Separation and Purity
Key idea: G3 Science lesson on separation methods, chromatography and purity evidence.
Choose a separation method from a difference in physical properties. State what passes through, what remains, and how the required substance is recovered.
1. Definition
Separation divides a mixture without forming new substances. Purification removes unwanted substances. A pure substance contains one substance and has characteristic physical properties.
2. Key Ideas
- Filtration separates an insoluble solid from a liquid.
- Crystallisation obtains a dissolved solid without heating the solution to dryness.
- Simple distillation obtains a solvent from a solution.
- Fractional distillation separates miscible liquids with different boiling points.
- Paper chromatography separates soluble substances because they travel differently with a solvent.
- A pure substance melts or boils sharply at its characteristic temperature.
3. Detailed Explanations
Match method to evidence
Use filtration when particle size and insolubility differ. Use crystallisation when a solid is soluble and may decompose on strong heating. Use distillation when volatility or boiling point differs.
In chromatography, place the sample on a pencil baseline above the solvent. The solvent moves through the paper and carries components different distances. At this course level, interpret the number and positions of spots; calculated Rf values belong to the full G3 Chemistry course.
Judge purity
An impurity usually lowers and broadens a solid’s melting range. A pure liquid boils at a sharp characteristic temperature at a stated pressure. One chromatogram spot supports purity only for the solvent and locating conditions used.
4. Common Mistakes
- Filtration cannot remove a dissolved solute.
- Evaporation to dryness is not the safest way to obtain every soluble solid.
- The chromatography baseline must be pencil, not ink.
- The sample spot must begin above the solvent level.
- “High melting point” does not prove purity; use a sharp characteristic value.
5. Exam Tips
Exam question 1: Separate with the required product in mindCore
A mixture contains sand, salt and water. Describe how to obtain dry sand and pure water.
Show Answer
Filter the mixture. Wash and dry the residue to obtain sand. Distil the filtrate and collect the condensed solvent as pure water.
Use the pattern property difference → method → named apparatus action → product obtained.
6. Worked Examples
Example 1: Recover a soluble solidCore
Explain how to obtain copper(II) sulfate crystals from its aqueous solution.
Show Answer
Heat gently to evaporate some water until the solution is nearly saturated, allow it to cool so crystals form, filter the crystals, wash them with a little cold distilled water, and dry them between filter papers.
Example 2: Interpret spotsCore
An unknown gives two spots. One aligns with reference A and the other with reference B in the same chromatogram. What can be concluded?
Show Answer
The unknown is a mixture and contains components consistent with A and B under these conditions.
7. Mind Stretchers
Mind stretcher 1: Question one-spot evidenceExtension
Why does one spot not prove that a sample is pure in every possible test?
Show Answer
Two components can travel together in one solvent or may not be visible with one locating method. A different solvent or locating condition may separate them.
Mind stretcher 2: Explain repeated separationExtension
Why does a fractionating column improve the separation of miscible liquids with fairly close boiling points?
Show Answer
Repeated condensation and vaporisation enrich the rising vapour in the more volatile liquid before it reaches the condenser.
8. Quiz
Before attempting the assessment, check that you can:
- select a method from solubility, boiling point or particle-size evidence;
- distinguish residue, filtrate, distillate and crystals;
- explain a paper chromatography setup and chromatogram;
- use melting and boiling behaviour as purity evidence;
- state the limits of one purity test.