Chemical Tests
5 marks · EasyA student tests a colorless liquid. She wants to find out whether it is pure water or contains dissolved chloride ions from an unknown salt. She also has some ammonia solution for use in any tests required.
Describe how the student could use a chemical test to show whether the liquid contains water, and explain what her result would mean about the presence of water.
The student measures the boiling point of the liquid and finds it to be 102°C (at 1 atmosphere pressure). Calculate the difference between this boiling point and the boiling point of pure water under these conditions, and explain what this result shows about the purity of the liquid.
To test for dissolved chloride ions, the student adds dilute nitric acid, followed by silver nitrate solution. She obtains a white precipitate. Explain why it is important to add the nitric acid before the silver nitrate solution in this test.
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.
Show answer
(a) Add a few drops of the liquid to white anhydrous copper(II) sulfate. If it turns blue, water is present. This only confirms water is present, not purity. · must show Describe using anhydrous copper(II) sulfate, State observation (white to blue), Explain shows presence but not purity
(b) Boiling point difference = 102°C - 100°C = 2°C. The higher boiling point indicates that the sample contains dissolved substances, so it is not pure water. °C · must show Calculation (102-100 = 2°C), Explanation: not pure as boiling point is higher
(c) Nitric acid must be added before silver nitrate to react with and remove other ions (like carbonate or hydroxide) that would also form precipitates with silver nitrate and give a false positive. This ensures that only halide ions are detected. · must show State that acid removes interfering ions, Explain avoids false positives, Reason for test order
Common mistakes: Using litmus for water test instead of anhydrous copper(II) sulfate; Not referring to the difference in boiling point; Not mentioning removal of interfering ions with nitric acid
Show mark scheme and worked solution
Marking points
A1 (a) · 1 mark · States that anhydrous copper(II) sulfate turns from white to blue if water is present, and that this test only shows water is present, not its purity.
M1 (b) · 1 mark · Calculates the difference between boiling point of sample and pure water correctly (2°C).
A2 (b) · 1 mark · Explains that boiling point above 100°C indicates sample is not pure water due to dissolved substances.
M2 (c) · 1 mark · States that nitric acid is added to remove other ions (interfering ions).
A3 (c) · 1 mark · Explains that this is to avoid false positives, so that only halide ions react to form the correct precipitate.
Worked solution
a) The student adds a few drops of the liquid to white anhydrous copper(II) sulfate. If the solid turns blue, this shows the presence of water in the sample. However, this does not prove the water is pure; impurities might also be dissolved.
b) The boiling point of pure water at 1 atm is 100°C. The measured value is 102°C. Difference = 102°C - 100°C = 2°C. A boiling point above 100°C indicates the water is not pure, since dissolved substances raise the boiling point.
c) Nitric acid is added to remove other ions, such as carbonate or hydroxide, which could otherwise react with silver nitrate to form misleading precipitates. Adding nitric acid first avoids false positive results, ensuring the test detects halide ions only.