heat, and leave no undesirable substances. The option that matches these characteristics is the correct answer.

Key Concept: Environmental Impact, Fuel Efficiency

c) Coal

[Solution Description] Coal produces significant amounts of pollution and leaves behind undesirable substances like ash and smoke. It is less efficient and has a higher environmental impact compared to other fuels, making it the least preferable for long-term use in urban areas.

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 charcoal is considered a good fuel because it burns easily in air at a moderate rate. This is true because charcoal meets the criteria of a good fuel, including burning easily and producing a large amount of heat. The reason given is that a good fuel burns easily in air at a moderate rate and produces a large amount of heat. This is also true and directly explains why charcoal is considered a good fuel. Therefore, both the assertion and reason are true, and the reason correctly explains the assertion.

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: Characteristics of a good fuel

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: Desirable Properties of Fuel

c) It should burn easily in air at a moderate rate

[Solution Description]

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

Key Concept: Fuel efficiency, Environmental impact

b) Natural gas

[Solution Description]

The fuel in question burns cleanly, leaves no harmful residues, is inexpensive, but is not readily available in all regions. Natural gas fits this description as it burns cleanly with minimal emissions, is relatively inexpensive, but access is limited in some rural areas due to infrastructure requirements.

Key Concept: Fuel Availability

c) Wood

[Solution Description]

In rural areas, availability of fuel depends on local resources. Wood and charcoal are commonly used in rural areas due to their easy accessibility and low cost compared to other fuels like petrol or natural gas.

Key Concept: Calorific Value

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

[Solution Description]

The assertion states that hydrogen has the highest calorific value among all fuels. This is true as per the calorific values provided in the syllabus, where hydrogen has a calorific value of $150000 kJ/kg$, which is the highest among all listed fuels. The reason given is that hydrogen produces a large amount of heat energy due to its high energy content per unit mass. This is also true because hydrogen's high calorific value directly results from its high energy density. 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: Combustion, CNG, Pollution Reduction

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

[Solution Description]

The assertion states that CNG reduces harmful emissions compared to diesel and petrol. This is true because CNG is a cleaner fuel and produces fewer pollutants like carbon monoxide and unburnt hydrocarbons. The reason given is that CNG combustion produces minimal amounts of carbon monoxide and unburnt hydrocarbons, which is also true. Furthermore, the reason correctly explains why CNG reduces harmful emissions, as these substances are major contributors to air pollution. Therefore, both the assertion and reason are true, and the reason is the correct explanation of the assertion.

Key Concept: Fuel Efficiency

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

[Solution Description]

CNG (Compressed Natural Gas) is indeed considered a cleaner fuel because it produces fewer pollutants such as carbon monoxide, nitrogen oxides, and particulate matter compared to petrol and diesel. This makes it environmentally friendlier. The reason states that CNG produces harmful substances in very small amounts, which is accurate as it emits lower levels of these pollutants. Both the assertion and reason are true, and the reason correctly explains why CNG is considered a cleaner fuel.

Key Concept: Classification of Fuels

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

[Solution Description]

The assertion states that natural gas is classified as a gaseous fuel, which is correct as per Table 4.3 in the syllabus. The reason given is that gaseous fuels burn more efficiently than solid fuels, which is also true based on the properties of fuels mentioned in the syllabus. However, the reason does not explain why natural gas is classified as a gaseous fuel. Therefore, both the assertion and reason are true, but the reason is not the correct explanation of the assertion.

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: Combustion Efficiency, Solid Fuels

b) 1,000 kg, More Pollutants

[Solution Description] First, find the total energy required to produce 21,000,000 kJ of effective heat energy, considering the combustion efficiency: $\text{Total Energy} = \frac{\text{Effective Energy}}{\text{Efficiency}} = \frac{21,000,000 \, \text{kJ}}{0.75} = 28,000,000 \, \text{kJ}$. Next, calculate the mass of coal needed: $\text{Mass} = \frac{\text{Total Energy}}{\text{Calorific Value}} = \frac{28,000,000 \, \text{kJ}}{28,000 \, \text{kJ/kg}} = 1,000 \, \text{kg}$. One disadvantage of coal is that it produces more harmful pollutants like sulfur dioxide and ash compared to wood.

