Water of appropriate quality is essential for life. For humans, drinking water should have sufficiently low levels of dissolved salts and microbes. Water that is safe to drink is called potable water. Potable water is not pure water in the chemical sense, because it still contains dissolved substances. Pure water contains only water molecules, H2O.
The methods used depend on the available supplies of water and local conditions. In the UK, rain provides fresh water with low levels of dissolved substances, and this collects in the ground and in lakes and rivers. Most potable water is made by choosing an appropriate source of fresh water, passing it through filter beds to remove solid particles, and then sterilised to kill microbes. Sterilising agents include chlorine, ozone and ultraviolet light.
If supplies of fresh water are limited, salty water or sea water can be treated by desalination. Desalination can be done by distillation or by processes that use membranes, such as reverse osmosis. These processes need large amounts of energy, so they are more expensive than treating fresh water.
In the required practical, you analyse and purify water samples. You test the pH, find the dissolved solids by evaporating a known volume of the sample and weighing the residue, and then purify the sample by distillation. The pure distilled water is collected in a cooled receiver and leaves the dissolved solids behind in the flask.