A student took four test tubes $P, Q, R$ and $S$ and filled about $8\,\text{ml}$. of distilled water in each. After that he dissolved an equal amount of $\mathrm{Na}_2\mathrm{SO}_4$ in $P, \mathrm{K}_2\mathrm{SO}_4$ in $Q, \mathrm{CaSO}_4$ in $R$ and $\mathrm{MgSO}_4$ in $S$. On adding an equal amount of soap solution and shaking each test tube well, a good amount of lather will be obtained in the test tubes: