Four solids, labelled E, F, G and H, are provided in small bottles, together with a nichrome wire, a watch glass of dilute hydrochloric acid and a Bunsen bu...
Question 1 Report
Four solids, labelled E, F, G and H, are provided in small bottles, together with a nichrome wire, a watch glass of dilute hydrochloric acid and a Bunsen burner. Each solid contains a different metal ion. Work through the solids in turn, cleaning the wire between the tests, and record what you see in Table 5.1. Aqueous sodium hydroxide and aqueous ammonia are also available on the bench, with distilled water and a rack of clean boiling tubes for a further test. Use only a few grains of each solid on the wire: a large lump does not become hot enough to give a clear colour. The colour appears as soon as the wire enters the hot flame and then fades, so look at the flame at once and record the first colour you see. If you are not sure of a colour, clean the wire and repeat the test on a fresh portion of the same solid.
Because each solid is to be identified by two independent tests, clean the nichrome wire in the dilute hydrochloric acid and hold it in the hot flame until the flame stays colourless before every flame test, since a trace of one solid left on the wire would colour the next flame and spoil the comparison. Take each of the four solids in turn, first testing a little of it in the flame and recording the colour seen, then dissolving a fresh portion in distilled water so that its cation can be confirmed with aqueous sodium hydroxide. Record both results for every solid, and keep the four solutions in a labelled rack so that they cannot be confused when the confirmatory tests are made.
Table 5.1
| solid | flame colour recorded |
|---|---|
| E | |
| F | |
| G | |
| H |
(a) Describe the method you use to carry out the flame test on solid E. [3]
(b) Record the flame colour of each of the four solids in Table 5.1. [4]
(c) Deduce the metal ion present in each of the four solids. [4]
(d) Two of the solids gave flame colours that are easily confused. State which two solids these are and describe a further test, using aqueous sodium hydroxide, that separates them. Give the observations expected. [3]
(e) A strong yellow flame can hide a faint lilac colour. State how the lilac colour can still be seen. [1]
(f) State one reason for using a nichrome wire rather than an iron wire. [1]
Answer Details
This task identifies four metal ions by their flame colours, with a confirmatory reaction where two colours could be confused. A flame test excites the metal ions, which emit a characteristic colour, so the colour names the metal.
(a) Method for the flame test on solid E [3]. Clean the nichrome wire by dipping it in dilute hydrochloric acid and heating it in the hot flame until the flame has no colour [1]; moisten the clean wire with the acid and dip it into a small portion of solid E [1]; hold the wire in the hot, roaring blue flame and record the colour at once [1].
(b) The flame colours [4]. E: red [1]; F: orange-red [1]; G: blue-green [1]; H: yellow [1].
(c) The metal ions [4]. E is lithium, \(\text{Li}^+\) [1]; F is calcium, \(\text{Ca}^{2+}\) [1]; G is copper(II), \(\text{Cu}^{2+}\) [1]; H is sodium, \(\text{Na}^+\) [1].
(d) The confusable pair and a sodium hydroxide test [3]. E and F, whose red and orange-red flames are easily confused [1]; dissolve a portion of each in distilled water and add aqueous sodium hydroxide dropwise until in excess: F (calcium) gives a white precipitate that stays in excess [1], while E (lithium) gives no precipitate [1].
(e) Seeing a faint lilac under a strong yellow [1]. View the flame through blue cobalt glass, which absorbs the yellow light and lets the faint lilac show [1].
(f) Nichrome rather than iron wire [1]. Any one: nichrome does not give a flame colour of its own, and it does not corrode or burn away in the hot flame [1].
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