In the test for sulphate, the solution is acidified with dilute $\mathrm{HCl}$ before adding $\mathrm{BaCl}{}_2$. The main reason is

1. Ato prevent hydrolysis of $\mathrm{BaCl}{}_2$ to $\mathrm{Ba}(\mathrm{OH}){}_2$
2. Bto raise the $\mathrm{Ba}{}^{2+}$ concentration
3. Cto dissolve $\mathrm{BaCO}{}_3$, $\mathrm{BaSO}{}_3$ and $\mathrm{Ba}{}_3(\mathrm{PO}{}_4){}_2$ so that only $\mathrm{BaSO}{}_4$ remains precipitated
4. Dto convert $\mathrm{BaSO}{}_4$ into a less soluble salt