Using an Appropriate Solvent
Solvent extraction involves dissolving one or more components of a mixture in a suitable solvent, leaving other components undissolved. This technique exploits differences in solubility.
For example, to separate salt from sand, water is used as a solvent because salt dissolves in water while sand does not. The mixture is stirred with water, dissolving the salt, and then the sand can be separated by filtration.
Choosing the correct solvent depends on the substances involved; the solvent must dissolve the target substance but not the impurities. This selective dissolution is the basis of many purification processes.
- Selective dissolution depends on solubility differences.
- Example: salt dissolves in water, sand does not.
- Solvent choice is critical for effective separation.
Filtration
Filtration separates an insoluble solid from a liquid by passing the mixture through a filter medium, such as filter paper, which allows the liquid to pass but traps the solid particles.
In the salt and sand example, after dissolving salt in water, the mixture is poured through filter paper in a funnel. The sand remains on the paper (residue), and the salt solution passes through (filtrate).
Filtration is a physical separation method and does not change the chemical composition of the substances involved.
- Separates insoluble solids from liquids.
- Filter paper traps solids; liquid passes through.
- Used after solvent extraction to separate undissolved solids.
Crystallisation
Crystallisation is used to obtain pure solid crystals from a solution. It involves evaporating some solvent to concentrate the solution until crystals form.
For example, after filtering salt solution from sand, gently heating the salt solution causes water to evaporate. As the solution becomes saturated, salt crystals start to form and can be collected by further filtration or decanting.
This method purifies solids because impurities remain dissolved in the solvent and do not crystallise with the pure substance.
- Concentrates solution by evaporation to form crystals.
- Used to purify solids from solutions.
- Impurities remain dissolved, crystals are pure.
Simple Distillation
Simple distillation separates a liquid from a solution or mixture based on differences in boiling points, typically when one component has a much lower boiling point than the others.
Apparatus includes a distillation flask, condenser, and receiving vessel. The mixture is heated; the component with the lower boiling point vaporises first, condenses in the condenser, and is collected separately.
For example, simple distillation can separate pure water from salt water. Water boils at 100°C, salt does not vaporise, so pure water vapour condenses and is collected, leaving salt behind.
- Separates liquids from solutions by boiling point differences.
- Collects vapour of lower boiling point component.
- Example: obtaining pure water from salt water.
Fractional Distillation
Fractional distillation separates mixtures of liquids with closer boiling points by repeated vaporisation and condensation cycles within a fractionating column.
The fractionating column provides a large surface area for multiple condensation and vaporisation steps, improving separation efficiency.
An example is separating ethanol (boiling point 78°C) from water (100°C). The mixture is heated; ethanol vapour rises first, condenses, and is collected, while water remains mostly in the distillation flask.
- Used for separating liquids with similar boiling points.
- Fractionating column allows repeated vaporisation-condensation.
- Example: separating ethanol from water in fermentation mixtures.
At-a-glance summary
Comparison table
| Technique | Purpose | Key Physical Property Exploited | Example Mixture Separated |
|---|---|---|---|
| Solvent extraction | Separate soluble from insoluble substances | Solubility | Salt and sand using water |
| Filtration | Separate insoluble solid from liquid | Particle size and solubility | Sand from salt solution |
| Crystallisation | Purify solid from solution | Solubility and saturation | Salt crystals from salt solution |
| Simple distillation | Separate liquid from solution with different boiling points | Boiling point difference (large) | Water from salt water |
| Fractional distillation | Separate liquids with close boiling points | Boiling point difference (small) | Ethanol and water mixture |
Active recall
Quick check
1. Why does salt dissolve in water but sand does not?
Because salt is soluble in water due to ionic interactions, while sand is insoluble as it is composed of silicon dioxide which does not interact with water molecules.
2. What is the purpose of filter paper in filtration?
Filter paper acts as a barrier that allows liquid to pass through while trapping insoluble solid particles.
3. How does crystallisation purify a solid?
By evaporating solvent to form pure crystals, leaving impurities dissolved in the remaining solution.
4. Why can simple distillation separate water from salt water?
Because water has a much lower boiling point (100°C) than salt (which does not vaporise), so water vaporises and condenses separately.
5. When is fractional distillation preferred over simple distillation?
When separating liquids with similar boiling points, as the fractionating column allows better separation through repeated vaporisation-condensation cycles.