Which of the following,on reaction with water,will have a $pH$ less than $7$?

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

Carborundum is obtained when silica is heated at high temperature with

The bleaching action of bleaching powder is due to the liberation of

The compound of copper which turns green on keeping in air is

Which of the following statements are correct?
$1.$ Charcoal can be obtained by heating wood in the absence of air.
$2.$ Charcoal is an impure form of graphite.
$3.$ Diamond is thermodynamically the most stable allotrope of carbon.
$4.$ Coke is used as an oxidising agent during the extraction of iron from its ores.