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.
