A solution is tested for nitrate. The solution is aqueous and colourless. Aluminium foil is used to bring about the change. Half fill a boiling tube with th...
Question 1 Report
A solution is tested for nitrate. The solution is aqueous and colourless. Aluminium foil is used to bring about the change.
Half fill a boiling tube with the solution. Make it alkaline using aqueous sodium hydroxide. Drop in a scrap of aluminium foil. Warm the boiling tube very gently. Hold damp red litmus just inside the mouth. Record what happens.
| stage | what happened |
|---|---|
| foil added and the mixture warmed | |
| damp red litmus at the mouth |
(a) Record both stages in Table 38.1. [3]
(b) Name the gas formed. [1]
(c) State the part played by the aluminium foil. [2]
Answer Details
This is the standard confirmatory test for the nitrate ion, in which aluminium in warm alkali reduces nitrate to ammonia. It tests careful observation and understanding of the reducing role of the metal.
(a) [3] The two stages to record in Table 38.1:
| stage | what happened |
|---|---|
| foil added and the mixture warmed | bubbles / effervescence as the foil reacts [1]; a gas is given off on warming [1] |
| damp red litmus at the mouth | the damp red litmus turns blue [1] |
Red litmus turning blue shows the escaping gas is alkaline.
(b) [1] The gas formed is ammonia [1]. It is the only common alkaline gas, which is why damp red litmus turning blue identifies it.
(c) [2] The part played by the aluminium foil: the aluminium reduces the nitrate ion [1], converting it into ammonia, which the litmus then detects [1]. The metal supplies the electrons that change nitrate into ammonia in the warm alkaline mixture.
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