6 exam-style questions with full mark schemes and model answers. Write your own answer and the AI examiner marks it against the mark scheme.
Learn this properly: Pure Substances and MixturesA forensic scientist is given a solution containing an unknown mixture of food colourings. They want to find out how many colourings are present and to identify them by comparing them with four known reference dyes.
Describe how the scientist could use paper chromatography to do this, and explain how the results would allow them to identify the dyes. (6 marks)
In a chromatography experiment, a spot of dye X moved a distance of 3.6 cm from the baseline. In the same time, the solvent front moved a distance of 6.0 cm from the baseline.
(a) Calculate the Rf value of dye X. Show your working. (2 marks)
(b) Explain why an Rf value can never be greater than 1. (1 mark)
(c) A second dye, Y, in the same experiment has an Rf value of 0.25. Calculate the distance moved by dye Y from the baseline. (1 mark)
A student is given a solution of an unknown ionic compound and carries out tests to identify the ions present.
(a) Describe the test, and the positive result, for a carbonate ion (CO32−). (2 marks)
(b) Describe how to test for the presence of carbon dioxide gas, and give the positive result. (1 mark)
Flame tests can be used to identify some metal ions. The table shows the flame colours of four metal ions.
| Metal ion | Flame colour |
|---|---|
| Lithium, Li+ | Crimson (red) |
| Sodium, Na+ | Yellow |
| Potassium, K+ | Lilac (purple) |
| Copper, Cu2+ | Green |
(a) A student performs a flame test on an unknown compound and observes a lilac flame. Identify the metal ion present. (1 mark)
(b) A mixture contains both sodium ions and potassium ions. Explain why a simple flame test might fail to detect the potassium ions in this mixture, and name the instrumental method that could be used instead to identify both ions accurately. (2 marks)
A pure substance in chemistry has a specific meaning that is different from its everyday meaning.
(a) State what is meant by a pure substance in chemistry. (1 mark)
(b) Explain how the melting point of a sample could be used to tell whether a substance is pure or is a mixture. (1 mark)
The test for hydrogen gas uses a lighted (burning) splint held at the mouth of a test tube.
State the positive result that shows the gas is hydrogen. (1 mark)