Water

Structure of water

Two hydrogens share electrons with an oxygen (two covalent bonds). Oxygen more electronegative so has a slight negative charge whilst hydrogen has a slight positive charge. This makes water polar.

Hydrogen bonding

Hydrogen bonds are the weak bonds that form between the hydrogen and oxygen in another molecule.

Key properties of water

• Good solvent

• High specific heat capacity

• High latent heat of vaporisation

• High cohesion and surface tension

• High adhesion

• Density and freezing properties

Good solvent

Dissolves most charged and polar substances, which is important in chemical reactions and as a transport medium

High specific heat capacity

As hydrogen bond can absorb lots of energy it takes a lot of energy to heat up 1g by 1^oc. This stops rapid temp rises and keeps temp fairly stable.

High latent heat of vaporisation

Lots of energy is required to break hydrogen bonds, so takes lots of energy to evaporate, making water great at cooling things.

High cohesion and surface tension

Very cohesive due to hydrogen bonds, helps water flow, so good for transporting substances. Cohesion generates surface tension

High adhesion

Water sticks to cell walls and xylem cells due to hydrogen bonding, so can be easily transported.

Density and freezing properties

Ice is less dense than water as below 4^oc density of water decreases. This means that ice floats insulating water below it so water below does not freeze.