why hardness of water is caused by salts of calcium and magnesium only and not by sodium or potassium salts?

Dear deeksha,

Hardness of water is usually attributed to calcium and magnesium salts but any cation with two or more charges can cause it (eg iron(II) here). The cation will form a precipitate (scum) with soap, eg:

calcium chloride + sodium stearate (soap) → calcium stearate (scum) + sodium chloride

This scum wastes soap and can cause abrasion to clothes, as well as causing unsightly deposits round baths and showers.

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hi

assume calcium carbonate to be present in the water .

when this water is treated with soap

calcium carbonate+sodium oliate(soap)-->sodiumcarbonate +calcium oliate

this calium oliate forms insoluble scum but sodium carbonate doesnot.