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 time they are made.
C is a mixture of two salts. Carry out the following exercises on C. Record your observations and identify any gas(es) evolved. State the conclusion you draw from the result of each test.
(a) Put all of C in a test tube and add about 10 cm\(^3\) of distilled water. Shake thoroughly and filter. Keep both the filtrate and the residue. Divide the filtrate into three portions.
(i) To the first portion, add NaOH\(_{(ag)}\) in drops and then in excess.
(ii) To the second portion, add NH\(_3\), solution in drops and then in excess.
(iii) To the third portion, add BaCl\(_{2(aq)}\) followed by dilute HCI.
(b) Divide the residue into two portions.
(i) Heat the first portion strongly in a test tube.
(ii) Add dilute HCI to the second portion.
Tests, observations and inferences
| Test | Observation | Inference |
|---|
| C was shaken with distilled water and filtered. | A blue filtrate and a green residue were obtained. | C contains a soluble salt and an insoluble salt. |
| (a)(i) To the first portion of the filtrate, NaOH(aq) was added dropwise and then in excess. | A blue gelatinous precipitate formed. It was insoluble in excess NaOH(aq). | Cu2+ ions are present. |
| (a)(ii) To the second portion of the filtrate, NH3(aq) was added dropwise and then in excess. | A pale-blue gelatinous precipitate formed. It dissolved in excess ammonia solution to give a deep-blue solution. | Cu2+ ions are confirmed. |
| (a)(iii) To the third portion of the filtrate, BaCl2(aq) was added, followed by dilute HCl. | A white precipitate formed and remained insoluble in dilute HCl. | SO42− ions are present. |
| (b)(i) The first portion of the residue was heated strongly. | The green solid changed to a black powder. A colourless gas was evolved; it turned limewater milky. | The residue contains a trioxocarbonate(IV). The gas is carbon(IV) oxide, CO2. |
| (b)(ii) Dilute HCl was added to the second portion of the residue. | Effervescence occurred. A colourless, odourless gas was evolved and turned limewater milky. | The gas is carbon(IV) oxide, CO2; hence CO32− is present. |
Conclusion
The soluble salt is copper(II) tetraoxosulphate(VI), CuSO4, while the green insoluble salt is copper(II) trioxocarbonate(IV), CuCO3. Therefore, C is a mixture of CuSO4 and CuCO3.
The relevant reactions are:
\[\mathrm{CuCO_3(s) \xrightarrow{heat} CuO(s) + CO_2(g)}\]
\[\mathrm{CuCO_3(s) + 2HCl(aq) \rightarrow CuCl_2(aq) + H_2O(l) + CO_2(g)}\]
Tests, observations and inferences
| Test | Observation | Inference |
|---|
| C was shaken with distilled water and filtered. | A blue filtrate and a green residue were obtained. | C contains a soluble salt and an insoluble salt. |
| (a)(i) To the first portion of the filtrate, NaOH(aq) was added dropwise and then in excess. | A blue gelatinous precipitate formed. It was insoluble in excess NaOH(aq). | Cu2+ ions are present. |
| (a)(ii) To the second portion of the filtrate, NH3(aq) was added dropwise and then in excess. | A pale-blue gelatinous precipitate formed. It dissolved in excess ammonia solution to give a deep-blue solution. | Cu2+ ions are confirmed. |
| (a)(iii) To the third portion of the filtrate, BaCl2(aq) was added, followed by dilute HCl. | A white precipitate formed and remained insoluble in dilute HCl. | SO42− ions are present. |
| (b)(i) The first portion of the residue was heated strongly. | The green solid changed to a black powder. A colourless gas was evolved; it turned limewater milky. | The residue contains a trioxocarbonate(IV). The gas is carbon(IV) oxide, CO2. |
| (b)(ii) Dilute HCl was added to the second portion of the residue. | Effervescence occurred. A colourless, odourless gas was evolved and turned limewater milky. | The gas is carbon(IV) oxide, CO2; hence CO32− is present. |
Conclusion
The soluble salt is copper(II) tetraoxosulphate(VI), CuSO4, while the green insoluble salt is copper(II) trioxocarbonate(IV), CuCO3. Therefore, C is a mixture of CuSO4 and CuCO3.
The relevant reactions are:
\[\mathrm{CuCO_3(s) \xrightarrow{heat} CuO(s) + CO_2(g)}\]
\[\mathrm{CuCO_3(s) + 2HCl(aq) \rightarrow CuCl_2(aq) + H_2O(l) + CO_2(g)}\]
