NCERT 8th Class (CBSE) Science: Combustion and Flame

Question: What is an explosion?

Answer: A combustion reaction in which sound is produced along with evolution of heat, light, and a large amount of gas is called an explosion. Example: Firecracker.

Question: Why is water not suitable for extinguishing a fire in an electrical appliance?

Answer: Fire in electrical equipments cannot be controlled with water as water is good conductor of electric current and a person trying to extinguish the fire can get an electric shock.

Question: Name and give chemical equations to show the products of the combustion reaction in: A limited supply of air, sufficient supply of air.

Answer: (a) Combustion reaction in limited supply of air causes incomplete combustion produce carbon monoxide, water and energy.
2CH₄ + 3O₂ ——-> 2CO + 4H₂O + Energy
(b) Combustion reaction in sufficient supply of air causes complete combustion produce carbon dioxide and water and
energy
2CH₄ + 3O₂ ———> 2CO₂ + 4H₂O + Energy

Question: List the different methods by which a fire can be extinguished.

Answer: A fire can be extinguished by applying different methods:

1. Removing the combustible substance.
2. Cooling the substance to below its ignition temperature using water.
3. Cutting off the supply of air or using fire extinguishers.

Question: What kind of fires can effectively be put out by water? What kind of fires is it not suitable for? Give reasons for both cases.

Answer: Water is suitable as extinguisher When wood or paper are on fire, water can be used to put out the fire effectively. As these substances are solids, heavier than water and not fuels by nature, they can be extinguished by water easily.

Water is not suitable to use as extinguisher on the fire caused by electricity as water conducts electricity. Also, oil or petrol fires cannot be put out by water as oil and petrol are lighter than water. They float on water and keep burning.

Question: How does carbon dioxide extinguish a fire?

Answer: Carbon dioxide being heavier than air forms a blanket around the fire cutting off the air supply. Also CO2 is not a supporter of combustion.

Question: What is calorific value of a fuel? Why is hydrogen not used as a fuel though it has highest calorific value?

Answer: Calorific value: The amount of heat produced on complete combustion of one kilogram of the fuel is called Calorific value of fuel.

Hydrogen is not used as a fuel because it has very low ignition temperature and highly explosive nature.