Give reasons: $A$ mixture of dilute $NaOH$ and aluminium pieces is used to open blocked drains.

(N/A) Sodium hydroxide and aluminium react to form sodium aluminate $(NaAlO_2)$ and hydrogen gas. The rapid evolution of hydrogen gas creates high pressure,which helps to clear the blockage in the drains.
The chemical reaction is:
$2 Al_{(s)} + 2 NaOH_{(aq)} + 2 H_2O_{(l)} \rightarrow 2 NaAlO_{2(aq)} + 3 H_{2(g)}$