Water Hardness

Water is classified as either soft or hard:

Soft water contains relatively few minerals and lathers easily.
Hard water is rich in minerals such as calcium and magnesium, which is the cause of “scale” in kettles.

Water hardness is usually expressed as the number of parts per million (ppm) of calcium carbonate present in the water (see Table 19).

Table 19 Water hardness (in ppm of calcium carbonate)
Type of Water	Hardness
Soft water	10–50 ppm
Slightly hard water	50–100 ppm
Hard water	100–200 ppm
Very hard water	Over 200 ppm

Regions with soft water (1 ppm to 50 ppm) include the Pacific Northwest from Oregon up through British Columbia. The hard water regions (100+ ppm) include the Canadian Prairies, the U.S. Midwest, and the southwestern states of New Mexico and Arizona. In a sense, the hardness of water is the other side of the coin to alkalinity. In general terms, rainy climates such as the Pacific Northwest have acid water. Rain leaches out much of the mineral ions in the soil, replacing them with hydrogen ions. The result is that the water is rich in hydrogen and thus acidic (soft). The reverse is the case in the dry regions, where moisture evaporates, leaving the minerals intact. The result is water rich in minerals and thus alkaline (hard). Note that this explanation is a simplification as other factors such as the type of bedrock have an effect on water hardness as well.

See: https://opentextbc.ca/ingredients/chapter/water-hardness-and-ph/