Temporary hardness of water is due to the presence of

The $O-H$ bond length in $H_2O$ in the gas phase is: (in $pm$)

Give reason: Hard water is not suitable for washing clothes.

Match the following.

Column-$1$ (Reaction)	Column-$2$ (Main product)
$A$. $SO_3 \xrightarrow{D_2O}$	$I$. $C_2D_2$
$B$. $CaC_2 \xrightarrow{D_2O \text{ (excess)}}$	$II$. $CD_4$
$C$. $Al_4C_3 \xrightarrow{D_2O \text{ (large excess)}}$	$III$. $D_2SO_3$
	$IV$. $C_2D_4$
	$V$. $D_2SO_4$

The correct match is:

The charring of sugar takes place when treated with concentrated $H_2SO_4$. What is the type of reaction involved in it?

When the first element of the periodic table is treated with dioxygen,it gives a compound whose solid state floats on its liquid state. This compound has an ability to act as an acid as well as a base. What products will be formed when this compound undergoes autoionisation?