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Unit 3: Forces, motion and pressure

Particles on the move — HIA International - Grade 7 Revision Notes & Topical Questions

5 min read 10 parts Lesson 8 of 8 3 quizzes

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Lesson summary

3.7 Particles on the move — HIA INTERNATIONAL notes: Explain diffusion in gases and liquids in terms of the random movement of particles.Diffusion = the overall random movement of particles from an area of higher concentration to an area of lower concentration.Concentration = the…

  • End of diffusion
  • Cooking food
  • Smell strength
  • Ink in water
  • Not in solids
  • Concentration difference

Part 1 of 8What diffusion is

Diffusion
Diffusion the overall random movement of particles from an area of higher concentration to an area of lower concentration.
Concentration
Concentration the number of particles in a particular volume.
Random movement
  • Random movement the speed and direction of gas particles are random.
Spreading
  • Spreading each gas spreads to fill the container.

Part 2 of 8Mixing two gases

Mixing two gases
At the start, one gas is at high concentration on the left and zero on the right, and the other gas is the opposite.
When the tap is opened, some particles of each gas pass through at random.
After some time, both gases are at equal concentration on both sides.
  • End of diffusion
    • happens when the concentrations are equal, but the particles keep moving.

Part 3 of 8Diffusion of smells

Cooking food
  • Cooking food some particles in the food change state to gas and diffuse through the air.
Smell strength
  • Smell strength stronger closer to the source, where the concentration is higher.
Animals
  • Animals use this change in smell strength to find food.

Part 4 of 8Diffusion in liquids

Liquids
  • Liquids particles also move at random.
Ink in water
  • Ink in water the ink spreads from high to low concentration until completely mixed.
Not in solids
  • Not in solids particles in solids only vibrate and do not move through the material.

Part 5 of 8Speed of diffusion

Speed of diffusion
Concentration difference
  • Concentration difference the bigger the difference, the faster the diffusion.
Temperature
  • Temperature the higher the temperature, the faster the particles move and the faster the diffusion.
Tea
  • Tea spreads through hot water faster than through cold water.
Lighter particles
  • Lighter particles diffuse faster than heavier ones.
Gases and liquids
  • Gases and liquids diffusion is faster in gases than in liquids.

Part 6 of 8Observing diffusion

Observing diffusion
Fill a transparent container with warm water and leave it to stand so the water stops moving.
Add one or two drops of dye to the bottom of the water.
Observe the dye spreading and draw labelled diagrams.

Part 7 of 8Temperature and rate of diffusion

1
Add equal volumes of water at different temperatures to identical beakers.
2
Measure the temperature of each.
3
Add a small volume of water-soluble ink to the bottom of each beaker, from lowest to highest temperature.
4
Time how long the ink takes to spread through all the water.
Result
  • Result diffusion is faster in warmer water.
Fair test
  • Fair test equal volumes of water are used.

Part 8 of 8Evidence for particle movement

Evidence for particle movement
Ammonia and hydrogen chloride
  • Ammonia and hydrogen chloride a white ring of ammonium chloride forms nearer the hydrogen chloride end because the lighter ammonia particles travel faster.
Brownian motion
  • Brownian motion random jerky movement of visible particles such as smoke caused by collisions with invisible particles.

Key terms

TermMeaning
DiffusionOverall movement of particles from high to low concentration.
ConcentrationThe number of particles in a volume.
RandomWithout a pattern or fixed direction.
ParticleA tiny piece of matter.
GasA state in which particles move freely and far apart.
LiquidA state in which particles move past each other.
Brownian motionRandom jerky movement of visible particles.
RateHow fast something happens.

Avoid these mistakes

1
Saying diffusion happens only in gases — it also happens in liquids.
2
Saying particles stop moving when diffusion stops — they keep moving.
3
Saying particles move from low to high concentration — they spread from high to low.
4
Saying diffusion needs a current or stirring — it happens through random movement.

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