CBSE Class 10 Science 2018 · Paper 31/1/C · Question 22
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 ?
Answer
Dissolve calcium chloride in tap water to make it hard; the longer the foam column, the greater the soap's cleaning capacity.
Salt: calcium chloride (or magnesium chloride or magnesium sulphate); a soluble calcium or magnesium salt dissolved in tap water makes it hard.
Comparison: take equal volumes of soft (distilled) water and of the hard water in two identical test tubes, add the same number of drops of the same soap solution to each, and shake both equally. Compare the length of the foam column in the two tubes: the longer the foam, the greater the cleaning capacity. Soft water gives a long foam column; hard water gives very little foam and a white curdy scum, so the soap cleans better in soft water.
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