Match the tests in Column I with the observations in Column II. Column I: (P) $\mathrm{NH}{}_4{}^{+}$ with Nessler's reagent; (Q) $\mathrm{Mg}{}^{2+}$ with $\mathrm{Na}{}_2\mathrm{HPO}{}_4$ in $\mathrm{NH}{}_4\mathrm{Cl}$/$\mathrm{NH}{}_4\mathrm{OH}$; (R) $\mathrm{Ca}{}^{2+}$ with $(\mathrm{NH}{}_4){}_2\mathrm{C}{}_2\mathrm{O}{}_4$; (S) $\mathrm{Ba}{}^{2+}$ with $\mathrm{K}{}_2\mathrm{CrO}{}_4$ in acetic acid. Column II: (1) white crystalline precipitate soluble in dilute acetic acid; (2) yellow precipitate insoluble in acetic acid; (3) white precipitate insoluble in acetic acid but soluble in dilute $\mathrm{HCl}$; (4) brown precipitate or coloration.

1. AP-4, Q-1, R-3, S-2
2. BP-4, Q-3, R-1, S-2
3. CP-1, Q-4, R-3, S-2
4. DP-4, Q-1, R-2, S-3