Practical Work — Question 4

(a) Ammonium dichromate — Orange red crystalline solid, on heating, swells up and decomposes violently with flashes of light leaving greenish residue. It also gives off steamy fumes, which condense on the cooler parts of the test tube to form tiny droplets of water.

$\underset{\text{[Ammonium dichromate (orange)]}}{(\text{NH}_4)_2\text{Cr}_2\text{O}_7} \xrightarrow{\space \Delta \space} \underset{\text{ [Chromic oxide(green)]}}{\text{Cr}_2\text{O}_3} + 4\text{H}_2\text{O} + \text{N}_2$

(b) Copper nitrate — Bluish green crystalline solid, on heating, melts to form a bluish green mass and gives off steamy vapours that condense on the cooler parts of test tube.

${\text{Cu(NO}_3)_2}.6\text{H}_2\text{O} \xrightarrow{\space \Delta \space} {\text{Cu(NO}_3)_2} + 6\text{H}_2\text{O}$

On further heating, the bluish green mass changes to a black residue of copper (II) oxide and brown coloured nitrogen dioxide gas is evolved along with oxygen gas.

$\underset{\text{[Copper nitrate (blue)]}}{2\text{Cu(NO}_3)_2} \xrightarrow{\space \Delta \space} \underset{\text{[copper oxide - black]}}{2\text{CuO}} + \underset{\text{[Nitrogen dioxide]}}{4\text{NO}_2\uparrow} + \underset{\text{[Oxygen]}}{\text{O}_2\uparrow}$

(c) Copper carbonate — Light green amorphous powder changes to black. Carbon dioxide gas is given off which turns lime water milky.

$\underset{\text{[Copper carbonate (light green)]}}{\text{CuCO}_3} \overset{\Delta}{\longrightarrow} \underset{\text{[copper oxide - black]}}{\text{CuO}} + \underset{\text{[Carbon dioxide]}}{\text{CO}_2\uparrow}$

(d) Zinc carbonate — On strong heating, light amorphous white solid changes to pale yellow. Carbon dioxide gas is given off which turns lime water milky.

The residue, on cooling, changes to a white colour. i.e., residue is yellow when hot and white when cold.

$\begin{matrix} \text{ZnCO}_3 &\xrightarrow\Delta & \text{ZnO} & + & \text{CO}_2\uparrow \\ \text{[Zinc} & & \text{[Zinc oxide]} & & \text{[Carbon} \\ \text{carbonate]} & & \text{[yellow - hot]} & & \text{dioxide]} \\ \text{[white]} & & \text{[white - cold]} \end{matrix}$

(e) Ammonium chloride — On heating ammonium chloride, white crystalline solid sublimates to form a basic gas (NH₃) and acidic gas (HCl). The dense white fumes are noticed that form a white mass on the cooler parts of the test tube. No residue is left behind.

$\text{NH}_4\text{Cl} \xrightleftharpoons[\text{cool}]{\text{heat}} \text{NH}_3 \uparrow + \text{HCl} \uparrow$