Lesson focus: filtrationSeparating an insoluble solid from a liquid by pouring it through filter paper, like sand from water. separates an insoluble solid from a fluid; crystallisationGetting solid crystals out of a solution by letting the liquid evaporate slowly, like making salt from salty water. recovers a dissolved solid by creating a saturated solutionA solution that cannot dissolve any more of the solid at that temperature. and controlled solvent loss.
Select and evaluate filtration and crystallisation methods for contrasting mixtures.
Build on earlier learning
FiltrationYear 7 used filtration. This lesson connects particle size, solubility, yield and crystal purity.
Practical
Filtration and crystallisation
- Filter a mixture to remove the insoluble solid.
- Heat the filtrate until crystals start to form at the edge.
- Leave it to cool and crystallise, then pat the crystals dry.
Record: Observations at each stage.
Safety: Wear eye protection.
You will be successful when you can…
- Identify residue and filtrate.
- Explain why dissolved solute passes.
- Describe safe crystallisation.
- Evaluate purity and yield.
Words that carry the science
- filtration
- Separation using a porous barrier that retains insoluble particles.
- residue
- Solid retained by the filter.
- filtrate
- Liquid and dissolved substances passing through the filter.
- crystallisation
- Formation of solid crystals from a solution.
- saturated solution
- A solution containing the maximum dissolved solute at a stated temperature.
- mother liquor
- Solution remaining around crystals after crystallisation.
Filtration depends on particle size and insolubility
01Filtration depends on particle size and insolubility
Filter paper contains pores. Liquid and dissolved particles pass through as filtrateThe liquid that passes through the filter paper., while an insoluble solid forms the residueThe solid left behind in the filter paper after filtration..
A dissolved solute is not trapped because its particles move with the solvent through the pores. filtrationSeparating an insoluble solid from a liquid by pouring it through filter paper, like sand from water. therefore cannot separate salt from salt solution.
02Crystallisation uses solubility changes
Wear eye protection. Gently evaporate some solvent to concentrate the solution, then allow it to cool so crystals form. Handle hot glassware with tongs or heatproof gloves; filter the crystals, wash with a small amount of cold solvent and dry them.
For copper sulfate, avoid skin or eye contact and keep the solution and washings out of drains and the environment: follow the laboratory disposal procedure. Do not heat to complete dryness. A warm saturated solutionA solution that cannot dissolve any more of the solid at that temperature. can yield crystals as solubility decreases on cooling.
03Yield and purity require trade-offs
Cooling further or evaporating more solvent can increase yield, but rapid formation can trap impurities. Washing improves purity but dissolves and loses some product.
A clear filtrateThe liquid that passes through the filter paper. is not proof that it contains no dissolved substance; clarity only shows an absence of sufficiently large suspended particles.
Filtration and crystallisation
Filtration separates an insoluble solid from a liquid: the filter paper has tiny holes that let water particles and dissolved salt through but stop sand. Crystallisation then gets the salt back by evaporating the water.
Brown: sand grains. Blue: water particles. Purple: dissolved salt.
Controls
Particle sizes are exaggerated. Stop heating when crystals start to form and leave the rest to evaporate slowly: slower evaporation gives bigger crystals.
Separate sand and salt
A beaker contains sand, sodium chloride and water.
- Filter the mixture: sand remains as residue.
- Wash and dry the sand if it is required.
- Gently heat the salt filtrate to concentrate it.
- Cool, filter, wash and dry the sodium chloride crystals.
How to make the reasoning visible: The sequence exploits sand insolubility first, then salt solubility and crystallisation.
Separation investigation
Choose the key outcome at each stage of an impure crystal preparation.
| Stage | Material kept | Material removed | Reason |
|---|---|---|---|
| first filtration | solution | insoluble impurity | solid cannot pass pores |
| partial evaporation | concentrated solution | some solvent vapour | approach saturation |
| cooling | crystals | solute leaves solution | lower solubility |
| cold wash | crystals | surface mother liquor | improve purity |
Why use a small amount of cold wash solvent?
Why not discard the first filtrate?
Check your understanding
Answer from memory, then check against the suggested answer.
Where is residue found?
What is filtrate?
Why cool a concentrated solution?
A student obtains very few crystals but much solution remains. Suggest an improvement.
Why can slower cooling produce purer, larger crystals?
Exam Question Practice
Exam Question Practice
Mark points shown after marking
Excess insoluble copper oxide has been reacted with dilute sulfuric acid. The mixture contains unreacted copper oxide and copper sulfate solution.
Describe how to remove the unreacted copper oxide.
(2)Describe how to obtain pure, dry crystals from the filtrate.
(4)Total for this question: 6 marks
Command word: For describeA command word: say what happens or what something is like, step by step, without needing to give reasons., give the steps in order and make clear which fraction is retained.