Water is said to be “hard” when it has certain minerals dissolved in it. The most noticeable effect of hard water is that soap does not lather well in it, instead forming a kind of scum. There are two kinds of water hardness, depending on which chemicals are dissolved in it. Temporary hardness can be removed by boiling the water. The chemicals become a solid, which is the scale that sometimes furs up kettles and shower heads. Permanent hardness can be removed by using a water softener, which exchanges the calcium and magnesium ions that cause the hardness with sodium ions.
Hard water (or water hardness) is a common quality of water which contains dissolved compounds of calcium and magnesium and, sometimes, other divalent and trivalent metallic elements.
The term hardness was originally applied to waters that were hard to wash in, referring to the soap wasting properties of hard water. Hardness prevents soap from lathering by causing the development of an insoluble curdy precipitate in the water; hardness typically causes the buildup of hardness scale (such as seen in cooking pans). Dissolved calcium and magnesium salts are primarily responsible for most scaling in pipes and water heaters and cause numerous problems in laundry, kitchen, and bath. Hardness is usually expressed in grains per gallon (or ppm) as calcium carbonate equivalent.
Oils and fats do not mix with water, so washing greasy clothes or hair in water alone will not clean them. Soap contains ions that are attracted to water at one end and to grease at the other. This end of each ion attaches itself to the grease, while the other end, attracted to the water, pulls the grease away from the fabric or hair. Hard water is a problem because the ions react with the chemicals in the water to form scum.
Picture Credit : Google