CBSE Class 10 Science 2019 · Paper 31/1/1 · Question 23
In three test tubes A, B, and C, three different liquids namely, distilled water, underground water and distilled water in which a pinch of calcium sulphate is dissolved, respectively are taken. Equal amount of soap solution is added to each test tube and the contents are shaken. In which test tube will the length of the foam (lather) be longest ? Justify your answer.
Answer
Test tube A has the longest foam, because distilled water is soft water and the other two are hard.
The foam (lather) is longest in test tube A, which contains distilled water. Justification: distilled water contains no dissolved calcium or magnesium salts (it is soft water), so all the soap forms lather. Underground water (B) and distilled water with calcium sulphate (C) are hard water: their calcium (and magnesium) ions react with the soap to form an insoluble scum, so much of the soap is wasted and little lather forms.
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