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Water hardness

Hardness in water is defined as the presence of multivalent cations. Hardness in water can cause water to form scales and a resistance to soap. It can also be defined as water that doesn't produce lather with soap solutions, but produces white precipitate (scum). For example, sodium stearate reacts with calcium:

2C17H35COONa + Ca2+ → (C17H35COO)2Ca + 2Na+

Hardness of water may also be defined as the soap-consuming capacity of water, or the the capacity of precipitation of soap as a characteristic property of water that prevent the lathering of soap.

Descriptions of hardness correspond roughly with ranges of mineral concentrations:

Water hardness [mg/l] or [ppm] [grains/gal]
Soft 0-17.1 0-1.0
Slightly hard 17.1-60 1.0-3.5
Moderately hard 60-120 3.5-7.0
Hard 120-180 7.0-10.5
Very hard >180 >10.5

Because it is the precise mixture of minerals dissolved in the water, together with the water's pH and temperature, that determines the behaviour of the hardness, a single-number scale does not adequately describe hardness.

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