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Crystallisation occurs when the solution solvent evaporates, and the concentration of the solute reaches saturation point. At this stage, the solute begins to precipitate out of solution. Under the right conditions, generally slow evaporation and a clear solution, the solute will crystallise. - thermally unstable substances, we should heat it very slowly.
We are trying to create a supersaturated solution.
Cooling it down will create crystals. Fast cooling = small crystals, Slow cooling = big crystals.
Evaporation Vs Crystallisation
Both methods are used to separate a soluble solid from a solution
Evaporation:
- Is used when the soluble solid does not decompose upon strong heating
- Drawback: any soluble impurities may also be present after evaporation.
Crystallisation:
- Is used when the soluble solid contains water of crystallisation - e.g. $CuSO_4.5H_2O$ - hydrated copper(II) sulfate - in comparison: anhydrous copper (II) sulfate = $CuSO_4$
- held by weak forces, using heat can break water of crystallisation (water evaporates), and you will be left with your desired substance
- Advantage: pure solid is collected in the form of crystals.
Centrifuge:
- Centrifuges rotate containers of liquids to separate suspended materials with different densities.
- Centrifuges separate different components of human blood or milk and to clarify solutions. A high speed separator can rotate at great speed to separate fat (cream) from milk.
- The spin drier in washing machines is a type of centrifuge that throws out the liquid by the “centrifugal force” of the rotation.
A magnet
- Can be used to separate a magnetic substance from a non-magnetic substance
Chromatography
- Separates components of a mixture based on ability of each component to be drawn across the surface of another material
- Mixture is usually liquid and is usually drawn across chromatography paper
- Separation occurs because various components travel at different rates
- Components with strongest attraction for paper travel the slowest.
- There is paper chromatography
- Separation by chromatography
- Ion chromatogram from an orange juice
Atomic Structure:
- Democritus - 460 BC
- Proposed the existence of the atom
- He pounded materials until he made them into smaller and smaller parts
- He called them atoma which is Greek for “indivisible”
Next week: STAWA 11 on Tuesday
