its all the game of common ion effect. when reaction quotient exceeds ksp we get ppt. so it is very imp that we perform them in order....

 as group 1 have lower ksp so when common ion effect takes place  they r the first to be ppted. the ksp values gradually increases so we need to use stronger reagents to have larger grade of common ion effect...

   moral- if we use reagents of higher groups no doubt we will have ppt. of that groupbut ppt. of lower groups r obtained. so we cant know which group r we really getting