A student has two unlabelled bottles. One contains solid sodium chloride and the other contains solid lithium chloride. Both are white solids. The student m...
Question 1 Report
A student has two unlabelled bottles. One contains solid sodium chloride and the other contains solid lithium chloride. Both are white solids. The student must plan tests to decide which bottle is which.
(a) Describe how a flame test can be used to distinguish the two solids. Give the method and the different results expected for each solid. [4]
(b) Explain why a test using aqueous silver nitrate would NOT distinguish the two solids. [2]
(c) Describe how the student could show that both solids contain the chloride ion. Give the reagents and the observation. [3]
(d) State one source of error in the flame test that could give a misleading result. [1]
Answer Details
This asks the student to plan tests to tell sodium chloride from lithium chloride. The two salts share the same anion (chloride) but have different metal ions, so a flame test distinguishes them while a chloride test does not.
(a) Using a flame test: clean a nichrome wire in dilute acid, dip it in the first solid and hold it in a hot Bunsen flame [1]; observe the flame colour [1]; sodium chloride gives a yellow flame [1]; lithium chloride gives a red flame [1]. The different colours identify which bottle is which.
(b) A silver nitrate test would not distinguish them because both are chlorides (both contain the chloride ion) [1], so both give the same white precipitate of silver chloride and cannot be told apart [1].
(c) To show both solids contain the chloride ion: dissolve each solid in water, add dilute nitric acid then aqueous silver nitrate [1] to a portion [1]; a white precipitate forms in each [1] (\(\text{Ag}^+ + \text{Cl}^- \rightarrow \text{AgCl}\)).
(d) One source of error in the flame test: the wire not being cleaned properly (contaminated with sodium), or using too much solid so the colour is masked [1]. Sodium contamination is especially misleading because its strong yellow can hide the red of lithium.
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