Why does a lump of cotton wool shrink in water?

Filling in the blank with most appropriate word.

_____ air is called wind. (Moving, Breeze)

Fill in the blank with the most appropriate word. 

Air is not a substance but it is a _____ of gases. (mixture, wind)

Fill in the blank with the most appropriate word.

Our earth is surrounded by _____. (air, wind)

Give the uses of following gas:

Oxygen

Give the use of the following gas:

Carbon dioxide

Give the use of following gas: Hydrogen

Give the use of following gas: Water vapour.