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What are combustible and non-combustible substances ? Explain with examples.


The substances in which combustion takes place are called combustible substances. For example : wood, paper, coal.

The substances in which no combustion takes place are called non-combustible substances. For example : Glass, iron nail. 

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Why is a match stick rubbed against a rough surface to light it ?


On rubbing the match stick in the rough surface, the friction converts this work into heat. The heat raises the temperature of the chemical present on the match stick head to its ignition temperature. Due to this the chemical substance catches fire and the match stick starts burning.

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Why does paper burn with a yellow flame ?


A yellow flame comes about when there is insufficient supply of oxygen. It is a luminous flame. In this case the hydrocarbons do not burn completely, the temperature of the flame is lower than that of the blue flame. The yellow flame also leaves behind a black soot or residue.

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Goldsmiths blow the outermost zone of a flame with a metallic blow pipe for melting gold and silver. Why do they use the outermost zone of the flame ?


The melting point of gold and silver is very high therfore goldsmiths need maximum temperature. Hence goldsmith use outermost part of the flame because it is the hottest part of flame.

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What is supposed to be the cause of global warming ?


Increasing amount of carbon dioxide is the cause of global warming.

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