WAEC SSCE - Chemistry - 2003 (Practical)
Question 1 Report
Credit will be given for strict adherence to instructions, for observations precisely recorded, and for accurate interference. All tests, Observations and inferences must be clearly entered in your answer book, in ink at the time they are made. C and D are aqueous solutions of simple salt and an inorganic compound respectively. Carry out the following exercises on C and D. Record your observations and identify the gasses evolved. State the conclusion and draw from the result in the test and add sodium hydroxide in excess.
(b)(i) Test D with litmus paper
(ii) Put about 5cm\(^3\) of D in a test tube and add sodium hydroxide solution in drops and then in excess
(iii) Add about 1cm\(^3\) of the soap solution provided to about 10 cm\(^3\) of D mixture. Repeat the test using distilled water in place D
Answer Details
Tests on C and D
Labelled sketch of the tests on D
| Test | Observation | Inference |
|---|---|---|
| (a)(i) Add aqueous sodium hydroxide to C and warm gently. | A colourless pungent gas is evolved. It turns damp red litmus paper blue and produces dense white fumes with a glass rod dipped in concentrated hydrochloric acid. | The gas is ammonia, \(\mathrm{NH_3}\). Ammonium ion, \(\mathrm{NH_4^+}\), is present in C. |
| (a)(ii) Add barium chloride solution to C, followed by excess dilute hydrochloric acid. | A white precipitate forms and remains insoluble in excess dilute hydrochloric acid. | Tetraoxosulphate(VI) ion, \(\mathrm{SO_4^{2-}}\), is present in C. |
| (b)(i) Test D with red and blue litmus papers. | Red litmus paper turns blue; blue litmus paper is unchanged. | D is alkaline. |
| (b)(ii) Add aqueous sodium hydroxide to D dropwise, then add it in excess. | A white precipitate is formed. The precipitate does not dissolve in excess sodium hydroxide solution. | \(\mathrm{Ca^{2+}}\) ions are present in D. The result is also consistent with \(\mathrm{Mg^{2+}}\), but the alkaline nature of D supports calcium hydroxide solution. |
| (b)(iii) Shake \(1\,\mathrm{cm^3}\) of soap solution with \(10\,\mathrm{cm^3}\) of D. Repeat using \(10\,\mathrm{cm^3}\) of distilled water. | With D, little or no lather is formed and a white scum appears. With distilled water, lather forms readily. | D is hard water. Its hardness is due to calcium ions. |
Equations:
\[\mathrm{NH_4^+(aq)+OH^-(aq)\xrightarrow{heat}NH_3(g)+H_2O(l)}\]
\[\mathrm{Ba^{2+}(aq)+SO_4^{2-}(aq)\rightarrow BaSO_4(s)}\]
Conclusion: C is ammonium tetraoxosulphate(VI), \(\mathrm{(NH_4)_2SO_4}\). D is an alkaline hard-water sample containing \(\mathrm{Ca^{2+}}\) ions, consistent with calcium hydroxide solution, \(\mathrm{Ca(OH)_2}\).
