Lesson focus: crystallisationGetting solid crystals out of a solution by letting the liquid evaporate slowly, like making salt from salty water. recovers soluteThe substance that dissolves, such as sugar in tea.; distillationSeparating a liquid from a mixture by boiling it and then cooling the gas back into a liquid. collects a volatile solventThe liquid that a substance dissolves in, such as water..
Compare evaporationWhen a liquid slowly turns into a gas from its surface, like a puddle drying up., crystallisation and simple distillation and explain apparatus design.
Build on earlier learning
FiltrationContinue separating the salt-solution filtrate.
You will be successful when you can…
- Compare evaporation and crystallisation.
- Trace solvent through distillation.
- Explain thermometer placement.
- Label condenser-water direction and open receiver.
Words that carry the science
- evaporation
- Surface vaporisation that removes solvent.
- crystallisation
- Controlled formation of solute crystals from solution.
- distillation
- Vaporisation followed by condensation to collect a component.
- condenser
- Device cooling vapour into liquid.
- distillate
- Condensed liquid collected after distillation.
Choose whether the desired product is solute or solvent
01Recovering solute
Concentrate solution by gentle heating, then cool so excess solute crystallises. Filter and dry crystals; heating to dryness may damage some solutes.
02Collecting solvent
In simple distillationSeparating a liquid from a mixture by boiling it and then cooling the gas back into a liquid., volatile solvent vaporises, enters the condenserA cooled glass tube that turns gas back into liquid during distillation., cools and is collected as distillateThe pure liquid collected at the end of distillation.; non-volatile salt remains. In simple steps: heat the salt water so the water boilingWhen a liquid turns to gas all the way through, with bubbles forming inside it. and turns into steam. The salt cannot turn into a gas, so it stays in the flask. The steam goes into the condenserA cooled glass tube that turns gas back into liquid during distillation., which is cold, so it turns back into liquid water. The pure water drips into a beaker.
03Apparatus details matter
Thermometer bulb sits at the side-arm entrance to measure vapour. Cooling water enters the lower condenser connection so the jacket fills. Receiver must not seal the system.
Simple distillation
Heat the mixture: the water boils, the steam cools and condenses in the condenser, and pure water drips into the beaker. The salt or dye is left behind.
Steam (gas) Water (liquid)
Controls
Rates are illustrative. The thermometer bulb is level with the side arm, so it reads the temperature of the steam going into the condenser.
Choose a method
You need pure water and salt from salt solution.
- Use simple distillation to vaporise water.
- Measure vapour temperature at the side arm.
- Condense and collect water distillateThe pure liquid collected at the end of distillation..
- Recover salt from concentrated flask residue by crystallisationGetting solid crystals out of a solution by letting the liquid evaporate slowly, like making salt from salty water. and drying.
How to make the reasoning visible: One setup can recover solvent, then solute.
Separation choice
Decide which product matters.
| Goal | Method | Collected product | Key control |
|---|---|---|---|
| Dry salt crystals | Crystallisation | Salt | Do not heat to violent dryness |
| Pure water | Distillation | Water distillate | Condenser cooling |
| Remove a little solvent | Evaporation | Concentrated solution | Gentle heating |
| Both salt and water | Distil then crystallise | Both | Track each fraction |
Why place thermometer at the side arm?
Why does cooling water enter low?
Use the table. Which method would you use to get pure water from salt water? Why?
Check your understanding
Answer from memory, then check against the suggested answer.
What is evaporation?
What is crystallisation?
What is distillation?
What does the condenser do in distillation?
What is the distillate?
Why is an open receiver important?
Why might crystallisation be better than heating to dryness?
Why is the salt left behind when salt water is distilled?
Exam Question Practice
Exam Question Practice
Mark points shown after marking
Answer the parts to explain how simple distillation gets pure water from salt water.
What do you do to the salt water first?
(1)What happens to the water when it is heated?
(1)What happens to the salt?
(1)What happens to the steam in the condenser?
(1)Where does the pure water end up?
(1)Total for this question: 5 marks
Command word: Follow the water from the flask, through the condenser, to the beaker.