The shape of the lesson
By the end of the lesson, learners can
- Choosing a method: Name the mixture, find the property that differs, then choose.
- Distillation and purity: The two distillations differ in one piece of apparatus: the fractionating column.
- Insoluble solid: filtration. Soluble solid: crystallisation.
- Solvent from solution: simple distillation.
- Miscible liquids: fractional distillation.
- Pure: sharp, fixed m.p. and b.p.; impure: lower m.p. over a range.
One picture every idea returns to
Choose a separation method from the difference between the substances. Filtration separates an insoluble solid from a liquid. Crystallisation obtains a soluble solid from its solution: heat to evaporate some solvent until the solution is saturated, then cool so crystals form. Simple distillation separates a solvent from a solution (for example water from salt water). Fractional distillation separates miscible liquids with different boiling points (for example ethanol and water). Dissolving in a suitable solvent separates a soluble solid from an insoluble one. A pure substance melts and boils at fixed, sharp temperatures; impurities lower the melting point and make it melt over a range, and raise the boiling point. Purity matters for food and drugs.
Forty-five minutes, phase by phase
| Time | Phase | What happens in the room | Grouping |
|---|---|---|---|
| 0 to 5 min | Hook: predict | Open with the question in the title, Pull the mixture apart.. Salt from sea water, pure water from ink, ethanol from a fermented mixture: each separation uses a difference in property, whether solubility, particle size or boiling point. Put the figure on the board: it is the one picture every idea in the lesson returns to. | Whole class, then pairs |
| 5 to 17 min | Teach and check: Choosing a method | Name the mixture, find the property that differs, then choose. | Whole class, then pairs |
| 17 to 30 min | Teach and check: Distillation and purity | The two distillations differ in one piece of apparatus: the fractionating column. | Cooperative Concept Mapping |
| 30 to 36 min | Check | The four Stage 1 questions from the student lesson, answered on mini whiteboards or by show of fingers. Re-teach the one most get wrong. | Individual, then whole class |
| 36 to 43 min | Plenary | The exam-style questions from Stage 2, one at a time: thirty seconds alone, then a random call. The feedback lines in the key give the examiner's wording. | Individual, random call |
| 43 to 45 min | Exit and homework | Set the worksheet below (6 questions, 14 marks). Learners self-assess against the objectives before they leave. | Individual |
Protect the parts that carry the learning
The sequence carries about fifty minutes of material, so the lab is driven by one student while the class predicts rather than explored freely. Times are a guide.
Protected: the check at 30 minutes. It is the only individual read on whether the key idea landed, and it is not cut.
Cut first if the build overruns: the last check in Distillation and purity, never the check or the plenary.
In a 60 minute block: run worksheet questions 1 to 3 in class as a Rally Coach set and model both the hardest worked example and its check live.
Cooperative Concept Mapping
Teams receive the key terms on cards and arrange them on a large sheet, drawing and labelling the links. Compare maps between teams before showing the model map.
The full structure, with its variations and the other twelve, is on the cooperative-learning page.
What to head off, and how
| Trap learners fall into | Teaching move that pre-empts it |
|---|---|
| Do not heat a solution to dryness when crystallising a hydrated salt: you lose the water of crystallisation and may decompose it. | Condenser water goes in at the bottom so the jacket stays full. Impurities lower the melting point (and widen the range) but raise the boiling point. |
Separation and purification: worksheet
6 questions, 14 marks. Written in the style of the papers; no past paper question is reproduced.
Describe how to obtain dry crystals of copper(II) sulfate from copper(II) sulfate solution.
Describe how to separate a mixture of salt and sand to obtain dry salt.
In simple distillation, explain why the thermometer bulb is placed level with the side arm of the flask.
A mixture of ethanol (b.p. 78 °C) and water (b.p. 100 °C) is fractionally distilled. Describe what the thermometer shows during the distillation.
Pure compound Q melts at 122 °C. A sample of Q melts between 115 and 119 °C. What does this show? Explain.
Explain why it is important that the substances in medicines are pure.
Answer key and marking notes
- Heat the solution to evaporate some water until it is saturated (crystals form on a cold glass rod).
- Leave it to cool so that crystals form.
- Filter off the crystals, wash with a little cold water and dry them between filter papers.
- Add water and stir so the salt dissolves.
- Filter: sand is left as the residue and salt solution passes through as the filtrate.
- Evaporate the filtrate and crystallise the salt (then dry it).
- It measures the temperature of the vapour passing into the condenser.
- This is the boiling point of the substance being collected.
- It stays at about 78 °C while ethanol distils over.
- It then rises to 100 °C when the ethanol has gone and water starts to distil.
- The sample is impure.
- Impurities lower the melting point and make the substance melt over a range rather than at a sharp temperature.
