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SavannahYear 8
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ScienceReproduction, plant gas exchange and particles on the move30 min★★ difficulty

Diffusion, solutions and solubility

Smell travels across a room with nothing pushing it, and sugar disappears into tea without vanishing. Both are the particle model at work. This lesson gives you the vocabulary to describe them precisely.

Part of your national curriculum
  • Chemistry: pure and impure substances: Understand diffusion in terms of the particle model, and the concepts of solutions, solvents, solutes and solubility

Lesson overview

What you'll learn in this lesson

Understand diffusion in terms of the particle model, and the concepts of solutions, solvents, solutes and solubility

Key learning points

  • • Why particles spread out
  • • What speeds diffusion up
  • • Dissolving is not disappearing
  • • How much will dissolve

This lesson at a glance

  • 30 minutes
  • 19 parts to scroll through
  • 4 quick checks
  • Marked quiz at the end
  • Gentle pace: short sittings with pauses

Words to know

diffusionparticleconceptssolutionssolvents

Scroll down — the lesson carries on below

Part 1 of 19 · Discover5%
1

Watch & discover

Part 1 of 19

Watch first

Diffusion, solutions and solubility

An English teaching video on this topic from Archie Science Show. Watch it, then scroll down for the lesson.

Do it for real

The practicals below use what the film shows, with a full method you can follow at home.

The hot-and-cold diffusion race

In a nutshell

Understand diffusion in terms of the particle model, and the concepts of solutions, solvents, solutes and solubility

2

What you already know

Part 2 of 19

Before we start

What you already know

You should know that matter is made of particles and that particles move faster when heated.

3

Key words

Part 3 of 19

Key words

Words you'll need today

Diffusion
The spreading of particles from a higher to a lower concentration.
Solvent
The liquid that does the dissolving.
Solute
The substance that dissolves.
Solution
A mixture of a solute evenly spread through a solvent.
Solubility
The mass of solute that dissolves in a set volume of solvent at a given temperature.
4

Learning cycle

Part 4 of 19

Learning cycle 1 of 2

Diffusion in the particle model

A short piece of teaching, then a check to make sure it has landed.

5

Explore the idea

Part 5 of 19

Learn

Why particles spread out

Particles in gases and liquids move constantly in random directions. Where there are more of them in one place, more happen to move outwards than inwards, so overall they spread until they are evenly mixed. Nothing pulls them; the pattern is the result of random motion. Diffusion is fastest in gases, slower in liquids and effectively absent in solids, where particles only vibrate in place.

6

Explore the idea

Part 6 of 19

Learn

What speeds diffusion up

Raising the temperature gives particles more kinetic energy, so they spread faster. A steeper concentration difference also speeds it up - which is exactly why alveoli, gills and leaves all keep one side low. Smaller, lighter particles diffuse faster than large ones. That is the whole model: random motion plus a difference in concentration.

7

Quick check

Part 7 of 19

Quick check

Diffusion happens because particles...

8

Quick check

Part 8 of 19

Quick check

In salty water, the solvent is...

9

Reset break

Part 9 of 19

Pause

That's sitting 1 of 2 done

Stretch, get a drink, look out of the window. There is no timer and nothing is counting down — your place is saved, so you can come back in five minutes or tomorrow.

Stop here for now
10

Learning cycle

Part 10 of 19

Learning cycle 2 of 2

Solutions, solutes and solubility

A short piece of teaching, then a check to make sure it has landed.

11

Explore the idea

Part 11 of 19

Learn

Dissolving is not disappearing

When a solute dissolves, its particles separate and slot between the solvent particles. Nothing is destroyed, so the total mass stays the same: 5 g of salt in 95 g of water gives 100 g of solution. That is a mixture, not a reaction - no new substance forms and you can get the solute back by evaporating the solvent.

12

Explore the idea

Part 12 of 19

Learn

How much will dissolve

Solubility has a limit. Keep adding solute and eventually no more dissolves - the solution is saturated and the extra sits at the bottom. For most solids, solubility rises with temperature, which is why hot tea takes more sugar than iced tea. Solubility is normally quoted as grams per 100 g of solvent at a stated temperature, because the temperature matters as much as the substance.

13

Quick check

Part 13 of 19

Quick check

Dissolve 10 g of sugar in 90 g of water. The solution's mass is...

14

Quick check

Part 14 of 19

Quick check

A saturated solution is one where...

15

Common mix-ups

Part 15 of 19

Common mix-ups

Things people often get wrong

  • Dissolved solute is still there - mass is conserved.
  • Dissolving is mixing, not a chemical reaction.
  • Insoluble is not the same as heavy; some heavy substances dissolve well.
  • Diffusion needs no pump or current, just random particle motion.
16

Challenge round

Part 16 of 19

Game · Sort it

Which of these are true?

Drag each card into the right column. Tap a card first if dragging is fiddly.

True

Not true

17

Challenge round

Part 17 of 19

Game · Fill the gaps

Finish the sentences

Choose the word that belongs in each gap.

____ is fastest in gases, slower in liquids and effectively absent in solids, where particles only vibrate in place.

18

Mastery quiz

Part 18 of 19

Marked quiz

End of lesson quiz: Diffusion, solutions and solubility

6 questions, marked with the reasoning shown. No timer.

  1. 1. Diffusion is fastest in...

  2. 2. Raising the temperature makes diffusion faster because particles...

  3. 3. Which term describes the substance that dissolves?

  4. 4. How can a dissolved solid be recovered from a solution?

  5. 5. For most solids, solubility in water...

  6. 6. Sand in water is not a solution because the sand...

19

Lesson round-up

Part 19 of 19

Lesson round-up

Ready when you are

Quiz score

Not sat

Games

Not played

Points this lesson

0

Best run

0 in a row

Luna: 0 out of 4 on the practice checks. Only if you feel up to it — one more?

Ask Luna

Do the work

Your written work for this lesson

This is the part you'd do in your book at school. Pick the pieces below, write here or on paper, and photograph anything you draw or work out.

Case study

Why the school greenhouse failed

A science club grew tomato plants in the school greenhouse. In June the plants grew well: leaves were dark green and the fruit set quickly. In July the club went away for two weeks and left the greenhouse door shut. When they came back, the leaves were curled and pale and most of the fruit had dropped.

The club had a data logger inside. It showed the temperature reached 46 °C on several afternoons and the soil moisture fell close to zero. The club knew photosynthesis needs light, water and carbon dioxide, and that enzymes in a plant work best in a narrow range of temperature. Above that range the enzymes stop working properly.

The club repeated the summer with the door propped open and a water bottle drip on each pot. Temperature peaked at 31 °C and the plants survived.

Written for Good Learning Co.

Reading questions

  1. Name the variable the club changed the second time, and one variable they kept the same.
  2. Explain, using the words photosynthesis and enzymes, why the plants were damaged in July.
  3. Write a conclusion for the club's report, saying how far the evidence supports it.

  • Method and results

    Writing

    Write the aim, what you changed, what you measured and what you kept the same, then record your results in a table.

    1 page + results table

  • Conclusion

    Writing

    Explain what your results show and why, using the scientific words from the lesson. Say how you know you can trust them.

    6–8 sentences

  • Labelled diagram

    Diagram

    Draw the apparatus or the process in pencil, label it fully, and photograph it.

    1 labelled diagram

  • Reading and notes

    Reading

    Read the lesson text again and write the key ideas in your own words.

    Half a page of notes

Saved as you type.

Part 1 of 19 · Watch & discover

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    Understand diffusion in terms of the particle model, and the concepts of solutions, solvents, solutes and solubility

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