1.10 Crystallisation
1.10 Purification by crystallisation
​Crystals can form when a molten solid cools and solidifies. They can also form when a saturated solution is cooled. This happens because the solubility of most solid solutes decreases as the temperature is lowered.
The solute can no longer stay in solution and therefore forms a solid precipitate. The precipitate can be crystalline.
Crystallisation can therefore be used to separate a solid solute from its solvent.
A crystal or crystalline solid is a solid material whose constituent particles (such as atoms, molecules, or ions) are arranged in a highly ordered structure
Crystals can be recognised by the fact that they have regular geometric shapes. Flat faces and straight edges are a common feature.
The regular shapes reflect the highly ordered structure of the particles of which they are made
Activity 1. Spot the error
- ​When recrystallising in order to purify a solid, how much solvent is used
- There is a mistake in the apparatus set up at the beginning of the video. Describe the mistake and explain why it would be dangerous to heat the apparatus as shown.
- Explain why the filter funnel is heated by a hot water jacket?
- Why do crystals then form in the filtrate?
- How are these crystals then separated?
The crystals obtained this way can then be dissolved and recrystallised to improve the purity further.​
Activity 2. Close packing
This video helps you to visualise how molecules ( or atoms) can stick together to form an orderly arrangement and therefore form crystals.
- ​What shape is used in the video to represent the sugar molecules ?
- The four images below show four different ways of "modelling" the arrangement of atoms or molecules forming a crystalline structure. Consider each model in turn. For each model try to say what is good about it and what might be "inaccurate" about it. This could be presented as a table of strengths and weaknesses.
- It is often said that crystallising something slowly will produce lager and more regular crystals. Can you explain this?
Activity 3. Modelling with balls
from r/oddlysatisfying
In this video you see how scientist use bubbles in a soap solution to model the behaviour of particles in a crystal.