Water exists in combined state in all organisms. A tree is about 60% water by weight, and most animals are about 5065% water. The human body contains approximately 70% of water. Each of us needs only about 2 liters of water per day to survive, but huge amounts of water are needed to supply us with food, shelter, and our other needs and wants. Water also plays a key role in sculpting the Earths surface, moderating climate and diluting pollutants
Water is also present in certain salts as water of crystallisation. (Many substances combine with water to form crystalline compounds known as "hydrates". These hydrated salts contain a definite amount of water known as water of crystallisation). For example, the crystals of copper sulphate contain five molecules of water of crystallization as CuSO4.5H2O, ferrous sulphate (green vitriol) contains seven molecules of water of crystallisation, as FeSO4.7H2O.
Some minerals also contain water of crystallisation such as:
Borax: Na2B4O7.10H2O (10 molecules of water of crystallisation).
Alumina: Al2O3.2H2O (2 molecules of water of crystallisation).