The diagram shows a school laboratory flow chart for identifying ions in an unknown white solid. A student dissolves a small mass of the solid in water befo...
Question 1 Report
The diagram shows a school laboratory flow chart for identifying ions in an unknown white solid. A student dissolves a small mass of the solid in water before splitting the solution between the tests.
The white precipitate dissolves in dilute ammonia. The warmed tube produces ammonia gas.
(a) Name the negative ion identified by the left-hand route. [1]
(b) Name the positive ion identified by the right-hand route. [1]
(c) Give the formula of the unknown compound. [1]
(d) Suggest why the solid is dissolved before testing. [1]
(e) Use the flow chart to name the reagent that is added before silver nitrate. [1]
A coach brings three unlabeled drinks powders to a sports-science laboratory. Powder A is known to contain sodium chloride. Powder B gives a yellow flame and a white precipitate when acidified silver nitrate is added. Powder C gives no flame colour but gives a white precipitate with barium chloride after acidification.
(a) Name the positive ion in powder B. [1]
(b) Name the negative ion in powder B. [1]
(c) Give the formula of the compound in powder B. [1]
(d) Name the negative ion in powder C. [1]
Answer Details
Unknown white solid
- (a) Nitric acid followed by silver nitrate gives a white precipitate that dissolves in dilute ammonia. This identifies chloride ions, \(\text{Cl}^-\). [1]
- (b) Warming with sodium hydroxide produces ammonia, so the positive ion is ammonium, \(\text{NH}_4^+\). [1]
- (c) Combining \(\text{NH}_4^+\) and \(\text{Cl}^-\) gives \(\text{NH}_4\text{Cl}\). [1]
- (d) The solid is dissolved so its ions are free to react in solution with the test reagents. [1]
- (e) The reagent added before silver nitrate is nitric acid. [1]
Drinks powders
- (a) The yellow flame from powder B identifies sodium ions, \(\text{Na}^+\). [1]
- (b) The white precipitate with acidified silver nitrate identifies chloride ions, \(\text{Cl}^-\). [1]
- (c) Powder B is therefore \(\text{NaCl}\). [1]
- (d) Acidified barium chloride giving a white precipitate identifies sulfate ions, \(\text{SO}_4^{2-}\). [1]
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