KS3 · Chemistry

Separating a soluble solid from a liquid (evaporation)

Stir salt into water and it vanishes. Pour it through a filter and the paper stays clean. So where did the salt go, and how do you get it back?

Where does the water go? Follow the salt.

1 · Salt solution

Stage: 1. State: 1 · Salt solution. static

Stage1

Salt dissolved in water. These 24 dots are the salt particles (the solute), spread right through the water (the solvent). The water particles aren't drawn. Count the dots now. You'll want that number at the end.

The salt particles of a salt solution at three stages as the water is removed: spread out in the solution, close together in a saturated solution, then locked into an ordered crystal. The water particles are not drawn.

Watch out: These dots are only the salt. The water particles aren't drawn, so the space between the dots isn't empty. In stages 1 and 2 it's water.

Chemistry · Which plan?

Which plan gets the salt back?

You've stirred salt into water until it has completely disappeared. Now you want the salt back.

Which plan would you pick?
How sure are you?

Evaporation vs boiling

EvaporationvsBoiling

Both turn a liquid into a gas. They're still not the same thing.

Focus

Where does the gas form?

Evaporation

Only at the surface of the liquid. The gas mixes with the air.

Boiling

Bubbles of gas form throughout the liquid.

The insight

This is the difference people mix up: surface only, or all the way through.

How much heat?

Evaporation

Gentle heating, or just the energy of the surroundings.

Boiling

The liquid is heated strongly.

How fast?

Evaporation

Slow. It can take several days.

Boiling

Quick. The solvent is removed quickly.

What happens to the solution?

Evaporation

No spitting. Evaporation avoids the spitting that boiling causes.

Boiling

The bubbles push through the solution and make it spit. Hot solution and solute can be thrown out of the container, which can be dangerous.

Chemistry · Making crystals

Put the crystal method in order

Order the steps for getting salt crystals from a salt solution

1 · First step4 · Last step

  1. Leave the dish in a warm place so the rest of the solvent evaporates slowly

  2. Heat gently (or stand the dish over a beaker of heated water) until small crystals appear at the edge

  3. Pour the solution into an evaporating dish

  4. Turn off the heat and let the dish cool a little

Predict, then check

Same saturated solution, two spots. Commit before you reveal.

A saturated salt solution is shared between two dishes. One sits on a cool window sill. The other sits near a heat source. Which dish ends up with the bigger crystals?

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

A filter can't do it. Taking the solvent away can.

What you need to know

  • Filtering can't separate a dissolved solute. Its particles are small enough to pass through the holes in the filter paper, so they stay in the filtrate. Only an insoluble solid is caught, as the residue.
  • To get a soluble solid back from its solution, remove the solvent. Solvent particles at the surface gain energy and escape as a gas into the air, and the solute particles are left behind.
  • Evaporation happens only at the surface. Boiling forms bubbles throughout the liquid and makes the solution spit, which can be dangerous. For crystals, heat gently, then let the rest evaporate slowly.
  • As the solvent leaves, the solution becomes saturated. The solute particles move closer, forces of attraction reform, and they lock into a crystal: a highly ordered, repeating pattern.

The big picture

Filtering can't catch a dissolved solid, because its particles are small enough to pass through the holes in the filter paper. To get a dissolved solid back you have to remove the solvent. Solvent particles at the surface gain energy and escape as a gas, and the solute particles are left behind. As the solution becomes saturated, those solute particles move closer together and lock into crystals. Gentle heating followed by slow evaporation does this safely, because boiling makes the solution spit.

Key points

1Dissolved particles pass through filter paper, so filtering cannot separate a solute from its solution.
2Remove the solvent and the solute is left behind: solvent particles escape as a gas, solute particles stay.
3Evaporation is at the surface only. Boiling is bubbles throughout, and it can make the solution spit.
4A saturated solution has no room for more solute. As the solvent goes, the solute particles form crystals with an ordered, repeating pattern.
5Slower evaporation of the solvent gives larger crystals, because the crystals have more time to develop.
6Rock salt: dissolve it, filter off the insoluble rock as the residue, then crystallise the salt from the filtrate.

Worked example

Problem

A pupil crushes rock salt (soluble salt mixed with insoluble rock), adds water and stirs. She filters the mixture, keeps the solid in the filter paper and pours the clear liquid down the sink. Her friend says, "You've just thrown away the salt!" Who is right?

⚠ Watch out

Calling evaporation 'boiling, but slower'. They aren't the same process. Boiling forms bubbles of gas throughout the liquid. Evaporation takes place only at the surface of the liquid.

🧠

Memory hook

Filter for lumps. For dissolved stuff, send the solvent away and the solute stays home.

✓

Check yourself

Cover the page. Why won't a filter get dissolved salt out of salty water? What leaves, what's left behind, and why does it end up as a crystal?

Flashcards

(15)
Solute
A substance that has dissolved into a solvent.
Solvent
A liquid into which a solute dissolves.
Mixture, and why a solution counts as one
A mixture is made of two or more different substances that can be physically separated. A solution is a mixture of a solute and a solvent.
Residue and filtrate
When you filter, the insoluble solid caught in the paper is the residue. The liquid that passes through is the filtrate.
Why can filtering not separate a dissolved solute?
Dissolved solute particles are small enough to pass through the holes in the filter paper, so they stay in the filtrate.
How do you separate a soluble solid from its solution?
The solvent has to be removed.
Evaporation
Particles of a substance in the liquid state at its surface have enough energy to change to the gas state and mix with the air.
Evaporation vs boiling: where does the gas form?
Evaporation: only at the surface of the liquid. Boiling: bubbles of gas form throughout the liquid.
What happens to the solvent particles and solute particles when a solvent is removed?
The solvent particles gain energy and change from liquid to gas, mixing with the air. The solute particles are left behind.
Why avoid boiling a salt solution to get the solvent out?
The bubbles push through the solution and make it spit, so hot solution and solute can be thrown out. That can be dangerous.
Saturated solution
A solution in which no more solute can dissolve in the volume of solvent present at that temperature.
Crystallisation
A process that forms solid crystals from a saturated solution by evaporating the solvent.
Crystal
A solid whose particles are arranged in a highly ordered, repeating pattern.
What does heating do to evaporation?
Heating a solution makes the solvent evaporate more quickly.
Faster or slower evaporation: which gives larger crystals?
Slower evaporation of the solvent. The crystals have more time to develop.

Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.

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