Water

Hard water contains dissolved compounds such as calcium and magnesium salts. There are two types of hard water:

- temporary (hydrogen carbonates)
- permanent (sulfate)

It forms when water flows over rocks containing calcium or magnesium which then dissolves into the water.

Benefits

- Calcium ions in drinking water help the development of strong bones and teeth.
- Some evidence suggests that hard water helps to reduce heart disease.

Scum

The calcium and magnesium ions in hard water can react with soap to form scum:

sodium stearate + Ca2+ ions = calcium stearate precipitate + Na+ ions
(soap)                   (calcium ions)             (scum)                   (sodium ions, soluble in…