Hard water contains relatively high amounts of dissolved minerals. The most important ones are calcium ions and magnesium ions.

These ions enter the water naturally. Rainwater moves through layers of rock underground and small amounts of minerals dissolve into the water. In regions where the ground contains limestone or similar rocks the water can absorb significant amounts of calcium and magnesium.

The higher the concentration of these ions the harder the water is considered to be.

Hard water affects how soap behaves. Calcium and magnesium ions can react with the fatty acid salts in soap and form substances that do not dissolve well in water.

Because of this reaction soap may produce less foam and small residues can remain after washing. These residues can also accumulate on hair.

Tiny mineral deposits may stay on the surface of the hair and change its texture slightly. As a result hair can sometimes feel rougher or drier after washing with hard water.

In areas with softer water where fewer calcium and magnesium ions are present this effect is much less noticeable.