Definition of hardness is water properties that is caused by the presence of ions or metal cations which has valence two. Cations are the main cause of hardness. The cations are Ca2+, Mg2+, SR2+, Fe2+ and Mn2+, while anions contained in the water are HCO3-, SO42-, Cl-, NO3-.
Water hardness is generally described as reaction of water to soap to produce foam or ability to cause scale or crust on hot water pipes, boilers, and other metal equipment. Water hardness is very important with respect to the various uses of water, because of the tendency to form crust. Water that has very low hardness, with only little soap will have a lot of foam, but if there is high water hardness, can cause the waste of soap. Water hardness should be optimum, because if too high will cause the crust, and if too low will cause corrosion at high temperatures.
Hardness of water varies from place to place. Generally, surface water has more hardness than ground water. Water hardness depends on the natural geological formations and with what water is in contact. The water hardness can be found in England, South Atlantic, North Pacific: Iowa, India, Arizona, Mexico because they are countries that has fairly high water hardness.