Separation techniques is one of the friendliest topics in the O-Level Combined Science Chemistry syllabus. It is also one of the most commonly under-scored, because students know the answer in their heads but write it in a way the mark scheme cannot reward. If you have ever written "use filtration to get the salt" and lost both marks, this guide is for you.
Why these questions lose marks
Cambridge does not want to know that you have heard of distillation. It wants to see that you can:
- Identify what kind of mixture you are dealing with.
- Choose the technique that exploits the correct physical property.
- Describe the steps in order, using the right technical nouns.
Most lost marks come from step three. Words like residue, filtrate, distillate, soluble, miscible and boiling point are the marks. Vague words like "the stuff left behind" are not.
The three questions to ask before you write
Before your pen moves, run through this quickly:
- Solid or liquid? Am I separating a solid from a liquid, two liquids, two solids, or dissolved substances?
- Soluble or insoluble? Does the solid dissolve in the solvent given?
- What do I actually want to keep? The solid, the liquid, or both?
That last question decides everything. Separating salt from sand and separating pure water from seawater use different techniques even though both involve salt water.
Match the technique to the mixture
- Filtration: insoluble solid from a liquid. The insoluble solid stays as the residue on the filter paper, the liquid passes through as the filtrate.
- Crystallisation: recovering a soluble solid that decomposes on strong heating, such as hydrated copper(II) sulfate. Heat the solution to evaporate some water until it is saturated, allow it to cool slowly so crystals form, filter, then dry between filter papers.
- Evaporation to dryness: recovering a soluble solid that is stable to heat, such as sodium chloride. Do not use this if the question mentions crystals or a substance that decomposes.
- Simple distillation: separating a solvent from a dissolved solid, for example pure water from seawater. The water evaporates, then condenses in the condenser and is collected as the distillate.
- Fractional distillation: separating two or more miscible liquids with different boiling points, for example ethanol and water, or crude oil fractions. Mention the fractionating column.
- Separating funnel: two immiscible liquids, for example oil and water. The denser layer is run off first through the tap.
- Paper chromatography: separating dissolved, usually coloured substances such as dyes, inks or food colourings.
- Sublimation: separating a solid that turns straight into a gas, for example ammonium chloride from sodium chloride.
- Magnetic attraction: iron filings from sulfur powder.
Worked example: salt, sand and water
Question: A mixture contains sand and sodium chloride. Describe how to obtain pure, dry samples of both. [4]
A full-mark answer:
Add water and stir, so that the sodium chloride dissolves and the sand remains insoluble. Filter the mixture. The sand is the residue, wash it with distilled water and dry it in an oven. Heat the filtrate to evaporate the water and obtain dry sodium chloride.
Notice four scoring moves: the property used (solubility), the named technique, the correct terms for both products, and the drying step. Students routinely forget "wash and dry", and that is a whole mark.
Chromatography questions
These come up almost every year in some form. Remember:
- Draw the baseline in pencil, because pencil does not dissolve in the solvent.
- The baseline must sit above the solvent level, otherwise the spots dissolve into the solvent directly.
- More soluble substances travel further up the paper.
- A pure substance gives one spot; a mixture gives more than one.
- If two spots travel the same distance in the same solvent, the substances are likely the same.
- For colourless substances, use a locating agent or view under UV light.
If asked to calculate R_f, use distance moved by the spot divided by distance moved by the solvent front, measured from the baseline to the centre of the spot. Give your answer to two decimal places with no units.
Diagram marks are free marks
When you are asked to complete or label distillation apparatus:
- The thermometer bulb sits at the level of the side arm, so it measures the vapour temperature.
- Cold water enters the condenser at the bottom and leaves at the top.
- The apparatus must not be completely sealed, or pressure builds up.
- Label the flask, condenser, thermometer and receiving beaker clearly.
Quick self-check before the exam
Cover your notes and write, from memory, one real-life example and one key term for each technique above. If you can do that in under five minutes, you are ready. Then attempt three past-paper structured questions and mark them strictly against the phrasing in this post.
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