(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.

$$\mathrm{CO}(g)+2 \mathrm{H}_2(g) \xrightarrow[\text { catalyst }]{\text { Cobalt }} \mathrm{CH}_3 \mathrm{OH}(l)$$

(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.

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.,

$$\mathop {{H_2}O(l)}\limits_{Aci{d_1}} + \mathop {N{H_3}(aq)}\limits_{Bas{e_2}} \to \mathop {NH_4^ + (aq)}\limits_{Aci{d_2}} + \mathop {O{H^ - }(aq)}\limits_{Bas{e_1}} $$

$$\mathop {{H_2}O(l)}\limits_{Bas{e_1}} + \mathop {{H_2}S(aq)}\limits_{Aci{d_2}} \to \mathop {{H_3}{O^ + }(aq)}\limits_{Aci{d_1}} + \mathop {H{S^ - }(aq)}\limits_{Bas{e_2}} $$

Water which is free from all soluble minerals salts is called demineralised water. Demineralised water is obtained by passing water successively through a cation exchange and an anion exchange resins.

In cation exchanger, $\mathrm{Ca}^{2+}, \mathrm{Mg}^{2+}, \mathrm{Na}^{+}$and other cations present in water are removed by exchanging them with $\mathrm{H}^{+}$ions while in anion exchanger, $\mathrm{Cl}^{-}, \mathrm{HCO}_3^{-}, \mathrm{SO}_4^{2-}$, etc., present in water are removed by exchanging them with $\mathrm{OH}^{-}$ions.

$$ \underset{\text { (Released in cation exchanger) }}{\mathrm{H}^{+}}+\underset{\text { (Released in anion exchanger) }}{\mathrm{OH}^{-}} \longrightarrow \mathrm{H}_2 \mathrm{O} $$

Synthetic ion exchange resins are of two types. Cation exchange resins contain large organic molecule with $\mathrm{SO}_3 \mathrm{H}$ group and are water soluble. It is changed to $R \mathrm{Na}$ by treating it with NaCl . The resin $R \mathrm{Na}$ exchanges $\mathrm{Mg}^{2+}$ and $\mathrm{Ca}^{2+}$ ions present in hard water to make the water soft.

$$2 R \mathrm{Na}(\mathrm{s})+\mathrm{M}^{2+}(\mathrm{aq}) \longrightarrow R_2 M(\mathrm{~s})+2 \mathrm{Na}^{+}(\mathrm{aq})\left(M=\mathrm{Ca}^{2+} \text { or } \mathrm{Mg}^{2+}\right)$$

The resin can be regenerated by passing NaCl (aqueous solution) in it. Pure demineralised (deionised) water is obtained by passing water successively through a cation exchange and anion exchange resins. In the cation exchange process,

$2 R \mathrm{H}(\mathrm{s})+\mathrm{M}^{2+}(\mathrm{aq}) \rightleftharpoons \underset{\begin{array}{c}\text { (Cation exchange } \\ \text { resin in the } \mathrm{H}^{+} \text {form) }\end{array}}{M R_2(\mathrm{~s})+2 \mathrm{H}^{+}(\mathrm{aq})}$

$\mathrm{H}^{+}$exchanges for $\mathrm{Ca}^{2+}, \mathrm{Mg}^{2+}$ and other cations present in water. This process results in proton release and thus, makes the water acidic. In the anion exchange process

$$R \mathrm{NH}_2(\mathrm{~s})+\mathrm{H}_2 \mathrm{O}(l) \rightleftharpoons R \stackrel{+}{\mathrm{N}} \mathrm{H}_3 \cdot \mathrm{OH}^{-}(\mathrm{s})$$

$R \stackrel{+}{N} \mathrm{H}_3 \cdot \mathrm{OH}^{-}$is substituted ammonium hydroxide anion exchange resin.

$$R \stackrel{+}{N} \mathrm{H}_3 \cdot \mathrm{OH}^{-}(\mathrm{s})+\mathrm{X}^{-}(\mathrm{aq}) \rightleftharpoons R \stackrel{+}{\mathrm{N}} \mathrm{H}_3 \cdot \mathrm{X}^{-}(\mathrm{s})+\mathrm{OH}^{-}(\mathrm{aq})$$

Molecular hydrides are classified according to the relative numbers of electrons and bonds in Lewis structure as follow

(i) Electron deficient hydrides These type of hydrides contain central atom with incomplete octet. These are formed by 13 group elements, e.g., $\mathrm{BH}_3, \mathrm{AlH}_3$, etc. To complele their octet they generally exist in polymeric forms such as $\mathrm{B}_2 \mathrm{H}_6, \mathrm{~B}_4 \mathrm{H}_{10}$, $\left(\mathrm{AlH}_3\right)_n$ etc. These hydrides act as Lewis acids.

(ii) Electron precise hydrides These hydrides have exact number of electrons required to form normal covalent bonds. These are formed by 14 group elements, e.g., $\mathrm{CH}_4, \mathrm{SiH}_4$, etc. These are tetrahedral in shape.

(iii) Electron rich hydrides These hydrides contain central atom with excess electrons, which are present as ione pairs.

These are formed by 15,16 and 17 group elements, e.g., $\mathrm{NH}_3, \mathrm{H}_2 \mathrm{O}, \mathrm{HF}$, etc. These hydrides act as Lewis bases.