- Impurities could be harmful (toxic) to the patient.
- The dose of active substance must be known exactly, so the drug works as intended.
8 multiple-choice questions on this lesson
From the topic question bank. Assign them in the app for a marked score in your gradebook, or run the topic as a class game.
Which method is used to obtain pure water from sea water?
- Asimple distillation
- Bfiltration
- Ccrystallisation
- Dchromatography
Which is the best method to separate a mixture of two miscible liquids with boiling points of 65 °C and 98 °C?
- Afractional distillation
- Bfiltration
- Ccrystallisation
- Dsimple distillation followed by filtration
Pure benzoic acid melts at 122 °C. Four samples are tested. Which sample is most likely to be pure?
- Amelts sharply at 122 °C
- Bmelts from 116 °C to 120 °C
- Cmelts from 118 °C to 122 °C
- Dmelts at 110 °C
Copper(II) sulfate crystals are made from copper(II) sulfate solution. What is the correct order of steps?
- Aheat until saturated, leave to cool, filter off crystals, dry
- Bfilter, heat to dryness, cool, dissolve in water
- Ccool, filter, heat until saturated, dry
- Dheat to dryness, filter, cool, dry
A student wants a dry sample of salt from a mixture of salt and sand. Which method should be used?
- Aadd water, stir, filter, then evaporate and crystallise the filtrate
- Badd water, stir, filter, then dry the residue
- Cheat the mixture strongly until the salt melts and pour it off
- Dfractionally distil the mixture
The diagram shows apparatus for fractional distillation. Which label shows the part that allows the liquids to separate by repeated evaporation and condensation?
- AA
- BB
- CC
- DD
A mixture of ethanol (b.p. 78 °C) and water (b.p. 100 °C) is fractionally distilled. How does the thermometer reading change?
- Ait stays at about 78 °C while ethanol distils, then rises to 100 °C
- Bit rises steadily from 78 °C to 100 °C while one mixed liquid distils
- Cit stays at 100 °C throughout
- Dit stays at 89 °C, the average of the two boiling points
Pure water boils at 100 °C. A sample of water boils at 102 °C at normal pressure. What does this show?
- Athe sample contains a dissolved impurity
- Bthe sample is pure water
- Cthe sample has been distilled
- Dthe thermometer must be broken, because impurities lower boiling points
Quiz key
In simple distillation the water boils off, leaving the salt behind, and the steam is condensed to pure water. Filtration cannot remove dissolved salt. Crystallisation keeps the salt and lets the water evaporate away, so the water is lost. Chromatography separates small amounts of dissolved substances for analysis, not large volumes of solvent.
Miscible liquids with different boiling points are separated by fractional distillation: the liquid boiling at 65 °C distils first, then the temperature rises. Filtration only removes insoluble solids. Crystallisation obtains a solid from solution. Simple distillation does not separate two volatile liquids cleanly, and filtration adds nothing.
A pure substance melts sharply at its fixed melting point, so the sample melting at exactly 122 °C is likely to be pure. Impurities lower the melting point and spread it over a range, so the samples melting over 116–120 °C and 118–122 °C are impure, and the one melting at 110 °C has a much lowered melting point.
The solution is heated to evaporate some water until it is saturated, then left to cool so crystals form; they are filtered off and dried. Heating to dryness drives off the water of crystallisation and does not give hydrated crystals, ruling out two options. Cooling before concentrating the solution gives few or no crystals to filter.
Salt dissolves in water and sand does not, so after filtering the salt is in the filtrate; evaporating and crystallising the filtrate gives the salt. Drying the residue gives sand, not salt. Salt melts at about 800 °C, which is impractical and unsafe. Fractional distillation separates liquids, not two solids.
A is the fractionating column, packed with glass beads that give a large surface for vapour to condense and evaporate repeatedly, so the more volatile liquid reaches the top. B is the thermometer, which shows the boiling point of the vapour leaving the column. C is the condenser, which turns vapour back into liquid. D is the distillate collected in the beaker.
Ethanol, with the lower boiling point, reaches the top of the column first, and the thermometer stays at about 78 °C until it has distilled; the temperature then rises to 100 °C as water distils. A steady rise would mean the liquids were not separating. It cannot stay at 100 °C while ethanol is distilling. Boiling points are not averaged.
Dissolved impurities raise the boiling point, so water boiling at 102 °C contains a dissolved substance such as salt. Pure water boils at exactly 100 °C at normal pressure. Distillation removes dissolved solids, so distilled water would boil at 100 °C. Impurities lower melting points but raise boiling points, so the reading is not a sign of a faulty thermometer.
Slides for this lesson
A projectable deck for every pack is on the way. Until then, present the student lesson full screen: its sections, figure and lab tabs follow the same order as the plan above.