Question

Which of the following metal carbonates decompose on heating?

• A. $L{i}_{2}C{O}_3$
• B. $N{a}_{2}C{O}_3$
• C. $K_{2}C{O}_3$
• D. $MgC{O}_3$

Answer 1

Answer: (A), (D)

$L{i}_{2}C{O}_3$
$MgC{O}_3$

Lithium carbonate and magnesium carbonate are thermally unstable. They decompose on heating. Properties of Li are similar to Mg because of diagonal relationship.

$L{i}_{2}C{O}_3\to L{i}_{2}O+C{O}_2$

$MgC{O}_3\to MgO+C{O}_2$

$L{i}_{2}C{O}_3$ and $MgC{O}_3$ are salts of weak acid $\left(C{O}_2\right)$ with weak bases $LiOH$ and $Mg(OH{)}_2$. On the other hand, NaOH and KOH are much stronger bases and can hold the $C{O}_2$ easily. Thus, sodium carbonate and potassium carbonate are thermally stable. They do not decompose on heating.