Water

Water dissolves an astonishing range of substances - sugar and salt (solids), alcohol and acids (liquids), and oxygen and carbon dioxide (gases). Because it dissolves more kinds of matter than any other liquid, water is called the universal solvent. The water molecule is slightly lopsided in charge (one end a little positive, the other a little negative), so it can pull apart and surround the particles of many substances. When a substance dissolves, the substance is the solute, the water is the solvent, and the clear mixture formed is the solution.

Solubility is the maximum mass of a solute that dissolves in 100 g of water at a stated temperature to make a saturated solution. A saturated solution can hold no more solute at that temperature. A graph of solubility against temperature is a solubility curve. For most solids, such as potassium nitrate, the curve climbs steeply: hot water holds much more. For gases, the curve falls - warm water holds less gas, which is why a warm fizzy drink goes flat. Gas solubility also increases with pressure.

When a hot saturated solution of a solid like copper sulphate is cooled, the excess solute can no longer stay dissolved and separates out as regular crystals - this is crystallisation. Many such crystals trap a fixed number of water molecules within their structure; this is the water of crystallisation. Blue copper sulphate is CuSO4·5H2O, washing soda is Na2CO3·10H2O. This combined water gives the crystals their shape and often their colour, even though they feel dry.

A salt holding its water of crystallisation is hydrated (blue CuSO4·5H2O). Heating drives this water off, leaving the white anhydrous salt; adding water turns it blue again, a test for water. Anhydrous salts that soak up moisture are drying agents. Water itself is hard if it contains dissolved calcium and magnesium salts: it wastes soap by forming scum. Temporary hardness, caused by bicarbonates, is removed simply by boiling; permanent hardness, caused by sulphates and chlorides, needs washing soda or other treatment.