Hardness of water can be detected with soap water. Water is described as being 'hard' if it does not lather readily with soap.
'Soft water', on the other hand, is described as the one, which lathers readily with soap. Chemically, natural water is never pure and contains varying amounts of the dissolved impurities absorbed from the natural or man made environment.
When appreciable concentrations of calcium ions (Ca2+) or the magnesium ions (Mg2+) in the form of their dissolved salts are contained in water, it becomes hard.