Water hardness

Water hardness is a measure mainly of dissolved calcium and magnesium ions in water. Certain other metal ions can contribute, but usually occur in smaller amounts. These minerals enter water as it interacts with rocks and soils .¹

Hardness is commonly reported in milligrams per litre as calcium carbonate, written mg/L as CaCO3. This is an equivalent measure that puts the contributions of different hardness ions on a common scale. It does not mean that all those minerals were present as calcium carbonate. Labels such as “soft” and “hard” describe ranges on this scale, although classification boundaries vary .¹

Hard water tends to require more soap to form a lather and can leave mineral scale in heated pipes or appliances. Hardness also differs from alkalinity, the water’s capacity to buffer added acid. The two can be related because dissolved carbonate and bicarbonate accompany hardness minerals, but they describe different chemical properties .¹