Chemical Tests
5 marks · EasyTwo unknown solutions, labelled X and Y, are tested. Solution X gives off a gas when warmed with sodium hydroxide. Solution Y forms a precipitate when tested with silver nitrate and dilute nitric acid.
State the name of the gas given off by X when warmed with sodium hydroxide, and describe how you would test for it (including the result expected).
Solution Y gives a yellow precipitate when tested with silver nitrate and dilute nitric acid. State which ion is present in Y, and the name and colour of the precipitate formed.
Describe how you could distinguish between the precipitates formed with chloride and iodide ions using this test.
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
Show answer
(a) The gas is ammonia. Test using damp red litmus paper or damp universal indicator paper; it will turn blue if ammonia is present. · must show Correct identification of ammonia, Testing with damp red litmus or indicator paper, State the paper turns blue
(b) Y contains iodide ions. The precipitate is silver iodide, which is yellow. · must show Identifies iodide ion, Names silver iodide, States precipitate is yellow
(c) The chloride ion gives a white precipitate (silver chloride), while the iodide ion gives a yellow precipitate (silver iodide). You can distinguish them by their colours. · must show Difference in colour: white for chloride, yellow for iodide
Common mistakes: Confusing ammonia with hydrogen when warming with sodium hydroxide; Mixing up precipitate colours
Show mark scheme and worked solution
Marking points
A1 (a) · 1 mark · Identifies ammonia as the gas given off by X or correct test/observation.
M1 (a) · 1 mark · Describes test: use damp red litmus/universal indicator paper; turns blue if ammonia present.
A2 (b) · 1 mark · States iodide ion present and the precipitate is silver iodide.
A3 (b) · 1 mark · States precipitate is yellow.
A4 (c) · 1 mark · Distinguishes between silver chloride (white) and silver iodide (yellow) precipitates by colour; states both.
Worked solution
a) The gas given off by X when warmed with sodium hydroxide is ammonia. It can be tested by holding a piece of damp red litmus paper or universal indicator paper near the mouth of the test tube; the paper will turn blue if ammonia is present.
b) Y gives a yellow precipitate with silver nitrate and dilute nitric acid. This means it contains iodide ions. The precipitate is silver iodide (AgI), which is yellow.
c) Silver chloride forms a white precipitate, while silver iodide forms a yellow precipitate in this test. You can distinguish them by their colours.