Question

Write the observations for the following:
(i) NaOH is added drop-wise till in excess to a solution of zinc sulphate.
(ii) NH₄OH is added first in a small quantity and then in excess to a solution of copper sulphate.
(iii) Excess NH₄OH is added to a substance obtained by adding hydrochloric acid in silver nitrate solution.
(iv) Moist starch iodide paper is put on the mouth of a test tube containing chlorine gas.
(v) A paper dipped in potassium permanganate solution is put on the mouth of a test tube containing sulphur dioxide gas.
(vi) Decomposition of bicarbonates by dil. H₂SO₄.
2NaHCO₃ + H₂SO₄ → Na₂SO₄ + 2H₂O + 2CO₂
2KHCO₃ + H₂SO₄ → K₂SO₄ + 2H₂O + 2CO₂

Answer 1

(i) When NaOH is added drop-wise till in excess to a solution of zinc sulphate first a white precipitate is formed that later dissolves in excess of sodium hydroxide.

$$\begin{gathered} {\mathrm{ZnSO}}_4+2\mathrm{NaOH}\to \mathrm{Zn}{\left(\mathrm{OH}\right)}_2+{\mathrm{Na}}_2{\mathrm{SO}}_4 \\ \mathrm{White}\mathrm{precipitate} \\ \mathrm{Zn}(\mathrm{OH}{)}_2+2\mathrm{NaOH}\mathit{(}excess\mathit{)}\to {\mathrm{Na}}_2\left[\mathrm{Zn}{\left(\mathrm{OH}\right)}_4\right] \end{gathered}$$

(ii) When NH₄OH is added first in a small quantity to a solution of copper sulphate a blue precipitate is formed and then in excess of ammonium hydroxide it dissolves to give a deep blue coloured solution.

$$\begin{gathered} {\mathrm{CuSO}}_4+2{\mathrm{NH}}_4\mathrm{OH}\to \mathrm{Cu}{\left(\mathrm{OH}\right)}_2+{\left({\mathrm{NH}}_4\right)}_2{\mathrm{SO}}_4 \\ \mathrm{Blue}\mathrm{precipitate} \\ \mathrm{Cu}{\left(\mathrm{OH}\right)}_2+2{\mathrm{NH}}_4\mathrm{OH}\left(excess\right)\to \left[\mathrm{Cu}{\left({\mathrm{NH}}_3\right)}_4\right]{\left(\mathrm{OH}\right)}_2 \\ \mathrm{Deep}\mathrm{blue}\mathrm{solution} \end{gathered}$$

(iii) Adding hydrochloric acid to silver nitrate produces a white precipitate, which dissolves in excess ammonium hydroxide.

$$\begin{gathered} {\mathrm{AgNO}}_3+\mathrm{HCl}\to \mathrm{AgCl}+{\mathrm{HNO}}_3 \\ \mathrm{White}\mathrm{precipitate} \\ \mathrm{AgCl}+2{\mathrm{NH}}_4\mathrm{OH}\mathit{\left(}excess\mathit{\right)}\to \left[\mathrm{Ag}{\left({\mathrm{NH}}_3\right)}_2\right]\mathrm{Cl}+2{\mathrm{H}}_2\mathrm{O} \end{gathered}$$

(iv) Chlorine gas turns moist starch iodide paper blue black.

$$\begin{gathered} {\mathrm{Cl}}_2+2{\mathrm{I}}^{-}\to {\mathrm{I}}_2+2{\mathrm{Cl}}^{-} \\ {\mathrm{I}}_2+\mathrm{Starch}\to \mathrm{Blue}\mathrm{black}\mathrm{coloured}\mathrm{complex} \end{gathered}$$

(v) Sulphur dioxide gas decolourises the pink colour of potassium permangante solution on the paper.

$2{\mathrm{KMnO}}_4+5{\mathrm{SO}}_2+2{\mathrm{H}}_2\mathrm{O}\to 2{\mathrm{MnSO}}_4+{\mathrm{K}}_2{\mathrm{SO}}_4+2{\mathrm{H}}_2{\mathrm{SO}}_4$

(vi) When the given bicarbonates are decomposed by dilute H₂SO₄ a brisk effervescence is observed. The evolved gas does not support combustion and when passed through calcium hydroxide solution turns it milky.

2NaHCO₃ + H₂SO₄ → Na₂SO₄ + 2H₂O + 2CO₂
2KHCO₃ + H₂SO₄ → K₂SO₄ + 2H₂O + 2CO₂
Ca(OH)₂ + CO₂ → CaCO₃ + H₂O
White precipitate

The white precipitate formed in the above reactions dissolves in excess concentrated HCl to form a colourless liquid.