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 wood is considered a good fuel for domestic use in rural areas. This is true because wood has been traditionally used as a fuel in rural areas due to its availability and affordability. The reason states that wood is easily available and cheap in rural areas, which is also true and explains why it is considered a good fuel in such regions. Both the assertion and reason are true, and the reason correctly explains the assertion.

Key Concept: Types of Fuels

b) Kerosene

[Solution Description]

Liquid fuels include substances like kerosene and petrol. These are used for various domestic and industrial purposes.

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: 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: Burning of Fuels Leads to Harmful Products

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

[Solution Description]

The assertion states that incomplete combustion of carbon fuels produces carbon monoxide, which is true as per the syllabus. The reason explains that carbon monoxide is dangerous because it can cause death when inhaled in closed spaces, 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: Global Warming, Acid Rain

b) Accelerated melting of polar glaciers due to global warming

[Solution Description]

The primary environmental impact in the short term would be global warming due to the increased concentration of carbon dioxide. While sulphur dioxide and nitrogen oxides contribute to acid rain, their levels remain constant, so the immediate effect is dominated by the rise in carbon dioxide, which traps heat in the atmosphere.

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: Environmental Impact

c) Deforestation

[Solution Description]

Burning wood as a fuel releases harmful smoke containing unburnt carbon particles, which causes respiratory problems. Additionally, it leads to deforestation, which negatively impacts the environment by reducing tree cover and biodiversity.

Key Concept: Incomplete combustion, Carbon monoxide toxicity

c) 0.5 mL

[Solution Description]

The concentration of carbon monoxide in the room is 0.1\%, which means there are 0.1 mL of CO in every 100 mL of air. To find out how much CO is inhaled per breath, we calculate 0.1\% of 500 mL:

$\text{CO inhaled} = \frac{0.1}{100} \times 500 = 0.5 \text{ mL}$

Therefore, 0.5 mL of carbon monoxide is inhaled per breath.

Key Concept: Carbon dioxide emissions, Fuel efficiency, Environmental impact

a) Car A by 39 kg

[Solution Description]

For Car A:

$\text{Gasoline used} = \frac{360\ \text{km}}{12\ \text{km/l}} = 30\ \text{l}$.

$\text{Carbon dioxide emitted} = 30\ \text{l} \times 2.5\ \text{kg/l} = 75\ \text{kg}$.

For Car B:

$\text{Gasoline used} = \frac{360\ \text{km}}{18\ \text{km/l}} = 20\ \text{l}$.

$\text{Carbon dioxide emitted} = 20\ \text{l} \times 1.8\ \text{kg/l} = 36\ \text{kg}$.

The difference in emissions is $75\ \text{kg} - 36\ \text{kg} = 39\ \text{kg}$.

Therefore, Car A emits more carbon dioxide by 39 kg.

Key Concept: Fuel Efficiency, Acid Rain, CNG

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

[Solution Description]

CNG (Compressed Natural Gas) is a cleaner fuel compared to diesel and petrol. When CNG is burned, it produces very small amounts of harmful gases like sulfur dioxide (SO$_2$) and nitrogen oxides (NO$_x$). Diesel and petrol, on the other hand, release significant amounts of these gases when combusted. Sulfur dioxide and nitrogen oxides are primary contributors to acid rain, as they dissolve in rainwater to form acids. Therefore, the assertion that CNG reduces the emission of these gases is true. The reason provided is also correct, as CNG combustion indeed produces negligible amounts of sulfur dioxide and nitrogen oxides. Moreover, the reason correctly explains why CNG is effective in reducing emissions of these harmful gases. Thus, both the assertion and reason are true, and the reason is the correct explanation of the assertion.