Hardness of natural waters is caused largely by calcium and magnesium salts and to a small extent by iron, aluminum, and other
metals. Hardness resulting from the bicarbonates and carbonates of calcium and magnesium is called temporary hardness and can be removed by boiling, which also sterilizes the water. The residual hardness
is known as noncarbonate, or permanent, hardness. The methods of softening noncarbonate hardness include the addition of sodium carbonate and lime and filtration through natural or artificial zeolites which absorb the hardness-producing metallic ions and release sodium ions to the water. Sequestering agents in detergents
serve to inactivate the substances that make water hard.
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