A student takes about $4 \mathrm{m}\mathrm{L}$ of distilled water in four test tubes marked $\mathrm{P}, \mathrm{Q}, \mathrm{R}$ and $\mathrm{S}$. He then dissolves in each test tube an equal amount of one salt in one test tube, namely sodium sulphate in $\mathrm{P}$, Potassium sulphate in $\mathrm{Q}$, calcium sulphate in $\mathrm{R}$ and magnesium sulphate in $\mathrm{S}$. After that he adds an equal amount of soap solution in each test tube. On shaking each of these test tubes well, he observes a good amount of lather (foam) in the test tubes marked: