Answer

Sulphur mainly exists in combined form in the earth’s crust primarily as sulphates [gypsum ( $\left.\mathrm{CaSO_4} \cdot 2 \mathrm{H_2} \mathrm{O}\right)$, Epsom salt $\left(\mathrm{MgSO_4} \cdot 7 \mathrm{H_2} \mathrm{O}\right)$, baryte $\left(\mathrm{BaSO_4}\right)$ ] and sulphides [(galena (PbS), zinc blends ( $\left.\mathrm{ZnS}\right)$, copper pyrites $\left(\mathrm{CuFeS_2}\right)$ ].

Answer

The thermal stability of hydrides decreases on moving down the group. This is due to a decrease in the bond dissociation enthalpy ( $\mathrm{H}-\mathrm{E}$ ) of hydrides on moving down the group.

Therefore,

$ \begin{array}{l} \mathrm{H_2} \mathrm{O} \\ \mathrm{H_2} \mathrm{S} \\ \mathrm{H_2} \mathrm{Se} \\ \mathrm{H_2} \mathrm{Te}^{2} \\ \mathrm{H_2} \mathrm{Po} \\ \end{array} \Bigg \downarrow \text{Thermal stability decreases} $

Answer

$\mathrm{H_2} \mathrm{O}$ has oxygen as the central atom. Oxygen has smaller size and higher electronegativity as compared to sulphur. Therefore, there is extensive hydrogen bonding in $\mathrm{H_2} \mathrm{O}$, which is absent in $\mathrm{H_2} \mathrm{~S}$. Molecules of $\mathrm{H_2} \mathrm{~S}$ are held together only by weak van der Waal’s forces of attraction.

Hence, $\mathrm{H_2} \mathrm{O}$ exists as a liquid while $\mathrm{H_2} \mathrm{~S}$ as a gas.