A molecule of water (H₂O) is made up of one oxygen atom (O) joined to two hydrogen atoms (H). These atoms are held together by 2 covalent bonds.

Water has both positive and negative poles, making it a dipolar molecule.

In a water molecule, the oxygen atom has a slightly negative charge whilst the hydrogen atoms have a slightly positive charge. 

Hydrogen bonding is the attraction between the partially positive hydrogen end of one water molecule and the partially negative oxygen end of another water molecule. 

1. Solvent
2. Temperature control
3. Habitat
4. Metabolite
5. Transport within an organism

Water is known as the universal solvent because it dissolves more substances than any other liquid.

When ionic compounds are added to water, the slightly negative oxygen end of water molecules are attracted to the positive ions whereas the slightly positive hydrogen ends are attracted to negative ions. 

The positive and negative ions are split apart and each is surrounded by water molecules to dissolve. 

Water being a solvent is useful as most biological reactions take place in solution and also so dissolved substances can be transported around the body. 

A high specific heat capacity means that a lot of energy is required to raise the temperature of water.

Water has a high specific heat capacity because there are many hydrogen bonds between water molecules. These hydrogen bonds can absorb a lot of energy before being broken. 

The high specific heat capacity of water allows many organisms to maintain a fairly stable temperature as their bodies are made up mostly of water.

A high latent heat of vaporisation means that a lot of energy is needed to break the hydrogen bonds to change water from a liquid to a gas. 

This allows organisms to use evaporation of water as a method of cooling without losing too much water. 

Water has a high specific heat capacity and a high latent heat of vaporisation which means it does not change temperature or evaporate easily.

This provides a stable environment for organisms to live in. 

Ice floats on water as it is less dense than water due to water molecules being further apart from one another.

This forms an an insulating layer that prevents the water below from freezing, allowing organisms to move and survive.

Cohesion is the tendency of water molecules to stick together, while adhesion is the tendency of water to stick to other materials.

Cohesion and adhesion allow water to flow through organisms, carrying substances along with it.