Chemical Tests
6 marks · NormalA student needs to check if water and carbonate ions are present in two different solutions. The first solution is labelled A (clear, colourless liquid). The second is labelled B (pale green solution).
Describe how the student could test solution A to check if it is pure water. Explain the result that would show if it is not pure.
The student adds a white powder to solution B and observes effervescence. The gas produced is bubbled through limewater which turns cloudy.
Explain what ions are present in the white powder and write the ionic equation for the reaction.
The student wishes to know if any ammonium ions are present in the white powder. Describe a test for ammonium ions and explain how the result confirms their presence.
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
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(a) Measure boiling or freezing point of solution A. Pure water boils at 100°C and freezes at 0°C at 1 atm. If the solution boils above 100°C or freezes below 0°C, it contains impurities (is not pure water).
(b) The white powder contains carbonate ions (CO3^2-), which react with acid in solution B to produce carbon dioxide gas. Ionic equation: CO3^{2-} (s or aq) + 2H^+ (aq) → CO2 (g) + H2O (l).
(c) Add sodium hydroxide solution to the white powder and warm gently. Test any gas evolved with damp red litmus paper: the paper turns blue if ammonia is present, confirming ammonium ions.
Also accept: Accept tests for purity by freezing or boiling point; Accept correct ionic equation with or without state symbols
Common mistakes: Stating only pH for water test instead of boiling/freezing point; Equations missing ions or wrong products; Not explaining why the litmus/universal paper must be damp
Show mark scheme and worked solution
Marking points
B1 (a) · 1 mark · Describes measuring freezing or boiling point as a purity test.
A1 (a) · 1 mark · Correctly states how results differ for pure/impure water (100°C/0°C for pure).
B2 (b) · 1 mark · Identifies carbonate ions as present in the powder.
A2 (b) · 1 mark · Gives correct ionic equation for carbonate-acid reaction.
B3 (c) · 1 mark · Describes adding sodium hydroxide and warming to test for ammonia.
A3 (c) · 1 mark · Explains blue litmus/universal indicator confirms ammonium ions.
Worked solution
a) To test purity of water, measure the boiling point or freezing point of solution A. Pure water boils at exactly 100°C and freezes at exactly 0°C at 1 atm. If the sample boils above 100°C or freezes below 0°C, the sample contains impurities.
b) Effervescence and limewater turning cloudy indicates CO₂ is produced; this shows carbonate ions in the white powder. The reaction (ionic equation):
c) To test for ammonium ions, add sodium hydroxide solution to the white powder and warm. Hold damp red litmus paper at the mouth of the test tube; it turns blue if ammonia is given off, confirming ammonium ions.