$\mathrm{ZnO}$ is white when cold and yellow when heated. It is due to the development of
$\mathrm{ZnO}$ is white when cold and yellow when heated. It is due to the development of
$\mathrm{Na}_2 \mathrm{CO}_3+\mathrm{NaOH}$
$\mathrm{NaHCO}_3+\mathrm{Na}_2 \mathrm{CO}_3$
$\mathrm{NaHCO}_3+\mathrm{NaOH}$
$\mathrm{NaOH}+\mathrm{NaCl}$
Solution
$\mathrm{NaHCO}_3$ is a sodium salt of weak acid, while $\mathrm{NaOH}$ is a strong base so, they react and cannot exist together in solution.
$\mathrm{NaHCO}_3+\mathrm{NaOH} \longrightarrow \mathrm{Na}_2 \mathrm{CO}_3+\mathrm{H}_2 \mathrm{O}$