Water hardness is a measure of mineral ions present in it. Primarily the dissolved of Magnesium and Calcium cause hardness in it which is expressed in milligrams/Liter. Hardness of water can be of two types temporary and permanent hardness. Temporary hard water contains carbonates or bicarbonates of Calcium and Magnesium whereas permanent hard water contains chlorides and sulfates of Calcium and Magnesium. The degree of hardness is determined as soft for 0 to 60 mg/ltr. Ions, slightly hard for 61 to 120 mg/ltr, hard for 121 to 180 mg/ltr and very hard for more than 180 mg/ltr dissolved carbonates or bicarbonates of calcium or magnesium in it. Temporary hardness can be removed by boiling whereas permanent hard water is treated differently to make it useful. Hard water can be the cause of formation of kidney stones in your body or may be the cause of reduced life of water heating equipments. But the effects of hard water on living and non-living systems are not always negative. The permissible hardness of water is also beneficial to supplement the mineral requirement of humans and other living beings.
There are different methods of estimation and determination of Hardness of water, simplest is comparing the lather produced with the soap or time taken in cooking of vegetables in open vessel, and other methods like titration and using electronic sensor etc.