Mixtures and separation · Process
Distillation
Evaporation throws the water away. What if the water is the thing you cannot live without?
Start here
You are on a life raft, surrounded by water, and dying of thirst.
You have a pan, a fire, a sheet of clear plastic and a cup. The sea is right there. Drinking it makes things worse, and filtering it does nothing at all — you proved that two lessons ago.
What do you do with the pan, the plastic and the cup?
Boil the sea water in the pan and hold the plastic above it, tilted, with the cup under the low corner. The water leaves the sea water as a gas; the salt cannot. Cool the gas on the plastic and it turns back into liquid water — fresh water — and runs down into the cup. Every desalination plant on Earth is a large, expensive version of that.
Distillation is evaporation with the vapour caught. Boil the solution: the solvent leaves as a gas and the dissolved solid stays behind, because it does not boil at anything like that temperature. Cool the gas on a cold surface and it condenses back to a liquid — and that liquid is the solvent on its own.
Evaporation keeps the solid. Distillation keeps the liquid. Same physics, opposite purpose.
Your turn · run the still
Pick a mixture, decide about the cooling water, then run it a stage at a time.
In the flask
Cold water through the condenser
Before you heat it: what comes out of the condenser?
Before you heat it: what colour is the liquid in the beaker?
Before you heat it: which liquid comes over first?
Thermometer at the side arm
20 °C
In the beaker
empty
Left in the flask
Sea water
Stage 1
The solution boils. The thermometer settles at just over 100 °C — a little above pure water, because of the dissolved salt, and it creeps up as the flask gets saltier.
Stage 2
Water leaves the solution as a gas and travels down the side arm. The salt cannot: it has no way of becoming a gas at this temperature, so it stays in the flask.
Stage 3
The vapour meets the condenser, whose outer jacket is full of cold water flowing the other way. It cools and condenses back to a liquid.
With no cold water in the jacket, the condenser is as warm as the vapour. Nothing condenses. The vapour goes straight out of the open end and into the room — you can smell it, and you will not collect it.
Stage 4
Clear drops run into the beaker. That is fresh water, and the flask now holds a stronger salt solution — eventually dry salt.
Nothing arrives in the beaker. The water left the flask as a gas, passed a condenser as warm as itself, and is now somewhere in the room. The flask is getting saltier all the same — the separating worked, the collecting never happened.
Stage 1
The ink boils at 100 °C. Ink is mostly water with a dye dissolved in it, and the dye is not going anywhere.
Stage 2
Water vapour comes off, colourless. The dye stays behind and the ink in the flask gets darker as it concentrates.
Stage 3
The vapour condenses in the cold jacket of the condenser.
With no cold water in the jacket, the condenser is as warm as the vapour. Nothing condenses. The vapour goes straight out of the open end and into the room — you can smell it, and you will not collect it.
Stage 4
Colourless drops collect in the beaker. The dye is a ring of solid left in the flask.
The beaker stays dry. The water has gone off into the room as a gas and the flask holds a darker, more concentrated ink. The dye never travelled — and this time neither did anything else, as far as you are concerned.
Stage 1
The mixture starts to boil at about 78 °C — the boiling point of ethanol, not of water.
Stage 2
The vapour coming off is mostly ethanol. Water is still in the flask, because 78 °C is not hot enough to boil it.
Stage 3
The vapour condenses in the condenser and the first drops arrive in the beaker.
With no cold water in the jacket, the condenser is as warm as the vapour. Nothing condenses. The vapour goes straight out of the open end and into the room — you can smell it, and you will not collect it.
Stage 4
The thermometer starts to climb towards 100 °C. From now on what is coming over is mostly water — so this is the moment to change the beaker.
The thermometer climbs towards 100 °C with an empty beaker underneath it. The ethanol came off first exactly as it should have, and every drop of it went out of the open end as a vapour instead.
Fresh water in the beaker, salt in the flask.
