Recovering solids and liquids from mixtures
Use dissolving, filtration, evaporation and distillation to collect the required solids or liquids.
On this page
Food Chemistry · 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 solution
For example: Water can dissolve salt while sand remains insoluble.- filtration
separating an insoluble solid from a liquid using a filter
For example: Sand remains as residue while water passes as filtrate.- evaporation
removing a liquid solvent as vapour
For example: Evaporation can leave dissolved salt behind.- distillation
boiling a liquid and condensing its vapour so the liquid can be collected
For example: Distillation can collect water from seawater.
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. If both water and dry salt are needed, distil most of the water, stop before the flask boils dry, then evaporate the remaining water under teacher supervision.
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: Dissolved salt is still present in the water. You can recover it by evaporating the water. Dissolving changes where the salt is distributed; it does not make the salt cease to exist.
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.
The complete dye investigation is now in paper chromatography
Worked Examples
Modelled example 1
Recover salt and water
Problem
A learner must obtain liquid water and dry salt from salt solution. Choose a method sequence that collects both safely.
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.
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, water from seawater, and dry salt from salt solution. State where each product is collected.
Need a hint?
Decide what must be collected before naming the method.
Check your answer
Answer: Filtration collects liquid tea as the filtrate; distillation collects water as the condensed liquid in a receiver; evaporation leaves dry salt in the evaporating container.
Check: Filtration cannot remove dissolved salt.
Explore why the baseline matters in the chromatography lesson
One-minute check
How does dissolving help you separate salt from sand, and where is each substance after filtration?
Why is condensation needed to collect water from salt solution?
Correct this common mistake: “Dissolving makes a substance cease to exist.”
The full four-mark method question is now in paper chromatography
After learning chromatography, try the complete multi-stage separation challenge
Syllabus and review details
- SEC G1 Science 2027 · 2027
Content structure and syllabus content, PDF pages 8–17
Last reviewed: