Common Separation Techniques
This topic covers the practical methods used to separate the components of a mixture by exploiting their different physical properties. For the ESAT, you must be able to select the correct technique for a given mixture, such as separating salt from water or different inks from each other.
Part of the ESAT Chemistry syllabus — revision for the Engineering and Science Admissions Test (ESAT), the UAT-UK admissions test for Cambridge, Imperial, Oxford and UCL.
Key points
- Use filtration to separate an insoluble solid from a liquid (e.g., sand from water). The solid is the 'residue', the liquid is the 'filtrate'.
- Use crystallisation or evaporation to separate a soluble solid (solute) from a liquid (solvent). Crystallisation yields purer, larger crystals; evaporation is faster.
- Use simple distillation to separate a liquid solvent from a dissolved solid, collecting the liquid (e.g., pure water from salt water). This works due to a large difference in boiling points.
- Use fractional distillation to separate two or more miscible liquids with different but close boiling points (e.g., ethanol from water). A fractionating column provides a large surface area for repeated condensation and vaporisation.
- Use a separating funnel to separate immiscible liquids (e.g., oil and water). The liquids form distinct layers based on density, which can be drained off one by one.
- Use paper chromatography to separate mixtures of soluble substances (e.g., different coloured inks). Separation occurs based on how strongly each component adheres to the paper versus how well it dissolves in the solvent.
Formulae
Rf = (distance moved by spot) / (distance moved by solvent front) To calculate a unique identifying value for a substance from a chromatogram. Both distances must be measured from the same starting baseline.
Definitions
- Miscible
- Describes liquids that can mix together completely to form a single, uniform solution (e.g., water and ethanol).
- Immiscible
- Describes liquids that do not mix together and form separate layers (e.g., oil and water).
- Rf value
- The 'retardation factor' in chromatography. It's the ratio of the distance a substance travels up the paper to the distance the solvent travels, used for identification.
- Filtrate
- The liquid or solution that has passed through the filter paper during filtration.
- Residue
- The insoluble solid material that is trapped by the filter paper during filtration.
Worked example
A student aims to separate a mixture containing sand, salt (NaCl), and water. They first pour the mixture through filter paper into a beaker. The contents of the beaker are then heated gently in an evaporating basin until crystals begin to form, before being left to cool. Which substances are the residue on the filter paper and the final crystals in the basin, respectively?
- 1
Step 1:
Identify the properties of the components.
Sand is insoluble in water.
Salt is soluble in water.
- 2
Step 2:
Analyse the first step - filtration.
Filtration separates insoluble solids from liquids.
Therefore, the insoluble sand will be trapped by the filter paper, making it the residue.
- 3
Step 3:
Analyse the second step.
The salt and water pass through the filter paper into the beaker as a salt solution.
Heating this solution evaporates the water.
- 4
Step 4:
The process described is crystallisation, which isolates a soluble solid from its solvent.
Therefore, the salt (NaCl) will form crystals in the evaporating basin.
- 5
Step 5:
Combine the findings.
The residue is sand, and the final crystals are salt.
Answer: Residue: sand. Crystals: salt.
Common mistakes
- ×Misreading diagrams is a frequent error. For distillation, always identify the condenser and note that the water inlet is at the bottom and the outlet is at the top to ensure it fills completely.
- ×Confusing simple and fractional distillation. Remember, fractional distillation requires a fractionating column and is used for miscible liquids with close boiling points.
- ×In chromatography, measuring distances from the wrong place. Always measure from the pencil baseline where the spots were initially placed, not from the bottom edge of the paper.
- ×Incorrectly identifying which separation technique to use for soluble vs. insoluble solids. Remember: filter an insoluble solid, crystallise a soluble solid.
No-calculator tips
- ✓Rf values must always be less than 1. If you calculate a value of 1 or more, you have likely divided the solvent distance by the spot distance. Swap the numerator and denominator.
- ✓When calculating an Rf value, simplify the fraction before converting to a decimal. For a spot that travels 6 cm when the solvent travels 8 cm, the Rf is 6/8. This simplifies to 3/4, which is easily identifiable as 0.75.