A student takes four test tubes marked $P$, $Q, R$ and $S$ of $25\,\mathrm{mL}$ capacity and fill $10\,\mathrm{mL}$ of distilled water in each. He dissolves one spoon full of four different salts in each as $-\mathrm{KCl}$ in $P, \mathrm{NaCl}$ in $Q, \mathrm{CaCl}_2$ in $R$ and $\mathrm{MgCl}_2$ in $S$. He then adds about $2\,\mathrm{mL}$ of a sample of soap solution to each of the above test-tubes. On shaking the contents of each of the testtubes, he is likely to observe a good amount of lather (foam) in the test tubes marked :