Name the classes of hydrides to which $\mathrm{H}_2 \mathrm{O}, \mathrm{B}_2 \mathrm{H}_6$ and NaH belong.
$\mathrm{H}_2 \mathrm{O}-$ Covalent or molecular hydride (electron rich hydride).
$\mathrm{B}_2 \mathrm{H}_6-$ Covalent or molecular hydride (electron deficient hydride).
NaH - lonic or saline hydride.
If same mass of liquid water and a piece of ice is taken, then why is the density of ice less than that of liquid water?
In ice, molecules of $\mathrm{H}_2 \mathrm{O}$ are not packed so closely as in liquid water. There exists vacant spaces in the crystal lattice. This results in larger volume and lower density (density = mass/volume).
In other words, density of ice is lower than liquid water and hence ice floats on water.
Complete the following equations
(i) $\mathrm{PbS}(s)+\mathrm{H}_2 \mathrm{O}_2(a q) \longrightarrow$
(ii) $\mathrm{CO}(\mathrm{g})+2 \mathrm{H}_2(\mathrm{g}) \xrightarrow[\text { catalyst }]{\text { Cobalt }}$
(i) When PbS react with hydrogen peroxide, then $\mathrm{PbSO}_4$ and water are formed.
$$\mathrm{PbS}(\mathrm{s})+4 \mathrm{H}_2 \mathrm{O}_2(a q) \longrightarrow \mathrm{PbSO}_4+4 \mathrm{H}_2 \mathrm{O}$$
(ii) When carbon mono-oxide reacts with hydrogen in the presence of cobalt catalyst, then methanol is formed.
$$\mathrm{CO}(g)+2 \mathrm{H}_2(g) \xrightarrow[\text { catalyst }]{\text { Cobalt }} \mathrm{CH}_3 \mathrm{OH}(l)$$
Give reasons
(i) Lakes freeze from top towards bottom.
(ii) Ice floats on water.
(i) Density of ice is less than that of liquid water. During severe winter, the temperature of lake water keeps on decreasing. Since, cold water is heavier, therefore, it moves towards bottom of the lake and warm water from the bottom moves towards surface. This process continues. The density of water is maximum at 277 K .
Therefore, any further decrease in temperature of the surface water will decrease in density. The temperature of surface water keeps on decreasing and ultimately it freezes. Thus, the ice layer at lower temperature floats over the water below it. Due to this, freezing of water into ice takes place continuously from top towards bottom.
(ii) Density of ice is less than that of liquid water, so it floats over water.
What do you understand by the term 'auto-protolysis' of water? What is its significance?
Auto-protolysis means self ionisation of water.
$\underset{\text { Acid }_1}{\mathrm{H}_2 \mathrm{O}(l)}+\underset{\mathrm{Base}_2}{\mathrm{H}_2 \mathrm{O}(l)} \rightleftharpoons \underset{\mathrm{Acid}_2}{\mathrm{H}_3 \mathrm{O}^{+}(a q)}+\underset{\mathrm{Base}_1}{\mathrm{OH}^{-}(\mathrm{aq})}$
Due to auto-protolysis, water is amphoteric in nature. It reacts with both acids and bases.
e.g.,
$$\mathop {{H_2}O(l)}\limits_{Aci{d_1}} + \mathop {N{H_3}(aq)}\limits_{Bas{e_2}} \to \mathop {NH_4^ + (aq)}\limits_{Aci{d_2}} + \mathop {O{H^ - }(aq)}\limits_{Bas{e_1}} $$
$$\mathop {{H_2}O(l)}\limits_{Bas{e_1}} + \mathop {{H_2}S(aq)}\limits_{Aci{d_2}} \to \mathop {{H_3}{O^ + }(aq)}\limits_{Aci{d_1}} + \mathop {H{S^ - }(aq)}\limits_{Bas{e_2}} $$