Separation techniques are the methods used to separate the components of a mixture. The right choice depends entirely on the physical properties of those components: particle size, boiling point, solubility, magnetism or whether two liquids mix.
Separation techniques are tested in every Lower Secondary Science examination and underpin all later chemistry. Knowing which method to choose, and why, is the core skill examiners target.
What these notes cover
- •Overview: choosing the right technique
- •Filtration
- •Evaporation
- •Crystallisation
- •Simple distillation
- •Paper chromatography and Rf values
- •Magnetic separation
- •Separating immiscible liquids
- •Common mistakes
- •Summary
Key definitions
- Filtrate
- The liquid that passes through the filter paper during filtration.
- Residue
- The solid that remains on the filter paper during filtration.
- Distillate
- The liquid collected in the receiving flask after the vapour cools and condenses in the condenser.
- Stationary phase
- In paper chromatography, the chromatography paper.
- Mobile phase
- In paper chromatography, the solvent that travels up the paper, carrying dissolved substances with it.
Key formulas
Rf value
Rf = distance moved by spot ÷ distance moved by solvent front
Both distances are measured from the pencil baseline, to the spot centre and to the solvent front. Rf is always between 0 and 1. A pure substance gives one spot at a fixed Rf for a given solvent.
Worked example
In a paper chromatogram, the solvent front travels 15.0 cm from the baseline. Spot X is 9.0 cm from the baseline and Spot Y is 12.0 cm from the baseline. Calculate the Rf values of X and Y, and state what they show.
- 1Use Rf = distance moved by spot ÷ distance moved by solvent front, with both distances measured from the baseline.
- 2Rf(X) = 9.0 ÷ 15.0 = 0.60.
- 3Rf(Y) = 12.0 ÷ 15.0 = 0.80.
- 4X and Y have different Rf values, so they are different substances.
- 5A reference dye with Rf = 0.60 under the same conditions would confirm that X is that dye.
Watch out: Rf > 1 is impossible. If your answer exceeds 1.0, you have swapped the spot distance with the solvent-front distance.
Common mistakes
Using filtration to recover a dissolved salt
Wrong: Filtering salt water to obtain the salt.
Right: Filtration removes insoluble solids only. A dissolved salt passes straight through the filter paper, so use evaporation or distillation to recover it.
Thermometer position in distillation
Wrong: Placing the thermometer bulb in the boiling liquid.
Right: Place the bulb level with the side-arm opening. It measures the vapour temperature, which is the boiling point of the distillate.
Chromatography baseline below the solvent
Wrong: Drawing the baseline so that it is submerged in the solvent.
Right: Draw the baseline above the solvent level. If it is submerged, the spot dissolves into the reservoir before it can travel up the paper.
Questions students ask
How do I choose the right separation technique?
Match the technique to the physical property of the components: particle size → filtration, boiling point → distillation, solubility vs temperature → crystallisation, relative solubility → chromatography. Magnetic solids need magnetic separation, and immiscible liquids need a separating funnel.
What is the difference between crystallisation and evaporation?
Crystallisation gives large, pure crystals by slow cooling, and suits a solid whose solubility decreases on cooling. Evaporation gives a crude solid quickly by heating off the solvent, and is used when crystal size is not critical and the solid is stable at high temperatures.
Why must the chromatography baseline be drawn in pencil?
Pen ink travels up the paper with the solvent and confuses the results. The baseline is drawn in pencil 2 cm from the bottom of the paper and must sit above the solvent surface.
How do you separate oil and water?
Use a separating funnel. Pour in the mixture and allow it to settle so that the denser liquid sinks, then slowly open the stopcock to drain the lower layer into a beaker. Close the stopcock and pour the upper layer out through the top into a separate beaker.
In the full PDF
- •Quick-reference table matching each technique to what it separates and the property it uses
- •Two more worked examples: obtaining dry sand from salt water by filtration, and growing large, pure copper(II) sulfate crystals from a mixture containing sand
- •Apparatus lists and step-by-step procedures for every technique, including when not to evaporate to dryness
- •Simple distillation of two miscible liquids and the iron filings and sulfur magnetic separation example
- •The remaining common exam mistakes, including crystallisation vs evaporation and filtrate vs residue
- •A five-point summary for last-minute revision