TEST OF PRACTICAL KNOWLEDGE QUESTION
Credit will be given for strict adherence to the instructions, for observations precisely recorded and for accurate inferences. All tests, observations, and inferences must be clearly entered in your answer book, in ink, at the i.itne time they are made.
C is a sample of copper (II) tetraoxosulphate (VI) crystals. Carry out the following exercises on C. Record your observations and identify any gases evolved. State the conclusion you draw from the result of each test.
(a) Put half of C in a test tube and heat strongly
(b) Make solution of about 10 cm\(^{-3}\) with the sécond half of C and divide it into three portions
(i) To the first portion, add Sodium hydroxide solution in drops and then in excess. Heat the mixture
(ii) To the second portion, add aqueous ammonia in drops and then in excess followed by few drops of moderately concentrated HCI.
(iii) To the third portion, add all the zinc dust provided and stir thoroughly until there is a visible change.
C is copper(II) tetraoxosulphate(VI) crystals, CuSO4·5H2O. The tests are recorded below in the standard Test / Observation / Inference form.
| Test |
Observation |
Inference |
| (a) Half of C is put in a dry test tube and heated strongly. |
The blue crystals gradually turn white. Colourless droplets of liquid condense on the cooler upper walls of the tube. On continued strong heating the white solid turns black, and a colourless gas with a sharp, irritating (choking) smell is evolved which turns moist blue litmus paper red. |
C is a hydrated salt: the liquid is the water of crystallisation (blue → white anhydrous CuSO4). The acidic, irritating gas is sulphur(IV) oxide, SO2, and the black residue is copper(II) oxide, CuO, indicating a tetraoxosulphate(VI) (sulphate) salt of copper. |
| (b) The second half of C is shaken with about 10 cm3 of water. |
C dissolves readily to give a clear blue solution. |
C is a soluble salt; the blue colour of the solution indicates the hydrated Cu2+ ion is present. |
| (b)(i) To the first portion, sodium hydroxide solution is added in drops, then in excess, and the mixture is heated. |
A pale (light) blue gelatinous precipitate forms; it is insoluble in excess sodium hydroxide. On heating, the pale blue precipitate turns black. |
The pale blue precipitate insoluble in excess NaOH is Cu(OH)2, confirming Cu2+ is present. On heating it dehydrates to black CuO. |
| (b)(ii) To the second portion, aqueous ammonia is added in drops, then in excess, followed by a few drops of moderately concentrated HCl. |
With ammonia in drops, a pale blue precipitate forms. In excess ammonia the precipitate dissolves to give a clear deep (royal) blue solution. When the HCl is added, the deep blue colour is discharged, leaving a pale blue–green solution. |
Cu2+ is present. The pale blue precipitate is Cu(OH)2, which dissolves in excess ammonia to form the deep blue tetraamminecopper(II) complex, [Cu(NH3)4]2+. The HCl neutralises the ammonia and destroys the complex, reforming the pale copper(II) chloride solution. |
| (b)(iii) To the third portion, all the zinc dust is added and stirred thoroughly until a visible change occurs. |
Effervescence is not the main feature; the blue colour of the solution fades until the solution becomes colourless, and a reddish-brown (pink) solid deposits on the zinc. |
A displacement (redox) reaction occurs: zinc, being higher than copper in the reactivity series, displaces copper from solution. Cu2+ is reduced to reddish-brown copper metal while zinc goes into solution, confirming Cu2+ in C. |
Supporting equations
Loss of water of crystallisation and decomposition on strong heating:
\[ \text{CuSO}_4\cdot 5\text{H}_2\text{O}_{(s)} \xrightarrow{\Delta} \text{CuSO}_{4(s)} + 5\text{H}_2\text{O}_{(g)} \]
\[ \text{CuSO}_{4(s)} \xrightarrow{\Delta} \text{CuO}_{(s)} + \text{SO}_{3(g)} \quad;\quad 2\text{SO}_{3} \rightarrow 2\text{SO}_2 + \text{O}_2 \]
With sodium hydroxide:
\[ \text{Cu}^{2+}_{(aq)} + 2\text{OH}^{-}_{(aq)} \rightarrow \text{Cu(OH)}_{2(s)}\ (\text{pale blue}) \]
\[ \text{Cu(OH)}_{2(s)} \xrightarrow{\Delta} \text{CuO}_{(s)}\ (\text{black}) + \text{H}_2\text{O}_{(l)} \]
With aqueous ammonia (excess) and then HCl:
\[ \text{Cu(OH)}_{2(s)} + 4\text{NH}_{3(aq)} \rightarrow [\text{Cu(NH}_3)_4]^{2+}_{(aq)}\ (\text{deep blue}) + 2\text{OH}^{-}_{(aq)} \]
With zinc dust:
\[ \text{Zn}_{(s)} + \text{Cu}^{2+}_{(aq)} \rightarrow \text{Zn}^{2+}_{(aq)} + \text{Cu}_{(s)}\ (\text{reddish-brown}) \]
Overall conclusion: C is a hydrated, soluble copper(II) salt containing water of crystallisation; the cation present is Cu2+ and the anion is the tetraoxosulphate(VI) (sulphate, SO42−) ion, i.e. C is CuSO4·5H2O.
