Calcium () and Magnesium () are the two most common ions that are found in hard water, while iron (Fe2+), Manganese (Mn2) and Aluminium (Al3+) can also contribute to hardness.
Hardness is defined as the sum of the calcium (Ca2+) and magnesium (Mg2+) concentrations, both expressed as calcium carbonate in milligrams per litre (mg/l). Iron (Fe2+), manganese (Mn2+) and Aluminium (Al3+) can also contribute to water hardness.
The water hardness has a significant effect on the performance of compounds. Soft water will cause foam; hard water will increase the compound usage and can cause spots on component parts during drying.