Combustion and Flame

Q38. Why does cooking oil catch fire if a frying pan is kept on the burning stove for a long time?

Ans. Cooking oil catch fire if a frying pan is kept on the burning stove for a long time because the cooking oil gets heated to its ignition temperature when kept over a burning stove for a long time.

Q39. What chemicals can put out a fire?

Ans. Carbon dioxide (CO₂) is the best extinguisher. Another way to get CO₂ is to release a lot of dry powder of chemicals like sodium bicarbonate (baking soda) or potassium bicarbonate. Near the fire, these chemicals give off CO₂.

Q40. Why do you have to use paper or kerosene oil to start fire in wood or coal?

Ans. Wood or coal has a high ignition temperature, so a wood or coal fire cannot be started by using a lighted matchstick directly. Thus, we use paper or kerosene oil to start fire in wood or coal.

Q41. Explain why we are advised not to sleep in a closed room with a coal fire burning.
Or
Why is it not advisable to sleep in a closed room with burning coal in the fireplace during winters?

Ans. Incomplete combustion of fuels gives carbon monoxide gas. It is a very poisonous gas. It is dangerous to burn coal in a closed room. The carbon monoxide gas produced can kill persons sleeping in that room.

Q42. Paper by itself catches fire easily whereas a piece of paper wrapped around an aluminium pipe does not. Give reason.

Ans. The heat supplied to the paper is transferred to aluminium pipe by conduction. So, in the presence of aluminium pipe, the ignition temperature of paper is not reached. Hence, it does not burn.

Q43. How is fire caused by electricity extinguished?

Ans. For fires involving electrical equipment, carbon dioxide (CO₂) is the best extinguisher. CO₂, being heavier than oxygen, covers the fire like a blanket. Since the contact between the fuel and oxygen is cut off, the fire is controlled.

Q44. What are inflammable substances? Give examples.

Ans. The substances which have very low ignition temperature and can easily catch fire with a flame are called inflammable substances. Examples of inflammable substances are petrol, alcohol, Liquefied Petroleum Gas (LPG), etc.

Q45. Which zone of a flame does a goldsmith use for melting gold and silver and why?

Ans. Goldsmith use the outermost zone of the flame for melting gold and silver because in the outermost zone (or non-luminous zone) of a flame, complete combustion of the fuel takes place and is the hottest part of the flame.

Q46. What is the colour of the different zones of a flame?

Ans. A flame consists of three zones.

The innermost zone of a flame is dark (or black)

The middle zone of a flame is yellow.

The outer zone of a flame is blue.

Q47. It is difficult to burn a heap of green leaves but dry leaves catch fire easily. Explain.

Ans. It is difficult to burn a heap of green leaves because it contains a lot of moisture and hence its ignition temperature is high. But dry leaves catch fire easily because it contains no moisture and hence its ignition temperature is low.

Q48. What is spontaneous combustion? Give one example.

Ans. The type of combustion in which a material suddenly bursts into flames, without the application of any apparent cause is called spontaneous combustion. Example: Burning of white phosphorous on its own at room temperature.

Q49. How does pouring water extinguish a fire?

Ans. Water cools the combustible material so that its temperature is brought below its ignition temperature. This prevents the fire from spreading. Water vapours also surround the combustible material, helping in cutting off the supply of air. So, the fire is extinguished.

Q50. What is explosion? Explain with the help of example.

Ans. When a cracker is ignited, a sudden reaction takes place with the evolution of heat, light and sound. A large amount of gas formed in the reaction is liberated. Such a reaction is called explosion. Explosion can also take place if pressure is applied on the cracker.