Why is it necessary to test for the acid radicals first (with dilute) `H_(2)SO_(4)` and then with conc. `H_(2)SO_(4)`?

Explanation

There are some ions like, `SO_(3)^(2-), S^(2-), NO_(2)^(-) " and" CH_(3)CO O^(-)` which can react with dilute/conc. `H_(2)SO_(4)` whereas ions like `Cl^(-), Br^(-), |^(-), NO_(3)^(ө)`, etc. react only with con. `H_(2)SO_(4)`.
Now if conc. `H_(2)SO_(4)` is used first then the anions of both the types will react. Hence, it is desired to test acid radicals first with dilute `H_(2)SO_(4)` and then with conc. `H_(2)SO_(4)`.