Separation and Purity

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.

For a solid–solid mixture, choose a solvent that dissolves only one solid. Add the solvent, stir, filter, and wash and dry the insoluble residue. Recover the dissolved solid from the filtrate by crystallisation or evaporation. For a solid–liquid mixture, filter if the solid is insoluble; if it is dissolved, use crystallisation, evaporation or distillation according to the product required. For miscible liquids, use fractional distillation when their boiling points differ.

Choose evaporation when the solvent is not required and the dissolved solid is stable on heating. Choose crystallisation when crystals are wanted or heating to dryness could decompose the solid. Choose simple distillation when the solvent itself must be collected.

Carry out simple and fractional distillation

For simple distillation, heat the solution so that the more volatile liquid boils. Its vapour enters the condenser, cools and changes back to liquid. Collect this liquid as the distillate; the less volatile dissolved substance remains in the flask. Keep the thermometer bulb level with the side arm so it measures the vapour entering the condenser. Cooling water enters the lower condenser port and leaves from the upper port, keeping the water jacket full.

For fractional distillation, place a fractionating column between the heated flask and condenser. Repeated condensation and vaporisation in the column allow vapour richer in the lower-boiling liquid to reach the condenser first. Use this method for miscible liquids, especially when their boiling points are close enough that simple distillation would separate them poorly.

Do not heat a closed apparatus. The receiver remains open to the air so pressure cannot build up.

Set up and read paper chromatography

In chromatography, place the sample on a pencil baseline above the solvent. The solvent moves through the paper and carries components different distances. Here, interpret the number of spots and their positions compared with known samples. Where your syllabus also requires calculated R_f values, the methods of separation and purification lesson teaches that calculation.

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

5. Exam Tips

Use the pattern property difference → method → named apparatus action → product obtained.

6. Worked Examples

7. Mind Stretchers

8. Quiz

Before attempting the assessment, check that you can:

• select a method from solubility, boiling point or particle-size evidence;
• use a suitable solvent to separate two solids and recover both products;
• 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.

Practise this: for each new mixture, write four parts before naming a method: the mixture type, the physical-property difference, the required product and the apparatus action.