Suppose you have a solution that might contain any or all of the following cations: Ni2+, Ag+, Sr2+, and Mn2+. Addition of HCl solution causes a precipitate to form. After filtering off the precipitate, H2SO4 solution is added to the resultant solution and another precipitate forms. This is filtered off, and a solution of NaOH is added to the resulting solution. No precipitate is observed.