Question

A white solid $Y$, on heating gives offa gas which turns lime water milky; the residue is yellow when hot; white when cold. The solid $Y$ is probably:

• A. $ZnC{O}_3$
• B. $PbC{O}_3$
• C. $ZnS{O}_4$
• D. $Zn(N{O}_3{)}_2$

Answer 1

The correct option is A $ZnC{O}_3$

The white solid $Y$ is probably zinc carbonate $ZnC{O}_3$.
On heating, it forms zinc oxide and carbon dioxide gas.
$ZnC{O}_3\stackrel{heat}{\to }ZnO+C{O}_2$
The carbon dioxide gas reacts with calcium oxide (from lime water) to form calcium carbonate which turns lime water milky.
$\underset{lime}{CaO}+C{O}_2\to CaC{O}_3$
The residue is zinc oxide which is yellow when hot and white when cold. The yellow colour of zinc oxide is due to defects in the solid structure which increases with temperature.