A technician clearing the store cupboard has found a specimen bottle whose label has fallen off. The bottle holds a white crystalline solid, solid W. Carry ...
Question 1 Report
A technician clearing the store cupboard has found a specimen bottle whose label has fallen off. The bottle holds a white crystalline solid, solid W. Carry out the two tests below on solid W and use what you see to name the metal in solid W.
Put one spatula measure of solid W into a boiling tube and pour in about 10 cm3 of distilled water. Shake the boiling tube until no more of solid W will dissolve, then stand the boiling tube in a rack. Keep this solution for the second test. Clean a nichrome wire by dipping the wire into concentrated hydrochloric acid and holding the wire in a hot Bunsen flame until the flame stays colourless. Dip the clean wire into the solution of solid W and hold the wire at the edge of the hot flame.
Table 1.1
| test | observations |
|---|---|
| appearance of solid W, and what happens when solid W is shaken with distilled water | |
| flame colour given by the solution of solid W |
(a) Record your observations for both tests in Table 1.1. [3]
(b) Deduce the metal ion present in solid W. [1]
(c) The nichrome wire is cleaned again before the next candidate uses it. Explain why. [1]
(d) Give one safety precaution needed when the concentrated hydrochloric acid is being used. [1]
Answer Details
This task identifies an unknown metal salt from its appearance, solubility and flame colour. A flame test excites the metal ions, which emit light of a characteristic colour as they return to their lower energy state, so the colour names the metal. A representative consistent set of observations is used below, in which solid W gives a lilac flame.
(a) Observations for both tests [3]. Appearance: white or colourless crystals or powder [1]; when shaken with distilled water all of solid W dissolves to give a colourless solution [1]; the flame colour recorded, for example lilac [1] (any single flame colour that is consistent with the cation you name in part (b)).
(b) The metal ion [1]. The cation matching the recorded flame colour [1]: a lilac flame indicates potassium, \(\text{K}^+\). For reference, yellow is \(\text{Na}^+\), red is \(\text{Li}^+\), orange-red is \(\text{Ca}^{2+}\), blue-green is \(\text{Cu}^{2+}\) and apple-green is \(\text{Ba}^{2+}\).
(c) Why the wire is cleaned again [1]. Any one: traces of the previous solid left on the wire would colour the next flame and give a false result; in particular sodium contamination gives an intense yellow that masks other colours [1].
(d) A safety precaution with concentrated hydrochloric acid [1]. Any one: wear eye protection; keep the acid in a small watch glass away from the flame; avoid skin contact or wear gloves; do not breathe the fumes [1].
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