Boiling separated them because only one of the two can become a gas. Cooling collected the one that did. Evaporate the distillate to dryness and nothing at all is left behind — which is exactly the point.
Colourless water out, dye left behind.
A dye that made the whole liquid blue turns out to be a solid dissolved in water. Distillation takes the water away from it, and the colour never travels.
Mostly ethanol first, then mostly water.
Two liquids, two boiling points, and the thermometer tells you which one is coming over. Mostly, not purely: a single still cannot fully separate them, which is what a fractionating column is for.
An empty beaker and a room that smells of it.
Boiling separated the mixture perfectly well — the flask proves that. Without cooling there was nothing to turn the vapour back into a liquid, so the separated substance left the apparatus and you have none of it. Distillation is two jobs: boil to separate, cool to collect. Doing one of them gets you nothing.
Key fact
Distillation separates a liquid from what is dissolved in it, or two liquids with different boiling points. Boil to separate, cool to collect — and the thing you keep is the one that came over as a gas.
Four words
Say your answer out loud before you turn each card over. If you cannot say it, you do not know it yet.
Think again
“Steam off boiling sea water tastes salty, so some salt must come over with it.”
Sea spray really does taste of salt, and a badly run still really does give a salty distillate. Commit before you read on.
There are two different things coming off a boiling pan and only one of them is a gas. Water vapour is single water particles, and salt cannot travel in it — the salt has no way to become a gas at 100 °C. What you can see above the pan is not vapour at all: it is tiny droplets of liquid, and a droplet of sea water is sea water, salt and all.
So the taste is real and the conclusion is wrong. Boil too hard and the mixture bumps, throws droplets up the neck of the flask, and they run into your distillate and make it salty. That is a fault in the method, not a property of steam — and it is why you heat gently, use anti-bumping granules, and never fill the flask more than half full.
Mastery ladder
Not started yet.
Rungs 3 and 4 you mark yourself.
Rung 1 · Recall
Sea water is distilled. What is collected in the beaker, and what stays in the flask?
Rung 2 · The one that catches people
A student distils sea water, evaporates their distillate to dryness, and finds a faint white residue. What is the most likely reason?
Rung 3 · Explain
Explain how distillation separates salt from water. Your answer must use boiling points and say where each substance is at the end.
Rung 4 · Take it somewhere new
You have a pan with a lid, a fire, a cup and no other equipment, and you need fresh water from sea water. Describe your still, say exactly where the cold surface has to be, and explain the one mistake that would make your water salty.
Key note
Distillation separates a dissolved solid from its solvent, or two liquids with different boiling points. Boil the mixture: the substance with the lower boiling point leaves as a gas. Cool that gas in a condenser and it condenses back to a liquid — the distillate. The solid stays in the flask because it cannot become a gas. Boil too hard and droplets are carried over, which is a fault in the method, not a property of steam.
Going further
Two liquids can be separated the same way if their boiling points differ. Heat a mixture of ethanol and water and the vapour coming off at 78 °C is mostly ethanol; once the thermometer climbs towards 100 °C, what is coming over is mostly water. Mostly, not purely — a single pass gives you an enriched mixture, and getting further needs a fractionating column, which is a still with the same trick repeated dozens of times up its length. A crude oil refinery is a column tens of metres tall, drawing off petrol near the top and bitumen at the bottom.
Distillation is honest about its cost. Boiling a tonne of sea water takes an enormous amount of energy, which is why most modern desalination plants avoid boiling altogether and force sea water through a membrane instead — around three to four kilowatt-hours per cubic metre, several times less than a thermal still. On a life raft you have no membrane and no fuel either, so the emergency kit contains a solar still: a black tray, a clear cover, and the sun doing the boiling slowly.
Before this lesson
Next in this unit
At GCSE this becomes
- Fractional distillation of crude oil and of liquid air, and distillation as a required practical.
Where to next
Ask Mr Badmus AI
Still not sure why the salt cannot come over with the steam?
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