Separating Mixtures
Required product → different physical property → method and collection point.
Continue where you stopped
The core idea
Food Chemistry · Lesson 2 of 5 · about 25–35 min
What you need to understand
Required product → different physical property → method and collection point.
Definitions
- dissolving
- mixing a soluble substance with a solvent to form a solutionFor example: Water can dissolve salt while sand remains insoluble.
- filtration
- separating an insoluble solid from a liquid using a filterFor example: Sand remains as residue while water passes as filtrate.
- evaporation
- removing a liquid solvent as vapourFor example: Evaporation can leave dissolved salt behind.
- distillation
- boiling a liquid and condensing its vapour so the liquid can be collectedFor example: Distillation can collect water from seawater.
- chromatography
- separating mixture components because a solvent carries them different distancesFor example: Ink dyes can form separate spots on paper.
Key idea
- LookA learner must obtain liquid water and dry salt from salt solution.
- ThinkRequired product → different physical property → method and collection point.
- DoName the desired component and use its physical properties to choose and sequence separation methods.
Explanation
Start by naming what you want to collect. Salt dissolves in water but sand does not, so adding water moves salt into a solution while sand remains solid. Filtering retains sand as the residue; salt solution passes through as the filtrate. Dissolved salt has not disappeared and passes through ordinary filter paper.
To collect salt, evaporate the water under suitable supervision. To collect the water instead, use distillation: boil water into vapour, then cool and condense it into a receiver. Simple evaporation lets the vapour escape. Paper chromatography separates soluble dyes because they move different distances with the solvent through the paper; compare standards under the same conditions.
Pause and say it: Required product → different physical property → method and collection point.
Common mistake
Tempting wrong idea: Dissolving makes a substance cease to exist.
Why it fails: Filtration cannot recover dissolved salt or separate dissolved dyes. Distillation needs condensation to collect liquid water.
Use this instead: Dissolving distributes a soluble substance through a solvent; it remains present.
Practical work
What to show: Name the desired component and use its physical properties to choose and sequence separation methods.
Before you finish: Filtration cannot remove dissolved salt.
Practical: Identify dyes in a coloured food mixture
Which known food dyes are present in an unknown soluble mixture?
Safety: Use only approved food dyes and solvent, wear eye protection, keep the beaker covered while it runs and do not taste any sample.
- Change
- food-dye sample placed on the baseline
- Measure
- number, colour and distance of separated spots
- Keep the same
- same solvent, same pencil baseline, same spot size, same solvent-front distance
View the apparatus, method and sample results
Apparatus
chromatography paper, pencil and ruler, capillary tubes or clean spotting sticks, covered beaker, safe food-dye standards, unknown food-dye mixture, water.
Method
- Draw a pencil baseline near the bottom of the paper and label positions for each standard and the unknown.
- Use a clean capillary for each sample to place small, concentrated spots on the baseline.
- Stand the paper in a shallow layer of solvent with every spot above the solvent level.
- Cover the beaker and let the solvent rise; remove the paper before it reaches the top and mark the solvent front immediately.
- Compare the unknown's spot colours and positions with standards run on the same paper.
| Sample | Spot distance / cm | Solvent distance / cm | Observation |
|---|---|---|---|
| red standard | 3.0 | 6.0 | one red spot |
| blue standard | 4.5 | 6.0 | one blue spot |
| mixture | 3.0 and 4.5 | 6.0 | red and blue spots |
Limitation: Similar spot positions alone may not uniquely identify an unknown substance.
Improvement: Run known standards beside the sample under identical conditions and repeat with another solvent if needed.
Worked Examples
Modelled example 1
Recover salt and water
Problem
A learner must obtain liquid water and dry salt from salt solution.
Study the worked solution
Reason from the evidence
Method
Start from the instruction to collect both substances, then reject methods that discard either one.
Reason
Name what must be collected, then choose the property—solubility, boiling or movement—that separates it.
Working
- Filtration cannot remove dissolved salt.
Evaporation alone leaves salt but does not collect the water vapour.
Distil most of the water into a receiving container, then stop before the flask boils dry.
Under teacher supervision, evaporate the small amount of water left with the salt.
State the conclusion
Working
Distillation collects the water; controlled evaporation of the concentrated solution then recovers the dry salt.
