Separation techniquesIB MYP Chemistry: Revision notes
Section 1
Choosing a separation method
A mixture can be separated because its parts keep their own physical properties. The right method depends on the property that differs:
- Solubility: one part dissolves and the other does not (filtration, crystallisation).
- Particle size: large insoluble particles can be caught by filter paper.
- Boiling point: the parts boil at different temperatures (distillation).
- Density or settling: a solid settles beneath a liquid (decanting).
These are all physical methods: no new substances are made.
Section 2
Filtration and decanting
Filtration separates an insoluble solid from a liquid. The mixture is poured through filter paper in a funnel. The solid stays on the paper as the residue and the liquid passes through as the filtrate. Filtration separates sand from water but cannot separate dissolved salt, because the particles are too small and pass through the paper.
Decanting means carefully pouring off the liquid from a solid that has settled at the bottom. It is quick but less complete than filtering.
Filtration does not remove dissolved substances. A salt solution passes straight through filter paper.
Section 3
Evaporation and crystallisation
Evaporation removes the solvent from a solution by heating, leaving the solid solute. It is used to get salt from salt solution. It is only suitable for solutes that do not decompose when heated.
Crystallisation gives larger, purer crystals. Heat the solution until it is saturated; you can test this by dipping a glass rod in and seeing crystals form as the drop cools. Then leave it to cool, so solubility falls and crystals form. Filter off the crystals and dry them between filter paper.
Section 4
Simple distillation
Simple distillation separates a liquid from a dissolved solid, and is used when you want to collect the liquid. For example, it gives pure water from seawater.
The solution is heated in a flask. The liquid boils, and the vapour passes into a condenser, which is cooled with cold water. The vapour condenses to a liquid called the distillate, which is collected. The dissolved solid stays behind in the flask.
Section 5
Fractional distillation
Fractional distillation separates a mixture of liquids with different boiling points, such as ethanol (78 °C) and water (100 °C).
The mixture is heated, and the liquid with the lowest boiling point evaporates first. The vapour rises up a fractionating column, which is hotter at the bottom and cooler at the top. Vapours of the liquid with the higher boiling point condense in the column and run back down. The vapour of the lower boiling point liquid reaches the top, passes into the condenser and is collected. Crude oil and liquid air are also separated this way.
In an exam answer, always name the property that is different (boiling point) and say which liquid comes off first (the one with the lower boiling point).
Section 6
Choosing a method: worked example
To separate a mixture of sand, salt and iron filings, plan the steps in order.
- Magnet: removes the iron, because only iron is magnetic.
- Add water and stir: the salt dissolves; the sand does not.
- Filter: the sand stays on the paper as the residue; the salt solution is the filtrate.
- Evaporate the filtrate (or crystallise it) to get the salt.
To check the success, add the masses recovered and compare them with the starting mass. If 1.0 g, 3.6 g and 4.9 g are recovered from 10.0 g, the percentage recovered is 9.5 ÷ 10.0 × 100 = 95%. The losses are usually solid left on equipment.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Separation techniques
- A school laboratory technician has four mixtures to separate. Mixture W is sand and water. Mixture X is salt dissolved in water, and she wants to collect the salt. Mixture Y is seawater, and she wants to collect pure water. Mixture Z is a liquid mixture of ethanol, which boils at 78 °C, and water, which boils at 100 °C, and she wants to collect both liquids.Explain why filtration cannot be used to collect the salt from mixture X.2 marks
- A student wants to obtain pure crystals of copper sulfate from a blue copper sulfate solution. She heats the solution in an evaporating basin until about half of the water has gone. Every so often she dips a clean glass rod in the hot solution and lets the drop cool, until small crystals form on the rod. She then stops heating, leaves the solution to cool, separates the crystals from the liquid and dries them between filter paper.Explain why the solution is left to cool before the crystals are separated, and why they are then dried.2 marks
- A student is given 10.0 g of a mixture of sand, salt and iron filings and must separate and recover all three substances. After she has dried each one, the masses she recovers are: iron 1.0 g, sand 3.6 g and salt 4.9 g.Outline a method that she could use to separate sand, salt and iron filings from the mixture.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).