What is observed when hot concentrated caustic soda solution is added to

(a) Zinc

(b) Aluminium?

Write balanced equations.

(a) When hot concentrated caustic soda solution is added to zinc, soluble salt of sodium zincate [Na₂ZnO₂] is formed and hydrogen gas is liberated.

The balanced equation is:

$\text{Zn} + \underset{\text{hot and conc.}}{2\text{NaOH}} \longrightarrow \underset{\underset{\text{colourless}}{\text{sodium zincate}}}{\text{Na}$2\text{ZnO}2} + \text{H}_2↑

(b) When hot concentrated caustic soda solution is added to aluminium, soluble salt of sodium meta aluminate [NaAlO₂] is formed and hydrogen gas is liberated.

$2\text{Al} + 2\text{NaOH} + 2\text{H}$2\text{O} \longrightarrow \underset{\underset{\text{colourless}}{\text{sodium meta aluminate}}}{2\text{NaAlO}2} + 3\text{H}_2↑