Question

Which compound is insoluble in water? (1) BaSO4 (3) KClO3 (2) CaCrO4 (4) Na2S

Answer 1

Answer: $BaSO_4$

Explanation:

A precipitation reaction is a special type of double displacement reaction.

$BaSO_4$ is an insoluble compound as it is does not dissolve in water, and thus formed as a soild or a precipitate.

The sulfate ion $SO_4{2-}$ form insoluble compounds when combined with $Ba^{2+}$.

The chlorate ion $ClO_3^-$ form soluble compounds with all positive ions.  

The chromate ion $CrO_4^{2-}$ form insoluble compounds with all ions but soluble with $Ca^{2+$.

The sulfide ion $S^{2-}$ form insoluble compounds with all ions except when combined with $Na^+$ or $NH_4^+$

Answer 2

 BaSO4 
I hope this helps!