Combustion and Flame

Exercises

Question 1

List conditions under which combustion can take place.

Answer

The conditions under which combustion can take place are:

1. Fuel.
2. Air (to supply oxygen).
3. Heat (to raise the temperature of the fuel beyond the ignition temperature).

Question 2

Fill in the blanks.

(a) Burning of wood and coal causes ............... of air.

(b) A liquid fuel, used in homes is ...............

(c) Fuel must be heated to its ............... ............... before it starts burning.

(d) The fire produced by oil cannot be controlled by ...............

Answer

(a) Burning of wood and coal causes pollution of air.

(b) A liquid fuel, used in homes is kerosene

(c) Fuel must be heated to its ignition temperature before it starts burning.

(d) The fire produced by oil cannot be controlled by water..

Question 3

Explain how the use of CNG in automobiles has reduced pollution in our cities.

Answer

The combustion of fuels such as coal, petrol and diesel results in the production of unburnt carbon particles and carbon monoxide gas, both of which are harmful pollutants that can enter the air and contribute to respiratory diseases. In contrast, Compressed Natural Gas (CNG) generates these harmful by-products in very less quantities, making it a cleaner fuel option. Consequently, the adoption of CNG has played a role in reducing pollution levels in our cities.

Question 4

Compare LPG and wood as fuels.

Answer

Question 5

Give reasons.

(a) Water is not used to control fires involving electrical equipment.

(b) LPG is a better domestic fuel than wood.

(c) Paper by itself catches fire easily whereas a piece of paper wrapped around an aluminium pipe does not.

Answer

(a) If electrical equipment is on fire, water being a good conductor, may conduct electricity and harm those trying to douse the fire. Hence, water is not a good alternative to stop such fires. Instead, carbon dioxide (CO₂) is the best extinguisher of fires involving electrical equipment.

(b) LPG is a better domestic fuel than wood because :

1. LPG doesn't produce smoke and causes less pollution. Wood on the other hand produces lots of smoke and causes more pollution.
2. No cutting of trees is required to produce LPG whereas wood is obtained by cutting trees which leads to deforestation.
3. LPG has a higher calorific value of 55000 kJ/kg whereas wood has a lower calorific value ranging between 17000 and 22000 kJ/kg.
4. LPG is easier to store and transport whereas wood requires a lot of storage space and transport is difficult due to more weight.
5. Ignition temperature of LPG is low and that of wood is high.

(c) Paper by itself catches fire easily because it has low ignition temperature. When a piece of paper is wrapped around an aluminium pipe and heated, its ignition temperature is not attained so soon because the heat supplied is absorbed by the aluminium pipe. Hence, it does not catch fire easily.

Question 6

Make a labelled diagram of a candle flame.

Answer

Labelled diagram of a candle flame is shown below:

Question 7

Name the unit in which the calorific value of a fuel is expressed.

Answer

The calorific value of a fuel is expressed in kilojoule per kg (kJ/kg).

Question 8

Explain how CO₂ is able to control fires.

Answer

CO₂ is stored at high pressure as a liquid in cylinders. When released from the cylinder, it expands enormously in volume and cools down. Being heavier than oxygen, it forms a blanket around the fire. As the contact between the fuel and oxygen is cut off, the fire is controlled. It also brings down the temperature of the fuel.

Question 9

It is difficult to burn a heap of green leaves but dry leaves catch fire easily. Explain.

Answer

A heap of green leaves contains a lot of moisture due to which its ignition temperature is high. Hence, it does not catch fire easily.

On the other hand there is no or very less moisture content in dry leaves, hence their ignition temperature is low and they catch fire easily.

Question 10

Which zone of a flame does a goldsmith use for melting gold and silver and why?

Answer

Goldsmith uses the outermost zone of a flame for melting gold and silver because it is the hottest zone of the flame undergoing complete combustion. Hence, it easily melts gold and silver metals.

Question 11

In an experiment 4.5 kg of a fuel was completely burnt. The heat produced was measured to be 180,000 kJ. Calculate the calorific value of the fuel.

Answer

Given,

Weight of fuel burnt = 4.5 kg

Amount of heat produced = 180,000 kJ

Calorific value = $\dfrac{\text{Amount of heat produced}}{\text{Weight of fuel burnt}}$

Substituting we get,

Calorific value = $\dfrac{180,000}{4.5}$ = 40,000 kJ per kg.

Hence, calorific value of the fuel = 40,000 kJ per kg

Question 12

Can the process of rusting be called combustion? Discuss.

Answer

Let's look at the similarities and differences between rusting and combustion that will help us to deduce if rusting can be called combustion:

Similarities between rusting and combustion:

1. Both involve oxygen — Both processes require oxygen from the air. In combustion, oxygen reacts with the fuel, and in rusting, it reacts with the metal (often iron).
2. Both are exothermic — Both processes release energy in the form of heat. However, the amount of heat released is significantly different. Combustion releases a much larger amount of heat, often accompanied by light. Rusting releases a minimal amount of heat that's usually undetectable.

Differences between rusting and combustion:

1. Rate of reaction — Combustion is a rapid process, often instantaneous or occurring within seconds. Rusting is a slow, continuous process that can take hours, days, or even years depending on the conditions.
2. Ignition temperature — For combustion to take place, the fuel should attain its Ignition temperature. Rusting does not require an ignition temperature and can occur at room temperature.
3. Products — Combustion typically produces water vapour, carbon dioxide, and other gases as products. Rusting produces iron oxide, commonly known as rust.

Therefore, while both involve oxygen and release heat, the significant differences in reaction rate, ignition temperature and products formed prevent us from calling rusting as combustion.

Question 13

Abida and Ramesh were doing an experiment in which water was to be heated in a beaker. Abida kept the beaker near the wick in the yellow part of the candle flame. Ramesh kept the beaker in the outermost part of the flame. Whose water will get heated in a shorter time?

Answer

Ramesh's beaker, positioned in the outermost part of the flame, will heat up more quickly. This is because the outer region of a non-luminous flame is the hottest, in contrast to the less hot luminous (yellow) part near the wick of the candle.