CBSE Class 10 Science 2018 · Paper 31(B)/C · Question 23
You have two samples of water marked A and B. How would you distinguish between the two if one sample is of soft water and the other is of hard water ?
Answer
Soap lathers easily in soft water; in hard water it forms a white curdy scum with very little lather.
Take equal volumes of samples A and B in two test tubes. Add the same number of drops of the same soap solution to each and shake both tubes equally.
The sample that forms a lot of lather easily is soft water.
The sample that forms very little lather and a white, curdy precipitate (scum) is hard water, because the calcium and magnesium salts in hard water react with soap to form insoluble scum.
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