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Drinking water is usually provided by precipitation.There are problems in supplying potable, drinkable water in some areas of the world, especially where there is drought.

Seawater is a very abundant source of water, but its high salt content makes it unsuitable as drinking water. However, pure water can be produced from seawater by distillation. This is also known as thermal desalination.

During distillation, the seawater is boiled. The water vapour is then cooled and condensed to form pure water - leaving the salt behind.

The disadvantages of producing drinking water this way include:

• it is expensive because large amounts of thermal energy are needed to heat the seawater
• it increases the use of fossil fuels - which are non-renewable resources
• carbon dioxide emissions from burning fossil fuels contribute to global warming

Distillation is common in some Middle Eastern countries that have little rainfall, but are wealthy due to their oil reserves.

Another method of desalination is reverse osmosis:

1. salt water is forced at high pressure into a vessel with a partially permeable membrane
2. the pressure causes water molecules to move in the opposite direction to osmosis from a concentrated salt solution (low water concentration) to a lower salt concentration (higher water concentration)
3. water molecules pass across the membrane leaving the salt behind, so pure water is available for drinking

Osmosis is the movement of water across a partially permeable membrane from a region of higher water concentration to a lower water concentration. Reverse osmosis water moves, due to pressure, in the opposite direction.