Potable water

Potable water is water that is safe to drink. It has low levels of dissolved salts and microbes.

Potable water is not pure in the chemistry sense: it still contains small amounts of dissolved substances, such as minerals. Pure water contains only water molecules.

In the UK, rain provides fresh water, which collects in the ground (aquifers), lakes, rivers and reservoirs. To make it potable:

1. Choose a suitable source of fresh water.
2. Pass it through filter beds to remove solid particles.
3. Sterilise it to kill harmful microbes, using chlorine, ozone or ultraviolet (UV) light.

Too much chlorine or ozone can be harmful, so the amounts are carefully controlled.

Where fresh water is scarce, potable water can be made from sea water by desalination — removing the salt:

• Distillation: the sea water is boiled, and the steam is condensed as pure water, leaving the salt behind.
• Reverse osmosis: sea water is forced at high pressure through a membrane that lets water molecules through but not salt ions.

Both methods need large amounts of energy, so desalination is expensive. It is mainly used in dry countries near the sea, such as those in the Middle East.

Compare water samples, such as tap water, rain water and sea water:

1. pH: test each sample with universal indicator or a pH meter.
2. Dissolved solids: weigh an empty evaporating basin, add 50 cm³ of the sample and heat until all the water has evaporated. Reweigh it; the increase in mass is the mass of dissolved solids.
3. Purify a sample of salty water by distillation: heat it in a flask, condense the steam in a condenser, and collect the distilled water. Check it: it should have a pH of 7 and leave no solid behind when evaporated.

Hazard: apparatus gets very hot — let it cool before touching it.

Better accuracy: heat gently so the solid doesn't spit out of the basin.