KS3 · Chemistry

Dissolving

Stir salt into water and it vanishes. Or does it? The salt is still in the glass, and its particles will show you exactly where it went.

Dissolving, particle by particle

1Collide2345

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Put a solute (like salt) into a solvent (like water) and its particles are soon surrounded by solvent particles. Everything is moving, so solute and solvent particles keep bumping into each other.

The solute is the substance that dissolves (like salt); the solvent is what it dissolves in (like water). Drag through the five stages and watch the solute come apart.

Where did the salt go?

Be honest: what do you think happened?

You stir a spoonful of salt into a glass of water. A minute later, you can't see any salt at all.

Which of these is closest to what you think happened to the salt?
How sure are you?

Particles and spaces

Two liquids, one surprise

You mix 40 cm³ of ethanol with 40 cm³ of water. What is the total volume of the mixture?

Your estimate

80 cm³

70 cm³90 cm³

When dissolving doesn't happen

?

Reason it through

Why doesn't flour dissolve in water?

Link 1 of 3

First link · your turn

Water particles collide with the flour. What do they try to do?

2
Locked — reveal the link above first
3
Locked — reveal the link above first

Solution or suspension?

Solution (salt in water)vsSuspension (flour in water)

Both are mixtures. What's different is what the particles are doing.

Focus

What the particles are doing

Solution (salt in water)

The salt particles have been pulled apart and spread out individually among the water particles.

Suspension (flour in water)

The flour hasn't dissolved. Its particles are dispersed (spread out) through the water, but bits of flour are still there.

The insight

This is the key difference. Dissolved particles are on their own; a suspended substance never came apart like that.

Can you see the substance?

Solution (salt in water)

No. Once dissolved, the salt can no longer be seen.

Suspension (flour in water)

Yes. You can still see bits of flour.

Is the mixture clear?

Solution (salt in water)

It can be clear.

Suspension (flour in water)

No, it isn't clear.

More examples

Solution (salt in water)

Salt dissolved in water.

Suspension (flour in water)

Flour or mud in water; dust in air.

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

The salt hasn't vanished. Follow its particles and see where it went.

What you need to know

  • A solute dissolves in a solvent to make a mixture called a solution. Once dissolved, the solute's particles are spread through the solvent and it can no longer be seen.
  • Dissolving is a tug of war between attractions: the solute dissolves only if solute–solvent attraction beats the attraction between the solute particles themselves.
  • Dissolving goes collide → pulled away → gap filled → new attractions → spread out by Brownian motion (the spreading is diffusion).
  • Dissolving is not melting, is not a change of state and makes no new substance. The solute's attractions are overcome, not broken.
  • An insoluble substance forms a suspension instead: bits can be seen, the mixture isn't clear, and the bits settle out over time.

The big picture

When a solute dissolves, solvent particles pull its particles away one at a time, because the solvent's pull on them is stronger than their pull on each other. Brownian motion then spreads the particles out through the solvent until the solute can no longer be seen, forming a solution. Nothing melts, no new substance forms, and the solute is still there. If the solvent's pull is too weak, the substance is insoluble in that solvent, and stirring it in makes a suspension, where bits can still be seen and settle out over time.

Key points

1Solute + solvent → solution. A solution is a mixture.
2The dissolving rule: the solute–solvent attraction must be stronger than the attraction between the solute particles.
3In this particle model, the particles of a solid solute attract each other more strongly than the particles of a liquid solvent do.
4Brownian motion is the random movement of particles in a liquid or gas, caused by collisions. Dissolving and diffusion both depend on it.
5The attractions between solute particles are overcome, not broken: they still act, just more weakly.
6Dissolving and melting are both physical processes, but only melting is a change of state. Dissolving makes no new substance.
7Insoluble means insoluble in a particular solvent: the same substance may dissolve in another solvent.
8Particles can fit into the spaces between other particles, so a mixture's volume can be less than the volumes added together.
9In a suspension, insoluble particles are dispersed, bits can be seen and the mixture isn't clear. Heavier particles settle out faster than lighter ones.

Worked example

Problem

Ella stirs a spoonful of white powder into a beaker of water. The water turns cloudy and she can see tiny bits floating in it. An hour later, most of the powder is sitting in a layer at the bottom. Has the powder dissolved? Explain using particles.

⚠ Watch out

Saying a solid 'melted' into a liquid when it dissolved. Melting is a change of state: a substance's particles gain energy and overcome their own attractions. When salt dissolves in water it never becomes liquid salt. The water particles overcome the salt particles' attractions and spread them out.

🧠

Memory hook

Dissolving is a tug of war. If the solvent pulls harder than the solute's particles hold on to each other, it dissolves. If the solute holds on harder, it doesn't.

✓

Check yourself

Without scrolling back: can you explain why a solute becomes invisible when it dissolves, say why it hasn't disappeared or melted, and give two things you'd look for to tell a suspension from a solution?

Flashcards

(16)
What is a solute?
The substance that dissolves, such as salt when it dissolves in water.
What is a solvent?
The substance a solute dissolves in, such as the water in salt water.
What is a solution?
The mixture that forms when a solute dissolves in a solvent.
When has a substance dissolved?
Its particles have separated and spread throughout the solvent's particles, so you can no longer see it. The solution can be clear.
What must be true for a substance to dissolve?
The attraction between solute and solvent particles must be stronger than the attraction between the solute particles themselves.
What makes a substance insoluble in a solvent?
The attraction between its particles and the solvent's particles is too weak to overcome the attraction between its own particles.
Can a substance be insoluble in one solvent but dissolve in another?
Yes. Soluble and insoluble always describe a particular solvent.
What is Brownian motion?
The random movement of particles in a liquid or gas, caused by collisions with the particles around them.
How is diffusion part of dissolving?
Once the solute particles have been pulled apart, diffusion spreads them randomly from where there are lots of them to all through the solvent. Both depend on Brownian motion.
When a solute dissolves, are the forces between its particles broken?
No: they are overcome, not broken. They still act, but they are weaker than before, giving the particles a different arrangement.
How is dissolving different from melting?
In melting, a substance's particles gain energy and overcome their own attractions, so it changes from solid to liquid. In dissolving, the solvent overcomes the solute's attractions and there's no change of state.
Does dissolving make a new substance?
No. The dissolved solute (for example salt in water) is still there in its normal form, broken up and mixed among the solvent particles.
Why can 40 cm³ of ethanol + 40 cm³ of water give only about 78 cm³?
Some particles of one liquid fit into the spaces between the particles of the other, so the total volume is less than expected.
What is a suspension?
A mixture where particles of an insoluble substance are dispersed through a medium. You can still see bits and it isn't clear, e.g. mud in water or dust in air.
What happens to a suspension if you leave it?
The insoluble particles settle out. Heavier ones like sand settle quickly; lighter ones like flour can take a very long time.
In this particle model, which attract more strongly: the particles of a solid solute or of a liquid solvent?
The particles of the solid solute attract each other more strongly than the particles of the liquid solvent do.

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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