CBSE Class 10 Science 2017 · Paper 31/1/1/C · Question 34

Take equal volumes of A and of B in two test tubes each. Add a few drops of C to one tube of A and one of B, and a few drops of D to the other two, and shake all four equally. (i) Soap and detergent: the solution that behaves differently in A and in B, forming a curdy white scum and little lather in one water but plenty of lather in the other, is the soap solution; the other, which lathers well in both waters, is the detergent solution. (ii) Hard and soft water: the water in which the soap forms the curdy scum and little lather is hard water; the water in which the soap lathers readily is soft water. The scum forms because the calcium and magnesium ions of hard water make insoluble salts with soap but not with a detergent.