Water
Water
- Water seems ordinary, but life depends on its unusual behaviour.
- Almost everything about it comes from one fact: the water molecule is polar.
- Polarity lets water molecules grip each other with hydrogen bonds.
Polar molecule, hydrogen bonds
- In water the oxygen end is slightly negative and the hydrogen ends slightly positive — it is a polar molecule.
- Opposite charges attract, so each molecule forms weak hydrogen bonds with its neighbours.
- These many small bonds are the reason for water's special properties.
Practice
Water molecules attract each other because water is:
Water is polar (slightly negative oxygen, slightly positive hydrogens), so neighbouring molecules form hydrogen bonds.
Three life-giving properties
- Solvent — many substances dissolve in water, so reactions can happen and substances can be transported around the body.
- High specific heat capacity — water needs a lot of energy to warm up, so temperatures stay steady and protect living things.
- High latent heat of vaporisation — water needs a lot of energy to evaporate, so sweating carries away a lot of heat and cools the body.
Practice
Water is described as a good solvent. This is useful because it:
As a solvent, water lets reactants meet and dissolved substances be carried around the body (e.g. in blood).
Practice
Water has a high specific heat capacity, which means it:
A high specific heat capacity means temperature changes slowly, protecting organisms from sudden changes.
Practice
Why does sweating cool the body?
Evaporating water needs a lot of energy (high latent heat of vaporisation); it takes that heat from the skin, cooling it.
You've got it
Key idea
- water is a polar molecule → forms hydrogen bonds between molecules
- solvent: dissolves substances for reactions and transport
- high specific heat capacity: temperature stays stable
- high latent heat of vaporisation: evaporation (sweat) cools the body