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?
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.
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
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
Q46. What is the colour of the different zones of a flame?
Ans. A flame consists of three zones.
innermost zone of a flame is dark (or black)
middle zone of a flame is yellow.
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.