Q29 of 38 Page 33

Cracker on ignition produces sound. Why?

On the ignition of a cracker, a very fast reaction takes place that leads to the evolution of various gases. The gases expand very rapidly and thus, give off a lot of heat and sound.


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People usually keep Angethi/burning coal in their closed rooms during winter season. Why is it advised to keep the door open?

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Write True/False against the following statements and also correct the false statement.

(a) A physical process in which a substance reacts with oxygen to give off heat is called combustion.


(b) Water is the best extinguisher for fires involving electrical equipment.


(c) Alcohol, CNG and LPG are inflammable substances.


(d) Increased concentration of nitrogen in air is believed to cause global warming.


(e) Greater the calorific value, better is the fuel.


(f) Middle zone is the hottest zone of a flame.


(g) The substances which vaporize during burning, give flame.

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What do you understand by fuel efficiency?

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You are provided with three watch glasses containing milk, petrol and mustard oil, respectively. Suppose you bring a burning candle near these materials one by one, which material(s) will catch fire instantly and why?