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1.3 Separating a Mixture

GCSE Chemistry · AQA · Chapter 1: Atomic Structure

A free GCSE Chemistry lesson for AQA (8462) from OBA School: mixtures and pure substances, and how filtration, crystallisation and simple distillation separate them. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

  • Explain the difference between a mixture and a compound
  • Describe what a pure substance is in chemistry and how melting points show purity
  • Describe and explain filtration, crystallisation and simple distillation
  • Choose the right method to separate a mixture and calculate a percentage of pure product

Key words

  • Mixture: Two or more substances (elements or compounds) that are not chemically combined.
  • Pure substance: A single element or a single compound, not mixed with any other substance.
  • Solution: A mixture made when a soluble solute (e.g. salt) dissolves in a solvent (e.g. water).
  • Filtrate: The liquid (or solution) that passes through the filter paper.
  • Residue: The insoluble solid left in the filter paper.
  • Crystallisation: Getting a soluble solid out of a solution by evaporating some solvent and letting crystals form.
  • Distillation: Boiling a liquid and then condensing the vapour to collect the pure liquid.

Mixtures and pure substances

In a mixture, the substances are not chemically combined. Each one keeps its own chemical properties, and they can be in any proportions. This means mixtures can be separated by physical processes: filtration, crystallisation, simple distillation, fractional distillation and chromatography. No chemical reaction is needed and no new substances are made.

  • Combined?; Mixture no – substances just mixed; Compound yes – elements chemically bonded
  • Proportions; Mixture any amounts; Compound fixed (e.g. always FeS)
  • Properties; Mixture same as the substances in it; Compound different from its elements
  • Separating; Mixture physical methods; Compound only by a chemical reaction

In chemistry, a pure substance is a single element or compound. A pure substance melts and boils at a fixed, sharp temperature. A mixture melts or boils over a range of temperatures. (In everyday life “pure” just means nothing added, e.g. “pure orange juice”, which is really a mixture.)

Filtration

Filtration separates an insoluble solid from a liquid or solution. The tiny holes in the filter paper let the liquid and dissolved particles through, but the larger solid particles are trapped.

To get the solid clean, wash the residue on the filter paper with a little distilled water to rinse away any solution still on it. Then dry it in a warm oven or between filter papers.

Exam tip: Two solids can sometimes be separated using a solvent that dissolves only one of them. Add the solvent, filter off the solid that does not dissolve, then crystallise the other solid from the filtrate. Never heat a flammable solvent with a naked flame – use a water bath.

Crystallisation

Crystallisation gets a soluble solid from a solution:

1. Pour the solution into an evaporating basin and heat it gently to evaporate some of the water. A water bath (or electric heater) heats more gently and evenly than a Bunsen flame, so the solution does not spit. 2. Stop heating when crystals start to form at the edge (or on a cold glass rod dipped in and taken out). 3. Leave the solution to cool, ideally in a wide, shallow dish so the rest of the water can evaporate slowly. More crystals form as it cools, because less solid can dissolve in cold water. 4. Filter off the crystals, then dry them between filter papers or in a warm oven.

Heating all the way to dryness (evaporation) is quicker, but it gives small crystals and can break down some salts. Crystallisation gives larger, purer crystals.

Worked example: percentage of salt in rock salt

A student dissolves 25.0 g of rock salt in water, filters out the sand and crystallises the salt. She gets 18.5 g of dry salt.
percentage of salt = (mass of salt ÷ mass of rock salt) × 100
= (18.5 ÷ 25.0) × 100 = 74.0%

Answer: 74.0%

Simple distillation

Simple distillation separates a solvent from a solution, e.g. pure water from salty water. The solution is heated and the water boils. The water vapour passes into the condenser, where it is cooled and condenses back into liquid water, which drips into the beaker. The salt has a very high boiling point, so it stays in the flask.

Key fact: The thermometer bulb is level with the side arm, so it measures the temperature of the vapour going into the condenser (100 °C for water). Cold water enters the condenser at the bottom so that the jacket stays full and cooling works best.

Choosing a method

Every method uses a difference in a physical property: filtration uses whether a solid dissolves (solubility), crystallisation and simple distillation use the very different boiling points of the solvent and the dissolved solid, fractional distillation uses the different (but close) boiling points of liquids, and chromatography uses how strongly each dissolved substance is attracted to the solvent and the paper.

Common mistakes

  • Saying salt is “filtered out” of salt water. Dissolved salt passes through the filter paper.
  • Saying a mixture is “chemically joined”. Only compounds are chemically combined.
  • Heating a solution to dryness when asked for crystals. Stop when crystals start to form and leave it to cool.
  • Putting the condenser water in at the top. It goes in at the bottom and out at the top.
  • Using everyday meanings of “pure”. In chemistry a pure substance is one element or one compound only.

Revision questions

Try these, then check your answers below each one.

Question 1. Which method is best for getting sand out of a mixture of sand and water? (Choose from: Filtration, Crystallisation, Simple distillation, Chromatography.)

Answer: Filtration

Working: Filtration separates an insoluble solid from a liquid. The sand is left in the filter paper.

Question 2. True or false? In a mixture, the substances are chemically combined.

Answer: False

Working: False. In a mixture the substances are not chemically combined, so they can be separated by physical methods.

Question 3. Which of these is a pure substance in chemistry? (Choose from: Sea water, Air, Distilled water, Orange juice.)

Answer: Distilled water

Working: Distilled water contains only water molecules. Sea water, air and orange juice are mixtures.

Related lessons

Continue in the full course

This lesson is section 1.3 (AGCCH0103) of our free GCSE Chemistry course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Chemistry course.

 

 

 

 

 

 

 

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