This mobile laboratory is used at a mineral spring open to visitors. Fig. 1 shows a flame-test wire loop being cleaned in hydrochloric acid before it is dip...
Question 1 Report
This mobile laboratory is used at a mineral spring open to visitors. Fig. 1 shows a flame-test wire loop being cleaned in hydrochloric acid before it is dipped into a dried sample.
The flame changes from colourless to an intense yellow. A second sample produces a lilac flame when viewed through cobalt glass.
(a) Name the ion in the first sample. [1]
(b) Name the ion in the second sample. [1]
(c) Give the formula of the first ion. [1]
(d) Suggest why the loop is cleaned before each test. [1]
(e) Use cobalt glass to state why it is used for the second sample. [1]
Fig. 1 shows a riverside monitoring kit. A volunteer tests a sample collected below an abandoned mine. The sample is first acidified, then barium chloride solution is added. A dense white precipitate forms.
(a) Name the ion identified by the white precipitate. [1]
(b) Give the formula of the precipitate. [1]
(c) Suggest why the sample is acidified before barium chloride is added. [1]
(d) Give one reason why sulfate ions may be monitored in river water. [1]
(e) Complete the formula of sulfuric acid: H2______. [1]
Answer Details
Flame tests and sulfate test
- (a) An intense yellow flame is the characteristic flame-test colour for sodium ions, \(\text{Na}^+\). [1]
- (b) A lilac flame seen through cobalt glass identifies potassium ions, \(\text{K}^+\). [1]
- (c) The formula of the first ion is \(\text{Na}^+\). [1]
- (d) The wire loop is cleaned to remove the previous sample and so avoid contamination. Even a trace of sodium could give a strong yellow flame and affect the next result. [1]
- (e) Cobalt glass masks the intense yellow sodium flame, allowing the lilac potassium flame to be seen clearly. [1]
Barium chloride test
- (a) Acidified barium chloride giving a dense white precipitate identifies sulfate ions, \(\text{SO}_4^{2-}\). [1]
- (b) The white precipitate is barium sulfate, \(\text{BaSO}_4\). [1]
- (c) Acidification removes carbonate ions, preventing a white barium carbonate precipitate from giving a false positive result. [1]
- (d) Sulfate ions may be monitored to check for contamination from mine drainage or to assess water quality. [1]
- (e) Sulfuric acid is \(\text{H}_2\text{SO}_4\), so the missing group is \(\text{SO}_4\). [1]
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