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: States of MatterA chemist is given a beaker containing a mixture of sand, copper(II) sulfate (a soluble blue salt) and water. The copper(II) sulfate is dissolved; the sand is insoluble.
The chemist needs to recover three separate things: the dry sand, pure dry copper(II) sulfate crystals, and a sample of pure water.
Describe and explain a sequence of separation techniques that would recover all three substances. For each step, name the technique, describe what you would do, and explain why it works in terms of the properties of the substances. (6 marks)
A student investigated paper chromatography of a green food dye. They placed a spot of the dye on the start line, ran the chromatogram, and the dye separated into a blue component and a yellow component.
The student measured the following distances from the start line:
| Measurement | Distance / cm |
|---|---|
| Solvent front | 8.0 |
| Blue spot (centre) | 2.4 |
| Yellow spot (centre) | 6.0 |
(a) Calculate the Rf value of the blue component. Show your working. (2 marks)
(b) State which component, blue or yellow, is more strongly attracted to the paper. Explain your answer. (2 marks)
The table gives the melting points and boiling points of four substances.
| Substance | Melting point / °C | Boiling point / °C |
|---|---|---|
| W | −7 | 59 |
| X | 801 | 1413 |
| Y | −210 | −196 |
| Z | 44 | 280 |
(a) State which substance is a gas at room temperature (25 °C). Explain your choice. (2 marks)
(b) Substance X is a pure compound. Predict whether its melting point would be a sharp value or a range, and explain why. (1 mark)
Sea water is a mixture of water and dissolved salts. In some countries, potable (drinking) water is produced from sea water by distillation.
(a) Explain why sea water cannot simply be filtered to make it safe to drink. (1 mark)
(b) Describe how distillation produces pure water from sea water. In your answer, refer to the processes of evaporation and condensation. (2 marks)
When solid iodine is heated gently it turns directly into a purple vapour without forming a liquid. This change of state is called sublimation.
Using the particle model, explain what happens to the arrangement and movement of the iodine particles as the solid sublimes to a gas. (2 marks)
In paper chromatography, the paper is described as the stationary phase.
Name the mobile phase in paper chromatography. (1 mark)