Separating Mixtures and Solutions
Separating mixtures and solutions relies on knowing what a substance is made of. A pure substance contains only one type of particle and melts and boils at fixed temperatures. A mixture contains two or more substances that are not chemically joined, so it can be separated by physical means and it melts and boils over a range. In a solution the solute dissolves in the solvent; the solution is saturated when no more solute will dissolve at that temperature. Solubility usually rises as temperature rises. The separation method chosen depends on what the mixture is made of. Filtration separates an insoluble solid from a liquid, leaving the residue in the filter paper and the filtrate underneath. Evaporation and crystallisation recover a dissolved solid. Simple distillation recovers the solvent from a solution, because the solvent evaporates, passes through a condenser and is collected as the distillate. Fractional distillation separates two or more liquids with different boiling points. Chromatography separates dissolved substances by how strongly they are attracted to the paper compared with the solvent.
Method
- Identify what the mixture contains before choosing anything: solid in liquid, dissolved solid in liquid, two liquids, or dissolved dyes.
- Match the mixture to the method. Insoluble solid in liquid means filtration. Dissolved solid you want to keep means evaporation or crystallisation. Liquid you want to keep means simple distillation. Two liquids means fractional distillation. Coloured dissolved substances means chromatography.
- Use the right words for what you collect. In filtration the solid left behind is the residue and the liquid that passes through is the filtrate. In distillation the liquid collected is the distillate.
- Explain a method through the particles: in distillation the solvent particles gain enough energy to evaporate, travel to the condenser, lose energy to the cold surface and condense back to a liquid.
- For chromatography, remember that the substance that travels furthest is the most soluble in the solvent and the least strongly attracted to the paper.
- Calculate an Rf value as the distance moved by the spot divided by the distance moved by the solvent front. The answer is always less than 1 and has no units.
Worked example
A student has a beaker containing sand, salt and water. Describe how to obtain a dry sample of sand and a dry sample of salt from this mixture, and explain each step.
- Work out what state each part is in: sand is an insoluble solid, salt is dissolved in the water, and water is the solvent.
- Filter the mixture. The sand is too big to pass through the pores in the filter paper, so it stays as the residue; the salt solution passes through as the filtrate.
- Wash the sand in the filter paper with a little distilled water to rinse off any salt solution clinging to it, then leave it in a warm oven to dry.
- Take the filtrate and heat it gently in an evaporating basin. The water particles gain energy and evaporate, leaving the salt behind.
- Stop heating when small crystals first appear at the edge and leave the rest to evaporate slowly at room temperature. Heating a solution dry can make the crystals spit out of the basin and can decompose some solids.
- Pat the crystals dry between filter papers. You now have dry sand and dry salt crystals.
Practice questions
Try each question, then tap to reveal the answer.
Q1Explain how you can tell from a melting point whether a substance is pure.Show answer
Answer: A pure substance melts at one fixed temperature. A mixture melts over a range of temperatures, so a range indicates an impurity.
Q2Name the residue and the filtrate when a mixture of chalk and water is filtered.Show answer
Answer: The residue is the chalk left in the filter paper. The filtrate is the water that passes through.
Q3Why does filtration fail to separate salt from salt solution?Show answer
Answer: The dissolved salt particles are far smaller than the pores in the filter paper, so they pass straight through with the water.
Q4In simple distillation, explain what happens to the particles in the condenser.Show answer
Answer: The vapour particles hit the cold surface of the condenser, lose energy, slow down and are pulled close together, so the vapour condenses back into a liquid.
Q5A spot of dye travels 4.5 cm while the solvent front travels 9.0 cm. Calculate the Rf value.Show answer
Answer: Rf = 4.5 / 9.0 = 0.5
Q6Two dyes are run on the same chromatogram. Dye X moves further than dye Y. What does this tell you about the two dyes?Show answer
Answer: Dye X is more soluble in the solvent and less strongly attracted to the paper than dye Y, so it is carried further up the paper.
Q7Explain why the pencil line on a chromatogram must be drawn in pencil rather than pen, and why it must stay above the solvent level.Show answer
Answer: Pencil is insoluble so it does not run and contaminate the results, while ink from a pen would separate into its own colours. If the line were below the solvent level, the samples would dissolve into the solvent in the beaker instead of travelling up the paper.
Exam-style questions
Written in the style of a KS3 Science exam paper, with a full mark scheme.
A student is given a sample of sea water and asked to produce pure water from it. (a) Name the separation technique required. (b) Describe the apparatus and the steps, explaining what happens to the particles at each stage. (c) State one way the student could check that the water collected is pure.
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Four food colourings, A, B, C and D, are spotted onto chromatography paper alongside an unknown green sweet coating. After the run, the solvent front is at 8.0 cm. Colouring A has a spot at 2.0 cm, B at 6.0 cm, C at 4.0 cm and D at 6.0 cm. The sweet coating gives two spots, at 2.0 cm and 6.0 cm. (a) Calculate the Rf value of colouring C. (b) Explain what the chromatogram shows about the sweet coating, and explain why the result is not conclusive.
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Free printable worksheet
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