Combustion and Flame — NCERT Solutions, AI Tutor & Practice

Understand: Combustion is a chemical process.

Definition: Combustion is a chemical reaction in which a substance reacts with oxygen to produce heat (and usually light). The substance that burns is called a combustible substance or fuel.

Word equation (example):

$\text{Carbon} + \text{Oxygen} \rightarrow \text{Carbon dioxide} + \text{Heat and Light}$

Two everyday examples:

1. Burning of wood or coal in a fire.
2. Burning of LPG (cooking gas) on a stove.

Conclude: In every case, oxygen is consumed and heat energy is released.

Understand: Combustion cannot start unless three things are present together. These form the fire triangle.

The three conditions:

1. A combustible substance (fuel) — something that can burn, e.g., paper, wood, kerosene.
2. A supporter of combustion (oxygen/air) — oxygen is needed to keep the reaction going.
3. Attainment of ignition temperature — the fuel must be heated to its lowest temperature at which it catches fire.

Conclude: If even one of these is removed, combustion stops. This is the basis of fire fighting.

Definition: The ignition temperature of a substance is the lowest temperature to which it must be heated before it catches fire and starts burning.

Analyze the iron case:

• Iron has a very high ignition temperature.
• A burning matchstick gives out only a small amount of heat.
• This small heat is not enough to raise the iron to its ignition temperature.

Conclude: Since the iron never reaches its ignition temperature, it does not catch fire. (Note: fine iron filings sprinkled into a flame do burn because they heat up quickly.)

Definition: A fuel is a combustible substance that burns to produce heat energy which can be used for domestic and industrial purposes.

Classification by physical state:

| Type of fuel | Example | |---|---| | Solid fuel | Coal, wood | | Liquid fuel | Kerosene, petrol | | Gaseous fuel | LPG, CNG |

Conclude: Fuels are chosen depending on cost, availability, and how much heat they produce.

Understand: Combustion can happen in different ways depending on speed and conditions.

1. Rapid combustion: The substance burns rapidly and produces heat and light in a short time. Example: burning of LPG on a stove.

2. Spontaneous combustion: The material catches fire on its own without any external heat being applied. Example: spontaneous burning of coal dust or phosphorus in air.

3. Explosion: A sudden reaction takes place with the evolution of heat, light, sound, and a large amount of gas, all at once. Example: bursting of crackers.

Conclude: All three are types of combustion but differ in rate and manner of release of energy.

Understand: A candle flame is not uniform; it has three distinct regions.

The three zones:

1. Innermost (dark) zone: Contains unburnt wax vapours. It is the least hot and appears black.
2. Middle (luminous/yellow) zone: Partial combustion occurs here due to insufficient oxygen. Unburnt carbon particles glow, giving the yellow bright light. It is moderately hot.
3. Outermost (blue) zone: Complete combustion takes place here because plenty of oxygen is available. This is the hottest zone of the flame.

Conclude: Goldsmiths use the outermost blue zone to melt gold and silver because it is the hottest part.

Understand: A closed room has a limited supply of air (oxygen).

Analyze:

• When fuel burns with insufficient oxygen, incomplete combustion takes place.
• This produces carbon monoxide (CO), a poisonous gas.
• Carbon monoxide is very dangerous because it reduces the ability of blood to carry oxygen.

Word equation for incomplete combustion:

$\text{Carbon} + \text{limited Oxygen} \rightarrow \text{Carbon monoxide}$

Conclude: Breathing in carbon monoxide in a closed room can cause suffocation and even death. Therefore, fuels should never be burnt in poorly ventilated rooms.

Understand: Water is a good fire extinguisher for many fires, but not all.

Analyze:

1. Electrical fires: Water is a conductor of electricity. If poured on live electrical equipment, it can give the person a severe electric shock.
2. Oil fires: Oil is lighter than water and does not mix with it. Water sinks below the burning oil, which continues to float and burn on top, spreading the fire.

Better solution: For such fires, carbon dioxide (CO₂) extinguishers are used. CO₂ is heavier than air, covers the fire, cuts off the oxygen supply, and cools the fuel.

Conclude: The correct extinguisher must be chosen according to the type of fire.

Definition: The calorific value of a fuel is the amount of heat energy produced when 1 kilogram of the fuel is completely burnt.

Unit: kilojoule per kilogram, written as $\text{kJ/kg}$.

Given:

• Heat produced $= 90000 \text{ kJ}$
• Mass of fuel $= 2 \text{ kg}$

Formula:

$\text{Calorific value} = \frac{\text{Heat produced}}{\text{Mass of fuel}}$

Substitute:

$\text{Calorific value} = \frac{90000 \text{ kJ}}{2 \text{ kg}} = 45000 \text{ kJ/kg}$

Conclude: The calorific value of the fuel is $45000 \text{ kJ/kg}$.

Understand: An ideal fuel is one that is best for use in daily life.

Characteristics of an ideal fuel:

1. It should be cheap and easily available.
2. It should have a high calorific value (produce a lot of heat).
3. It should have a proper ignition temperature — not too low (unsafe) and not too high (hard to burn).
4. It should produce little smoke, ash, and harmful gases so it does not pollute the environment.

Conclude: No single fuel is perfect in all respects, but a good fuel meets most of these conditions.

Understand: Burning fuels releases several gases into the air, some of which harm the environment.

Global warming:

• Combustion of fuels produces carbon dioxide ($CO_2$).
• Increased $CO_2$ traps more heat in the atmosphere (greenhouse effect).
• This raises the Earth's average temperature — called global warming, which can melt polar ice and raise sea levels.

Acid rain:

• Coal and diesel contain sulphur and nitrogen impurities.
• Burning them releases sulphur dioxide ($SO_2$) and nitrogen oxides.
• These gases dissolve in rainwater to form acids, producing acid rain.
• Acid rain damages crops, buildings, and soil.

Conclude: To reduce these effects, we should use cleaner fuels like CNG and reduce fuel consumption.

Understand: CNG means Compressed Natural Gas.

Analyze — why CNG is cleaner:

1. CNG burns more completely, producing far less smoke and unburnt particles.
2. It releases fewer harmful gases like carbon monoxide and no significant sulphur compounds.
3. It causes less air pollution in cities, helping keep the air healthier.
4. It has a fairly high calorific value and is efficient.

Comparison: Petrol and diesel produce more smoke, carbon monoxide, and sulphur/nitrogen oxides, which pollute the air.

Conclude: Because it burns cleanly with minimal pollution, CNG is a better and eco-friendly fuel for vehicles in large cities.