Hardness of water is expressed per litre (m Eq/L), one unit which is equivalent to which one of the following.
Hardness of water

Hardness of water is defined as the soap-destroying power of water.
Water is considered to be bard if large amounts of soap are required to produce lather.
The hardness is caused by high mineral content, Mainly calcium and magnesium cations.
Hardness may be : -

i) Temporary (Carbonate) hardness → Calcium bicarbonate & Magnesium bicarbonate.

It is called temporary because it can be removed by boiling or addition of lime.

ii) Permanent (Non-Carbonate) hardness → Calcium Sulfate, magnesium sulfate, Calcium chloride, magnesium chloride.

It is called permanent because it cannot be removed by boiling.
Hardness is expressed in terms of mili-equilents per litre (mEq/L).
One mEq/L of hardness - producing ion is equal to 50 mg CaCO3 (50ppm) in one litre of water.
20 mg CaCO3 (20 ppm)
30 mg CaCO3 (30 ppm)
40 mg CaCO3 (40 ppm)
50 mg CaCO3 (50 ppm)