Fig. 1 shows the labels on three sealed sample bottles in a forensic laboratory. A powder from an unknown bottle is heated strongly in a flame. The scientis...
Question 1 Report
Fig. 1 shows the labels on three sealed sample bottles in a forensic laboratory. A powder from an unknown bottle is heated strongly in a flame. The scientist observes that it gives a green flame colour. The samples contain compounds of lithium, sodium and copper. The scientist uses flame tests because each metal ion gives a characteristic colour.
(a) Name the metal ion in the unknown powder. [1]
(b) Describe how to carry out a flame test safely. [2]
(c) Explain why the chloride ion does not determine the flame colour in these three samples. [2]
A battery company tests metals for use as electrodes. Fig. 1 shows a reactivity-series card supplied with a prototype battery kit. The card includes potassium, magnesium, copper and gold. The engineers need a metal that transfers electrons readily, but they must also consider whether it reacts dangerously with water or acid during manufacture.
(a) Name the least reactive metal shown. [1]
(b) Use Fig. 1 to state which metal reacts most strongly with cold water. [1]
(c) Describe what is likely to happen when magnesium is added to dilute hydrochloric acid. [2]
(d) Explain why copper can be found as an uncombined metal but potassium is not. [1]
Answer Details
Flame tests
(a) A green flame identifies the copper ion, \(\mathrm{Cu^{2+}}\). [1]
(b) Use a clean wire loop. [1] Dip it into the sample and place it in a non-luminous Bunsen flame while wearing eye protection. [1]
(c) All three samples contain chloride ions. [1] The different flame colours are caused by the different metal ions, not by chloride ions. [1]
Reactivity series
(a) The least reactive metal shown is gold. [1]
(b) Potassium reacts most strongly with cold water. [1]
(c) Magnesium reacts or dissolves in dilute hydrochloric acid [1] and hydrogen gas is produced. [1]
(d) Copper is much less reactive, so it can occur uncombined. Potassium reacts readily with substances such as water and oxygen, so it is not found uncombined. [1]
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