Mixtures and separation · Process
Evaporation and crystallisation
The salt is dissolved and invisible. How do you get it back, and does the way you do it matter?
Start here
Same solution, same salt, two dishes. One took four minutes and one took a week.
Both dishes started with 20 cm³ of the same salt solution. One was boiled over a Bunsen until it was dry. The other sat on a windowsill until the water had gone. Both ended up with exactly the same mass of salt.
If you had to guess, which dish has the bigger crystals in it?
Fair guess. Heat speeds everything up. But a fast boil starts thousands of tiny crystals at once, and none of them gets big.
Good guess. Slow evaporation lets the salt particles join up in an orderly way. A few crystals start, and they grow big.
A big, clean crystal is also good evidence that you have one substance, not a dried-out puddle of everything. Next: three ways of removing the water to try for yourself.
Evaporation removes the solvent: it leaves the solution as a gas and the solute stays behind. Crystallisation is what the solute does as it goes — once the solution is saturated, solute particles start joining a growing, regular arrangement, and that arrangement is a crystal.
So the two words are not alternatives. Evaporation is the thing you do; crystallisation is the thing the solute does while you do it. How fast you do it decides what you get.
Your turn · the crystallising bench
Pick a solution and a way of removing the water. Predict, then run it.
The solution in the dish
How you remove the water
Before you run it: what will be left in the dish?
Time taken
What is in the dish
Mass of solute recovered
5.0 g
In the evaporating dish
All three methods, and the mass of solute recovered was the same every time. Only the crystals changed. Speed does not buy you more product — it costs you the quality of it.
Key fact
Evaporating the solvent leaves the solute behind. The slower the solvent goes, the larger and more regular the crystals — and the solvent itself is lost to the air, so this method keeps the solid and throws the liquid away.
Five words
Say your answer out loud before you turn each card over. If you cannot say it, you do not know it yet.
Three jobs · pick the method
Same equipment, three different things wanted
Commit to a method for each, then read what actually happens. One of the three cannot be done this way at all.
A geologist has rock salt dissolved and filtered, and needs the salt back today to weigh it.
Warm it gently — or boil it, if all you need is the mass.
Nobody is looking at these crystals; the answer is a number on a balance. Speed is free here, and a crust weighs the same as a cube.
A student wants one large, sharp copper sulfate crystal to keep.
Leave it to evaporate slowly. Days, not minutes.
Big crystals are made of time. Boiling gives a powder, and heating a dry dish of copper sulfate turns it white and ruins it.
A village needs the fresh water out of a barrel of sea water. The salt can be thrown away.
Not this method at all. Evaporation keeps the solid and loses the liquid.
Here it is the water that is wanted, and evaporation sends the water into the air. You need to catch it as it comes off — which is distillation, in the next lesson.
Think again
“The water has gone. It evaporated, so it is not anywhere any more.”
The dish is dry and you can see it is dry. Commit before you read on.
The water is in the room. Its particles left the surface one at a time and joined the air, and they are still water — the same particles, the same mass, spread out where you cannot see them. Hold something cold above a warm dish and they come back as liquid on it, which is the whole of the next lesson.
This is the same wrong idea as "the puddle dried up and the water was destroyed", met in the particle model and met again here. Evaporation is a change of state and a change of place. It destroys nothing. The only reason it feels like a loss is that this method deliberately throws the solvent away.
Mastery ladder
Not started yet.
Rungs 3 and 4 you mark yourself.
Rung 1 · Recall
A salt solution is left in an open dish until the dish is dry. What is left, and what happened to the rest?
Rung 2 · The one that catches people
Two students crystallise identical solutions. One boils hers dry in four minutes; the other leaves his for a week. What is the difference in what they get?
Rung 3 · Explain
Explain, in terms of particles, why slow evaporation produces larger crystals than fast boiling — and why the mass of solid recovered is the same either way.
Rung 4 · Take it somewhere new
A coastal village can boil sea water over a wood fire but has no other equipment. They need drinking water. Explain why evaporating the sea water in an open pan is exactly the wrong thing to do, and what the smallest change to their method would be.
Key note
Evaporation removes the solvent as a gas and leaves the solute behind; crystallisation is the solute coming out of a saturated solution into a regular arrangement. Fast boiling starts thousands of crystals at once and gives a fine crust; slow evaporation starts a few and grows them large. The mass recovered is the same either way — and the solvent is lost, so this method is only for keeping the solid.
Going further
Slow crystallisation has been running for far longer than anyone's windowsill. Salt pans around the Mediterranean flood a shallow bed with sea water, let the sun take the water over weeks, and rake up crystals — the same process, at the scale of a field. Under Naica in Mexico, a cave of gypsum crystals grew in mineral-rich water at a steady 58 °C for hundreds of thousands of years; the largest beams are metres long. Nothing exotic happened. It was simply slow for a very long time.
One warning about heating crystals you have just grown. Copper sulfate crystals are blue because water is built into the crystal itself, and heating them hard drives it out and leaves a white powder. The substance is still there, but the crystals are gone and cannot be undone by cooling. It is a good reason to stop heating a dish while there is still liquid in it and let the last of it go on its own.
Before this lesson
Next in this unit
At GCSE this becomes
- Crystallisation as a required practical step in making a soluble salt, and hydrated versus anhydrous compounds.
Where to next
Ask MrBadmus
Still not sure why slow evaporation gives bigger crystals?
Evaporating to dryness over a naked flame spits hot solution. Safety glasses, a gentle flame, and take the heat away while there is still liquid in the dish rather than baking it dry.
Lesson content © MrBadmus.