Guided practice 2
Obtain dry salt from salt and sand
Problem
Describe how to obtain dry salt from a mixture of salt and sand using water, filtration and evaporation.
Put the separation stages in order
Hints
Hint 1: use solubility first
Move only the salt into water.
Hint 2: name residue and filtrate
Decide where sand and salt solution are after filtering.
View solution step by step
Dissolve and filter
Method
Add water and stir, then filter the mixture.Reason
Salt dissolves while sand remains insoluble.Working
Sand stays as residue; salt solution passes through as filtrate.
Recover the salt
Method
Heat the filtrate gently to evaporate the water.
Reason
The required product is the dissolved solid rather than the solvent.
Working
Water leaves as vapour and dry salt remains.
Common misconception 3
Can filter paper trap dissolved salt?
Learner response
“Filter salt solution and the salt crystals will remain on the paper.” Spot and correct the error.
Explain what passes through
View solution step by step
Use the dissolved state
Method
State that dissolved salt travels through the filter with the water.
Reason
Filtration separates an insoluble solid from a liquid; it does not remove a dissolved substance.
Working
Use evaporation for salt, or distillation when the water must also be collected.
Examiner practice 4
Separate dyes by paper chromatography
Examination question
Describe how to use paper chromatography to separate the soluble dyes in a drink. Include two method details that prevent an invalid result. [4 marks]
Write a four-mark method
View solution step by step
Prepare the paper
2 marksMethod
Draw a pencil baseline and place a small sample spot on it.
Reason
Pencil does not dissolve to add extra spots, and a small spot gives clearer separation.
Working
Use pencil; let the spot dry if another small drop is added.
Run and record
2 marksMethod
Stand the paper with the baseline above the solvent, allow the solvent to rise, then mark its front.
Reason
A submerged sample washes into the solvent, while an unmarked solvent front may disappear during drying.
Working
Remove before the solvent reaches the top and observe the separated spots.
Self-mark with the mark scheme
Compare your response with each mark point. Select a point only when your response contains that evidence.
Self-mark pencil, sample placement, solvent position and observation.
Challenge 5
Recover water and an insoluble herb
Problem
A food mixture contains an insoluble herb, water and a dissolved colouring. The herb and water must be collected, and the colouring only needs to be identified. Choose a sensible sequence of methods and explain each choice.
Work backwards from the required products
Hints
Hint 1: remove the insoluble solid
Begin with the property that distinguishes the herb.
Hint 2: treat the filtrate by purpose
Collect the solvent with distillation; identify soluble colours with chromatography on a separate sample.
View solution step by step
Collect the herb
Method
Filter the mixture.Reason
The herb is insoluble and remains as residue.Working
Wash and dry the residue if a clean dry herb sample is required.
Collect water and identify colouring
Method
Distil part of the filtrate to collect water and run paper chromatography on a small separate sample.
Reason
Distillation recovers the solvent; chromatography separates or identifies dissolved colour components.
Working
Do not use filtration again for the dissolved colouring.
Guided practice
Try it with support
A mixture contains salt and sand. Describe how to obtain dry salt using dissolving, filtration and evaporation.
- Use water to separate soluble from insoluble material.
- Name residue and filtrate.
Check the guided answer
Answer: Add water to dissolve the salt, filter so sand remains as residue and salt solution passes as filtrate, then evaporate water from the filtrate to obtain salt.
Check: Use distillation instead of simple evaporation if the water must also be collected.
Practise and continue
Practise this
Choose a method to collect each product: tea without leaves, pure water from seawater, and the different dyes in a drink.
Need a hint?
- Decide what must be collected before naming the method.
Check your answer
Answer: Use filtration for tea without leaves, distillation for water from seawater and paper chromatography to separate the dyes.
Check: Filtration cannot remove dissolved salt.
Think like a scientist
Why must a chromatography baseline be drawn in pencil above the solvent?
Check the reasoning
Pencil graphite does not dissolve and add moving spots, while keeping the samples above the solvent lets the rising solvent carry them through the paper.
Remember: Mark the solvent front before it evaporates if distances will be compared.
One-minute check
- Hide the page and explain separating mixtures in your own words.
- Give a new example that is different from the worked example.
- Correct this common mistake: “Dissolving makes a substance cease to exist.”