Degree of Hardness
Small amounts of calcium and magnesium, the most common minerals that make water hard, dissolve in water as it moves through rock and soil and remain there. As the minerals increase in content the degree of water "hardness" is greater, according to water-treatment.org.
Water as a Solvent
Being a solvent, water can pick up impurities easily. Combining it with the carbon dioxide in the air, it becomes an even better solvent through the formation of weak carbonic acid.
The traditional treatment for hard water is a mechanical water softener which works by ion exchange in which sodium replaces the minerals in the water. While magnesium and calcium are the most common minerals found in hard water, iron, zinc and copper can also be present. Water softeners can be pricey, but in addition to replacing the most common minerals, they also treat the scarce minerals, according to water-treatment.org.
Magnetic Water Conditioner
Another available treatment is a magnetic water conditioner, in which a magnetic field is created around the pipe work that alters the water, causing it to lose the ability to cause scale. Both conditioner and softener will reduce lime scale.
Precipitating Water Softener
Precipitating and non-precipitating chemicals can also help control hard water problems. Borax and washing soda are present in precipitating water softeners, according to hardwater.org. An insoluble precipitate is formed with magnesium and calcium ions. The precipitate can build up on surfaces and cause cloudiness in the water, but the mineral ions will not interfere with cleaning efficiency.
Non-Precipitating Water Softener
Complex phosphates are used by non-precipitating water softeners, which sequester magnesium and calcium ions, according to hardwater.org. The alkalinity is not increased and no precipitate forms. When used in enough quantity for a period of time, the non-precipitating water softener will help dissolve soap curd.