Question

why we conclude basic radicals on the basis of ppt formation

Answer 1

Positively charged radicals are basic radicals. Acid radicals are formed by removal of hydrogen ion $(H^+)$ Basic radicals are formed by the removal of hydroxide ion $(OH^-)$

Examples include;

$Ag^-,Na^+,Fe^+,Al^+$

The solubility products $(K_{sp})$ of basic radicals is less than the solubility products of other radicals in the same group in a given solution hence tasted through precipitation.