More things will dissolve in water than in any other known substance. For this reason pure water is never found in nature. Water in lakes, rivers, and wells always contains minerals dissolved from the earth. Even rainwater, the purest natural water, contains chemicals dissolved from the air. Pure water is obtained only by special methods of distillation and when water is produced directly from hydrogen and oxygen.

Minerals dissolved in water help nourish living things. Harmful substances, such as decaying animal and vegetable matter and poisonous chemicals, may also be dissolved, and for this reason it is important that sources of drinking water be tested. It is because water is such a good solvent, and therefore dissolves dirt, that it is used for washing.

Water that contains calcium and magnesium is said to be hard. (Soft water lacks these minerals.) Hard water, which occurs in many parts of the United States, wastes soap and forms a scale inside pipes, water heaters, kettles, and other containers. Hard water can be softened by chemical means.

Every river that runs to the sea carries minerals in solution. Some minerals, such as salt, bromine, and magnesium, can be recovered from the sea. Other minerals help nourish fish and other living things in the sea. A number of minerals and some rocks, including slate and sandstone, are obtained from the beds of prehistoric seas.