Class 8th Chapter 4 Combustion and Flame

Class 8 Science – Combustion and Flame

1. List conditions under which combustion can take place.

Combustion needs fuel, oxygen, and heat. Without any one of these, burning will not happen. These are the three main things required.

2. Fill in the blanks:

• (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) Fire produced by oil cannot be controlled by water.

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

CNG burns cleanly and does not release harmful gases like petrol or diesel. It helps keep air clean in cities.

4. Compare LPG and wood as fuels.

LPG is cleaner, burns faster and doesn’t produce smoke. Wood creates smoke and leaves ash. LPG is easier to store.

5. Give reasons:

• (a) Water conducts electricity. It can cause electric shocks. That's why it is not used on electric fires.
• (b) LPG gives more heat, burns cleanly, and does not create smoke. It is easier to use and store than wood.
• (c) Aluminium absorbs the heat and keeps paper cool. Paper alone heats up fast and burns, but not when wrapped on metal.

6. Make a labelled diagram of a candle flame.

[Students should draw a diagram showing: Outer zone (blue), Middle zone (yellow), Inner zone (black/unburnt).]

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

Calorific value is measured in kilojoules per kilogram (kJ/kg).

8. Explain how CO₂ is able to control fires.

Carbon dioxide cuts off the air supply and cools the burning material. This stops the fire from spreading further.

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

Green leaves have water which stops them from catching fire. Dry leaves are dry and burn easily when heat is given.

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

A goldsmith uses the outermost zone. It is the hottest part of the flame and melts metals easily.

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.

Calorific value = Total heat / Mass of fuel = 180,000 ÷ 4.5 = 40,000 kJ/kg

12. Can the process of rusting be called combustion? Discuss.

No, rusting is a slow process. It does not produce heat or light. Combustion is fast and gives heat and light.

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?

Ramesh’s water will get heated faster because the outermost part of the flame is the hottest.