1.1.2Principles of Chemistry: Part 1
Diffusion and Dilution — Edexcel International GCSE Science Double Award (Modular) - Chemistry Unit 1 Revision Notes & Topical Questions
5 min read 7 parts Lesson 3 of 23
Section 1 of 7Diffusion
What it is
- What it is Diffusion is the net movement of particles from a region where they are at a higher concentration to a region where they are at a lower concentration.
- It continues until the particles are spread evenly through all the space available.
Why it happens
- Why it happens Particles in gases and liquids are in constant random motion, so over time they spread out and mix.
- No stirring or energy input is needed, because the particles' own movement does the work.
Effect of temperature
- Effect of temperature At a higher temperature the particles have more kinetic energy and move faster, so diffusion is quicker.
Section 2 of 7Diffusion compared in gases and liquids
| Feature | Gases | Liquids |
|---|---|---|
| Spacing of particles | Very far apart with large gaps | Close together but able to slide past each other |
| Speed of particles | Fast | Slower |
| Rate of diffusion | Fast | Slower than in gases |
| Example | Red-brown bromine vapour spreading through air in a gas jar | Purple potassium manganate(VII) spreading through water |
Section 3 of 7Bromine in a gas jar
1
A few drops of liquid bromine are placed in a gas jar, where they vaporise into a red-brown gas.
2
A second gas jar containing only air is placed upside down on top, with a lid between them that is then removed.
3
Bromine particles move randomly and pass through the large gaps between air particles.
4
After a few minutes the colour appears in the upper jar, and eventually both jars are the same colour all the way through, showing even mixing.
Section 4 of 7Dilution
Dilution spreads particles, it does not remove them
Definition
- Definition Dilution means adding more solvent, usually water, to a solution so the concentration of the solute falls.
What changes
- What changes The solute particles become further apart, and the colour of a coloured solution becomes fainter.
What stays the same
- What stays the same The number of solute particles is unchanged, because none are destroyed when water is added.
Repeated dilution
- Repeated dilution If a purple potassium manganate(VII) solution is diluted again and again, the colour is still visible after many dilutions,
- so a tiny crystal must contain a huge number of very small particles.
Section 5 of 7Evidence for particle theory
1Observation
- Observation Colour spreads through gases and liquids without stirring, and a small amount of dye colours a large volume of water.
2Interpretation
- Interpretation Matter must be made of particles that move randomly and are far smaller than anything we can see.
3Conclusion
- Conclusion Diffusion and dilution support the particle (kinetic) model of matter in solids, liquids and gases.
Section 6 of 7Key terms
| Term | Meaning |
|---|---|
| Diffusion | The net movement of particles from a region of higher concentration to a region of lower concentration. |
| Concentration | The amount of a substance in a given volume of a mixture or solution. |
| Dilution | Adding extra solvent to a solution so that the concentration of the solute decreases. |
| Solute | The substance that dissolves in a solvent to form a solution. |
| Kinetic energy | The energy a particle has because it is moving. |
| Random motion | Movement in unpredictable, constantly changing directions. |
Section 7 of 7Exam technique
1
Saying diffusion happens because particles are pushed or because of stirring. It is caused by the random motion of the particles themselves.
2
Thinking diffusion stops when the colour looks even. The particles keep moving, but there is no longer a net movement.
3
Writing that dilution makes the solute disappear. The particles are still there, just more spread out.
4
Forgetting to say why diffusion is slower in a liquid: the particles are closer together and move more slowly than gas particles.
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