Water evapourates from rivers, lakes and oceans and condense to form clouds, returning to the surface of Earth as rain. Most ionic substances are soluble in water, but many covalent compounds are not. A saturated solution contains the maximum amount of solute that will dissolve at that given temperature into the solution. The solubility of most solid solutes increases with the temperature, although the solubility of gasses decreases as the temperture increases. Solubility curves show how the solubility of a substance changes with temperature.

Hard watercontains dissolved substances such as calcium and magnesium salts. The calcium and/or magnesium ions in hard water react with soap producing a precipitate called scum. The calcium salts and/or magnesium salts also decompose to form scale when water is heated. Hard water may have benefits for human health when comsumed, contrary to popular belief.