C is copper(II) tetraoxosulphate(VI) crystals, CuSO4·5H2O. The tests are recorded below in the standard Test / Observation / Inference form.
| Test |
Observation |
Inference |
| (a) Half of C is put in a dry test tube and heated strongly. |
The blue crystals gradually turn white. Colourless droplets of liquid condense on the cooler upper walls of the tube. On continued strong heating the white solid turns black, and a colourless gas with a sharp, irritating (choking) smell is evolved which turns moist blue litmus paper red. |
C is a hydrated salt: the liquid is the water of crystallisation (blue → white anhydrous CuSO4). The acidic, irritating gas is sulphur(IV) oxide, SO2, and the black residue is copper(II) oxide, CuO, indicating a tetraoxosulphate(VI) (sulphate) salt of copper. |
| (b) The second half of C is shaken with about 10 cm3 of water. |
C dissolves readily to give a clear blue solution. |
C is a soluble salt; the blue colour of the solution indicates the hydrated Cu2+ ion is present. |
| (b)(i) To the first portion, sodium hydroxide solution is added in drops, then in excess, and the mixture is heated. |
A pale (light) blue gelatinous precipitate forms; it is insoluble in excess sodium hydroxide. On heating, the pale blue precipitate turns black. |
The pale blue precipitate insoluble in excess NaOH is Cu(OH)2, confirming Cu2+ is present. On heating it dehydrates to black CuO. |
| (b)(ii) To the second portion, aqueous ammonia is added in drops, then in excess, followed by a few drops of moderately concentrated HCl. |
With ammonia in drops, a pale blue precipitate forms. In excess ammonia the precipitate dissolves to give a clear deep (royal) blue solution. When the HCl is added, the deep blue colour is discharged, leaving a pale blue–green solution. |
Cu2+ is present. The pale blue precipitate is Cu(OH)2, which dissolves in excess ammonia to form the deep blue tetraamminecopper(II) complex, [Cu(NH3)4]2+. The HCl neutralises the ammonia and destroys the complex, reforming the pale copper(II) chloride solution. |
| (b)(iii) To the third portion, all the zinc dust is added and stirred thoroughly until a visible change occurs. |
Effervescence is not the main feature; the blue colour of the solution fades until the solution becomes colourless, and a reddish-brown (pink) solid deposits on the zinc. |
A displacement (redox) reaction occurs: zinc, being higher than copper in the reactivity series, displaces copper from solution. Cu2+ is reduced to reddish-brown copper metal while zinc goes into solution, confirming Cu2+ in C. |
Supporting equations
Loss of water of crystallisation and decomposition on strong heating:
\[ \text{CuSO}_4\cdot 5\text{H}_2\text{O}_{(s)} \xrightarrow{\Delta} \text{CuSO}_{4(s)} + 5\text{H}_2\text{O}_{(g)} \]
\[ \text{CuSO}_{4(s)} \xrightarrow{\Delta} \text{CuO}_{(s)} + \text{SO}_{3(g)} \quad;\quad 2\text{SO}_{3} \rightarrow 2\text{SO}_2 + \text{O}_2 \]
With sodium hydroxide:
\[ \text{Cu}^{2+}_{(aq)} + 2\text{OH}^{-}_{(aq)} \rightarrow \text{Cu(OH)}_{2(s)}\ (\text{pale blue}) \]
\[ \text{Cu(OH)}_{2(s)} \xrightarrow{\Delta} \text{CuO}_{(s)}\ (\text{black}) + \text{H}_2\text{O}_{(l)} \]
With aqueous ammonia (excess) and then HCl:
\[ \text{Cu(OH)}_{2(s)} + 4\text{NH}_{3(aq)} \rightarrow [\text{Cu(NH}_3)_4]^{2+}_{(aq)}\ (\text{deep blue}) + 2\text{OH}^{-}_{(aq)} \]
With zinc dust:
\[ \text{Zn}_{(s)} + \text{Cu}^{2+}_{(aq)} \rightarrow \text{Zn}^{2+}_{(aq)} + \text{Cu}_{(s)}\ (\text{reddish-brown}) \]
Overall conclusion: C is a hydrated, soluble copper(II) salt containing water of crystallisation; the cation present is Cu2+ and the anion is the tetraoxosulphate(VI) (sulphate, SO42−) ion, i.e. C is CuSO4·5H2O.
