2.12 - Drinking water and laboratory water

Potable does not mean pure

Chemically pure water contains only water, H₂O, with no dissolved salts or other substances. Potable water can be a mixture: small amounts of dissolved minerals may be safe. Harmful levels of microorganisms or toxic substances would make it unsuitable for drinking.

For example, water containing a safe concentration of dissolved magnesium salts can be potable but is not chemically pure. Appearance alone cannot establish either property: clear water may contain dissolved substances or harmful microorganisms that you cannot see.

Treating waste and ground water

Waste water and ground water may contain suspended solids and microorganisms. Three important treatment stages are sedimentation, filtration and chlorination. Each has a different purpose.

Sedimentation followed by filtration removes solids before chlorination disinfects the water. Chlorination does not separate dissolved salts, and ordinary filtration does not remove them either: dissolved ions pass through with the water. Water sources must therefore be checked for harmful dissolved substances as well; these three stages do not guarantee that every contaminated source becomes potable.

Removing dissolved salts from sea water

Sea water contains too much dissolved salt to be potable. Filtering it would leave that salt in the water. Distillation separates water from dissolved salts by changing the water's state and collecting it again.

1. Heat sea water so water boils or evaporates to form water vapour.
2. Cool the vapour so it condenses back to liquid water.
3. Collect the condensed water, called the distillate, in a separate container.

The salts do not vaporise with the water under these conditions, so they remain in the heated flask. Initially they remain dissolved in the water left there; solid salt is left if the water is removed. The collected distillate has had the salts removed.