The huge reservoirs of water which is formed by the collection of groundwater between the layers of hard rocks inside the earth are known as aquifers. It is one of the most important sources of water on Earth and it is the largest reservoir of usable fresh water. We can use this water for drinking and other domestic purposes.

Conversion of water vapours which is the gaseous state to water its liquid state on cooling, is called condensation. When we put ice-cold water in a glass, we can see water droplets outside the glass. This is due to the condensation of water vapour when it comes in contact with the cold surface. So, the process of changing a gas (or vapour) to a liquid by cooling is known as condensation.