Fig. 1 shows a forensic scientist comparing a clear liquid from a damaged battery with a sodium chloride reference solution. Nitric acid and silver nitrate ...
Question 1 Report
Fig. 1 shows a forensic scientist comparing a clear liquid from a damaged battery with a sodium chloride reference solution. Nitric acid and silver nitrate solution are used for both samples.
The battery liquid gives a cream precipitate. The reference gives a white precipitate. The cream precipitate dissolves only slightly in dilute ammonia solution.
(a) Name the halide ion in the battery liquid. [1]
(b) Give the formula of the cream precipitate. [1]
(c) Name the halide ion in the reference solution. [1]
(d) Suggest why nitric acid is used before silver nitrate solution. [1]
(e) Use the ammonia result to support the identification of the cream precipitate. [1]
Table 1 shows tests made on a white powder spilled from a delivery sack at a swimming-pool store. Fig. 1 shows the gas from the acid test being bubbled through limewater.
| test | observation |
|---|---|
| add dilute acid | fizzing |
| pass gas into limewater | limewater turns milky |
| add silver nitrate after nitric acid | no precipitate |
(a) Name the gas made in the first test. [1]
(b) Name the ion identified in the powder. [1]
(c) Complete the word equation: carbonate + acid → salt + water + ______. [1]
(d) Suggest why the silver nitrate result is useful. [1]
(e) Give the formula of the carbonate ion. [1]
Answer Details
Halide-ion tests
- (a) A cream precipitate with acidified silver nitrate identifies bromide ions, \(\text{Br}^-\). [1]
- (b) The cream precipitate is silver bromide, \(\text{AgBr}\). [1]
- (c) The white precipitate from the sodium chloride reference identifies chloride ions, \(\text{Cl}^-\). [1]
- (d) Nitric acid removes carbonate ions, preventing formation of a silver carbonate precipitate that could be mistaken for a halide result. [1]
- (e) Silver bromide is only slightly soluble in dilute ammonia. This agrees with the observation that the cream precipitate dissolves only slightly. [1]
Carbonate-ion tests
- (a) Fizzing with acid produces carbon dioxide. [1]
- (b) Carbon dioxide turning limewater milky identifies carbonate ions. [1]
- (c) carbonate + acid \(\rightarrow\) salt + water + carbon dioxide. [1]
- (d) No precipitate with acidified silver nitrate rules out chloride, bromide and iodide ions. This makes a halide impurity unlikely. [1]
- (e) The carbonate ion is \(\text{CO}_3^{2-}\). [1]
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