- Define and explain Separating by Evaporating in your own words
- Use key terms such as evaporate accurately
- Apply what you have learned to new examples and questions
- Avoid the common mistakes learners make with this topic
This lesson focuses on Separating by Evaporating: use gentle heat to leave dissolved salt behind when water evaporates.
Use gentle heat to leave dissolved salt behind when water evaporates.
Key ideas
Dissolved is not disappeared
Stir salt into water and the grains seem to vanish, but the salt is still there, spread invisibly through the water — this is dissolving. Taste the water and it is salty, which proves the salt remains. Warm the water gently so it evaporates, and the salt is left behind as crystals.
Choose the method that fits
Each separation method suits a particular mixture: sieving separates solids of different sizes, filtering separates an insoluble solid from a liquid, and evaporation recovers a dissolved solid from its solution. Scientists look at the properties of the parts — size, solubility, state — and then pick the method. Using the wrong method, like trying to sieve salt water, simply does not work.
Key term — evaporate: To change a liquid into a gas by warming it, leaving any dissolved solid behind.
You stir sugar into tea until you cannot see it. Has the sugar disappeared? How could you get it back?
No, it has dissolved. Evaporate the water and the sugar is left behind.
Answer: No, it has dissolved. Evaporate the water and the sugar is left behind.
- Forgetting that evaporation needs the liquid to escape Evaporation only separates a mixture if the vapour can leave. Warming salt water in a sealed jar just makes it steamy — leave it open so the water vapour escapes and the salt stays behind.
- Trying to filter a dissolved solid Filter paper traps solid lumps but dissolved salt passes straight through with the water. To get dissolved salt back, evaporate the water instead.
Practice
Some salt was dissolved in the water and passed through the filter. Evaporate the water to recover the salt.
The salt is dissolved into pieces far too small to be caught — everything passes through the holes.
Sieving — the sand falls through the holes but the pebbles are too big.
Pour the mixture through filter paper: the sand is trapped and the water passes through.
Quick check
Which of these best defines "evaporate"?
Is a fruit salad a mixture? Explain why.
- Separating by Evaporating: use gentle heat to leave dissolved salt behind when water evaporates.
- Dissolved is not disappeared: Stir salt into water and the grains seem to vanish, but the salt is still there, spread invisibly through the water — this is dissolving.
- mixture: Two or more materials mixed together that are not joined chemically and can be separated again, such as sand and water.
- Watch out for: forgetting that evaporation needs the liquid to escape