In the laboratory of a school , the sample of hard water is not available " to study the comparative cleaning capacity of a sample of soap in soft and hard water". Which salt from the laboratory can be added to tap water to make it hard ? In the experiment how is cleaning capacity of soap compared ?

Explanation

Calcium and Magnesium salts (like ` CaCl_(2), MgCl_(2))` are added to the tap water to make it hard. Take 10 ml of soft water (tap water) in a test tube and 10 ml of hard water ( just prepared above) in a separate test tube. Add few drops of soap solution in both the test tubes. It is observed that,l the test tube containing soft water produces large amount of form but the test tube with hard water will have white and curdy appearance of percipilate.
Soap does not give lather with hard water as it reacts with the calcium and magnesium ions present in hard water to form insoluble precipitates of calcium and magnesium salft of fatty acids.
The scum ( or precipitate) formed by the action of hard water on soap stickes to the clothes being washed and it interferes with the cleaning ability of soap. This makes the cleaning of clothes difficult.