•      Like air, water is also essential for life.

•      Water is the only substance on Earth that is in liquid form at the temperatures commonly found on the surface of our planet.

Figure 6: Water is in liquid state on the surface of the Earth

•      Water is a universal solvent, i.e. most of the substances can be easily dissolved into it.

•      This allows water to carry nutrients to cells, and carry waste away from them.

•      In addition to this, water has the unique property of expanding as it freezes.

•      The water that we use mainly comes from rivers, ponds, lakes, and wells.

•      Often sand, dirt, mud, germs, etc., make this water unfit for consumption. We call these substances impurities.


Impurities are of two Kinds:

•      Soluble impurities are the ones that dissolve in water (e.g., salt)

•      Insoluble impurities are the ones that do not dissolve in water (e.g., sand and germs).