Hardness in water, or "hard water", refers to specific minerals that consume soap and cause scaling in water heaters and boilers. The more minerals, the harder the water. Soft water refers to the absence of these minerals.
The term hardness comes from an expression of how difficult or "hard" it is to wash clothes with the water. When soap is mixed with hard water, these minerals combine with the soap and form a precipitate, or a solid. This decreases the cleaning efficiency of the soap and forms soap scum. As more soap is added, solids continue to form until the minerals are depleted. When the minerals are no longer available, the soap forms a lather and works as a cleaning agent.
The minerals that precipitate with soap are polyvalent cations such as calcium, magnesium, iron, manganese, and zinc. The concentration of calcium and magnesium in natural waters generally far exceeds that of any other polyvalent cation. Therefore, hardness is generally considered to be the concentration of calcium and magnesium in water.
Hardness is removed from water systems by precipitation or ion exchange. The treatment method varies depending on the relative amounts of carbonate vs. non-carbonate hardness.
Hach offers many solutions for hard water testing including hard water test kits, hard water test strips, analyzers for hard water, as well as ion-selective eletrodes.