Question

Sodium hydroxide solution is added first in small quantity, then in excess to the aqueous salt solutions of copper (II) sulphate, zinc nitrate, lead nitrate, calcium chloride and iron (III) sulphate. Copy the following table and write the colour of the precipitate in (i) to (v) and the nature of the precipitate (soluble or insoluble) in (vi) to $\left(x\right)$.

Aqueous salt solution	Colour of precipitate When NaOH is added in a small quantity	Nature of precipitate (soluble or insoluble) when NaOH is added in excess
Copper (II) sulphate	(i)	(vi)
Zinc nitrate	(ii)	(vii)
Lead nitrate	(iii)	(vii)
Calcium chloride	(iv)	(ix)
Iron (III) sulphate	(v)	(x)

Answer 1

Copper (II) sulphate: When NaOH is added to it, an insoluble and pale blue precipitate, i.e. Copper (II) hydroxide is formed.

$$\begin{gathered} 2\mathrm{NaOH}\left(\mathrm{aq}\right)+{\mathrm{CuSO}}_4\left(\mathrm{s}\right)\to {\mathrm{Na}}_2{\mathrm{SO}}_4\left(\mathrm{s}\right)+\mathrm{Cu}(\mathrm{OH}{)}_2\left(\mathrm{s}\right) \\ \mathrm{Pale}\mathrm{blue} \\ \mathrm{precipitate} \end{gathered}$$

Zinc nitrate: When NaOH is added to it, a soluble and white precipitate, i.e. Zinc hydroxide is formed.

$$\begin{gathered} \mathrm{Zn}{\left({\mathrm{NO}}_3\right)}_2\left(\mathrm{s}\right)+2\mathrm{NaOH}\left(\mathrm{aq}\right)\rightleftharpoons \mathrm{Zn}{\left(\mathrm{OH}\right)}_2\left(\mathrm{s}\right)+2{\mathrm{NaNO}}_3\left(\mathrm{s}\right) \\ \mathrm{White}\mathrm{precipitate} \end{gathered}$$

Lead nitrate: When NaOH is added to it, a soluble and curdy white precipitate, i.e. Lead hydroxide is formed.

$$\begin{gathered} \mathrm{Pb}{\left({\mathrm{NO}}_3\right)}_2\left(aq\right)+2\mathrm{NaOH}\left(aq\right)⟶2{\mathrm{NaNO}}_3\left(aq\right)+\mathrm{Pb}\left(\mathrm{OH}{)}_2\right(s) \\ \mathrm{Curdy}\mathrm{white} \\ \mathrm{precipitate} \end{gathered}$$

Calcium chloride: When NaOH is added to it, an insoluble and white precipitate, i.e. Calcium hydroxide is formed.

$$\begin{gathered} {\mathrm{CaCl}}_2\left(\mathrm{aq}\right)+2\mathrm{NaOH}\left(\mathrm{aq}\right)⟶\mathrm{Ca}(\mathrm{OH}{)}_2\left(\mathrm{s}\right)+2\mathrm{NaCl}(\mathrm{aq}) \\ \mathrm{White} \\ \mathrm{precipitate} \end{gathered}$$

Iron (III) sulphate: When NaOH is added to it, an insoluble and reddish-brown precipitate, i.e. Iron (III) hydroxide is formed.

$$\begin{gathered} {\mathrm{Fe}}_2{\left({\mathrm{SO}}_4\right)}_3\left(aq\right)+6\mathrm{NaOH}\left(aq\right)\to 2\mathrm{Fe}\left(\mathrm{OH}{)}_3\right(\mathrm{s})+3{\mathrm{Na}}_2{\mathrm{SO}}_4(aq) \\ \mathrm{Reddish}-\mathrm{brown} \\ \mathrm{precipitate} \end{gathered}$$

Aqueous salt solution	Colour of precipitate When NaOH is added in a small quantity	Nature of precipitate (soluble or insoluble) when NaOH is added in excess
Copper (II) sulphate	(i) Pale blue	(vi) Insoluble
Zinc nitrate	(ii) White	(vii) Soluble
Lead nitrate	(iii) Curdy white	(vii) Soluble
Calcium chloride	(iv) White	(ix) Insoluble
Iron (III) sulphate	(v) Reddish-brown	(x) Insoluble