Question

An aqueous solution of a substance gives a white precipitate on treatment with dilute hydrochloric acid, which dissolves on heating. When hydrogen sulphide is passed thorugh the hot acidic solution, a black precipitate is obtained. The substance is a:

• A. $H{g}^{2+}\text{salt}$
• B. $C{r}^{2+}\text{salt}$
• C. $A{g}^{2+}\text{salt}$
• D. $P{b}^{2+}\text{salt}$

Answer 1

The correct option is D $P{b}^{2+}\text{salt}$

The given substance is a salt of lead (II). It reacts with dil. $HCl$ to form white precipitate of lead chloride.
$P{b}^{2+}+2HCl\to PbC{l}_2\downarrow \text{(black)}+2H^{+}$
Lead chloride has low solubility in cold water and it dissolves in hot water.
It reacts with sulphide ions from hydrogen sulphide to form a black precipitate of lead sulphide.

$PbC{l}_2+H_{2}S\to PbS\downarrow \text{(black)}+2HCl$