(i) Lead in +2 oxidation state as $\mathrm{Pb}^{2+}$ reacts with hydrogen sulphide gas $\left(\mathrm{H}_2 \mathrm{~S}\right)$ to form black coloured precipitate of lead sulphide $(\mathrm{PbS})$.

$\begin{aligned} & \mathrm{Pb}^{2+}(a q)+\mathrm{H}_2 \mathrm{~S}(g) \rightarrow \mathrm{PbS}(s) \downarrow\text { (Black } \text { ppt) }+2 \mathrm{H}^{+}(a q) \\\\ & \mathrm{PbS} \text { is black coloured precipitate. } \\ & \end{aligned}$

(ii) Silver in +1 oxidation state as $\mathrm{Ag}^{+}$reacts with hydrogen sulphide in neutral or acidic medium to form black coloured precipitate of silver sulphide $\left(\mathrm{Ag}_2 \mathrm{~S}\right)$.

$$ 2 \mathrm{Ag}^{+}(a q)+\mathrm{H}_2 \mathrm{~S}(g) \xrightarrow[\text { or acidic medium }]{\text { Neutral }} \mathrm{Ag}_2 \mathrm{~S}(s) $$(Black ppt)

$\mathrm{Ag}_2 \mathrm{~S}$ is black coloured precipitate.

(iii) Mercury in +2 oxidation state as $\mathrm{Hg}^{2+}$ reacts with hydrogen sulphide in dilute hydrochloric acid to form black coloured precipitate of mercury sulphide (HgS).

$$ \mathrm{Hg}^{2+}(a q)+\underset{\begin{array}{c} \text { (saturated aq. } \\ \text { solution or gas) } \end{array}}{\mathrm{H}_2 \mathrm{~S}} \xrightarrow{\text { dil. } \mathrm{HCl}} \underset{\begin{array}{c} \text { Black } \\ \text { ppt } \end{array}}{\mathrm{HgS}(s)} $$

HgS is black coloured precipitate.

(iv) Copper in +2 oxidation state as $\mathrm{Cu}^{2+}$ reacts with dilute hydrochloric acid to form black coloured precipitate of copper sulphide (CuS).

$$ \begin{aligned} & \mathrm{Cu}^{2+}(a q)+\mathrm{H}_2 \mathrm{~S}(\mathrm{~g}) \text { (saturated aq. solution) } \xrightarrow[\text { HCl }]{\text { dil }} \mathrm{CuS}(\mathrm{s}) \text { (black ppt) } \\ & \end{aligned} $$

CuS is black coloured precipitate.

(v) Cobalt in +2 oxidation state as $\mathrm{Co}^{2+}$ reacts with hydrogen sulphide in neutral or alkaline solution to form black coloured precipitate of cobalt sulphide.

$\mathrm{Co}^{2+}(a q) \text { (ammonical hydrogen sulphide) }+\mathrm{H}_2 \mathrm{~S} \longrightarrow \mathrm{CoS}(\mathrm{s})\text { (Black ppt.) }$

$(\mathrm{CoS})$ is black coloured precipitate.

(vi) Nickel in +2 oxidation state as $\mathrm{Ni}^{2+}$ reacts with hydrogen sulphide in neutral or slightly alkaline solution to form, black coloured precipitate of nickel sulphide.

$$ \mathrm{Ni}^{2+}(a q)+\mathrm{H}_2 \mathrm{~S} \longrightarrow \underset{\substack{\text { Black } \\ \text { ppt. }}}{\mathrm{NiS}(\mathrm{s})} $$

NiS is black coloured precipitate.

(vii) Bismuth in +3 oxidation state as $\mathrm{Bi}^{3+}$ reacts with hydrogen sulphide in cold dilute hydrochloric acid to form crystalline dark brown coloured precipitate, but appears black coloured solid of $\mathrm{Bi}_2 \mathrm{~S}_3$.

$\mathrm{Bi}^{3+}(a q) \text { (Saturated solution) }+\mathrm{H}_2 \mathrm{~S} \longrightarrow \mathrm{Bi}_2 \mathrm{~S}_3(\mathrm{~s})\text { (Dark brown or Black precipitate brownish) }$

$\mathrm{Bi}_2 \mathrm{~S}_3$ may appear black in colours.

Either compounds of sulphur are black in colour.

(viii) In mildly acidic, medium tin in +2 state, i.e., $\mathrm{Sn}^{2+}$ reacts with Hydrogen sulphide $\left(\mathrm{H}_2 \mathrm{~S}\right)$ to form brown coloured tin (II) sulphide which further reacts with excess of hydrogen sulphide to form light yellow coloured tin (IV) sulphide $\left(\mathrm{SnS}_2\right)$.

$\begin{aligned} \mathrm{Sn}^{2+}(a q)+\mathrm{H}_2 \mathrm{~S} \rightleftharpoons \underset{\text { Brown }}{\mathrm{SnS}(s)} \\ \mathrm{SnS}(\mathrm{s})+\mathrm{H}_2 \mathrm{~S} \rightleftharpoons \underset{\text { Yellow }}{\mathrm{SnS}_2(\mathrm{~s})}+2 \mathrm{H}^{+}\end{aligned}$

Hence, tin (IV) sulphide or $\mathrm{SnS}_2$ is yellow in colour.

(ix) MnS is known to be dirty pink coloured.