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1.2.1 Diffusion

What diffusion is

Definition

Diffusion

The net movement of particles from a region of higher concentration to a region of lower concentration as a result of their random motion.

Definition

Concentration gradient

The difference in the concentration of a substance between two regions.

  1. Diffusion is the process that lets the smell of cooking spread through a house, or a drop of ink colour a whole glass of water, without any stirring.
  2. During diffusion, the particles of one substance spread out among the particles of another substance until the two are evenly mixed.
  3. The overall movement is from where the particles are more concentrated to where they are less concentrated, which is described as moving down a concentration gradient.
  4. Diffusion takes place in gases and in liquids, because the particles in both states are free to move from place to place.
  5. Diffusion does not take place to any noticeable extent in solids, because their particles only vibrate about fixed positions.
  6. Diffusion needs no stirring, shaking or pumping, because it is caused only by the movement of the particles themselves.

Explaining diffusion with particles

Definition

Kinetic particle theory

The theory that all matter is made of tiny particles in constant motion, whose energy and forces of attraction explain the properties of solids, liquids and gases.

Definition

Random motion

The continual movement of particles in unpredictable directions, which change every time the particles collide.

  1. Kinetic particle theory states that the particles in liquids and gases are in constant, random motion.
  2. Each particle travels in a straight line until it collides with another particle, and then moves off in a new, unpredictable direction.
  3. The direction of each particle keeps changing, so no single particle travels directly from one region to another.
  4. Where particles are crowded together, more of them happen to move outwards than move inwards, simply because there are more of them there to move.
  5. This produces a net movement of particles from the region of higher concentration to the region of lower concentration.
  6. As the particles spread out, the difference in concentration becomes smaller, so the net movement becomes slower.
  7. Once the particles are evenly spread, they still move randomly, but equal numbers move in every direction and there is no further net movement.
  8. The particles themselves do not change during diffusion, so they do not grow, shrink, break apart or disappear.
Common Mistake
  • Do not say that particles "want" to move to a region of lower concentration or are pushed there.
  • Every particle moves randomly, and the net movement happens only because more particles leave the crowded region than enter it.

Diffusion in gases and liquids

  1. Diffusion in a gas
    1. Gas particles move quickly and have large spaces between them, so each one travels a relatively long way before it collides with another particle.
    2. As a result, diffusion in a gas is fast, and a smell can spread across a classroom within a few minutes.
    3. When a gas jar of air is placed upside down on top of a gas jar containing brown bromine gas, the brown colour spreads upwards until both jars are the same pale brown.
    4. The bromine spreads upwards even though it is denser than air, which shows that the mixing is caused by the random motion of the particles and not by the gas flowing.
  2. Diffusion in a liquid
    1. Liquid particles are close together, so a diffusing particle collides with its neighbours very often and changes direction after travelling only a tiny distance.
    2. As a result, diffusion in a liquid is much slower than diffusion in a gas.
    3. A purple crystal of potassium manganate(VII) placed in still water slowly dissolves, and the purple colour spreads through the water over several hours until it is evenly coloured.
  3. Effect of temperature
    1. At a higher temperature, the particles have more kinetic energy and move faster.
    2. Faster particles spread out more quickly, so diffusion is faster at a higher temperature.
Practical

Observing diffusion in water

  • Aim: to observe diffusion in a liquid and to compare how quickly it happens in cold water and in hot water.
  • Apparatus: two identical 250 cm3250\ \text{cm}^3250 cm3 beakers, cold tap water, hot water at about 60 ∘C60\,^{\circ}\text{C}60∘C, a 100 cm3100\ \text{cm}^3100 cm3 measuring cylinder, a thermometer, forceps, small crystals of potassium manganate(VII) of similar size, a sheet of white paper, a stopwatch, eye protection and gloves.
  • Method:
    • Measure 200 cm3200\ \text{cm}^3200 cm3 of cold water into one beaker and 200 cm3200\ \text{cm}^3200 cm3 of hot water into the other, then record the temperature of each.
    • Stand both beakers on the white paper and leave them for one minute so that the water is completely still.
    • Using forceps, gently drop one crystal of potassium manganate(VII) into the centre of each beaker at the same moment and start the stopwatch.
    • Do not stir, knock or move the beakers at any point during the experiment.
    • Look at each beaker from the side every two minutes and note how far the purple colour has spread from the crystal.
    • Record the time taken for the purple colour to spread evenly through the water in each beaker.
  • Variables: the independent variable is the temperature of the water, the dependent variable is the time taken for the colour to spread evenly, and the controlled variables are the volume of water, the size of the crystal and the size of the beaker.
  • Results: a dark purple region forms around each crystal and spreads slowly outwards, and the colour becomes evenly spread much sooner in the hot water than in the cold water.
  • Conclusion: the particles of potassium manganate(VII) move randomly among the water particles and spread from a region of higher concentration to a region of lower concentration, and they spread faster at a higher temperature because they move faster.
  • Sources of error: crystals of different sizes give different starting amounts, any movement of the beaker stirs the water, and deciding when the colour is evenly spread is a matter of judgement.
  • Safety: wear eye protection and gloves, handle the crystals only with forceps because potassium manganate(VII) is harmful and stains skin and clothing, and take care with the hot water.
Exam technique

Explaining diffusion

  • Use the words random motion or the particles move randomly, because this is the central idea in every diffusion explanation.
  • Give the direction of the net movement, from a higher concentration to a lower concentration.
  • Mention that the particles collide with other particles and change direction, especially when you explain why diffusion in a liquid is slow.
  • Write that the particles spread out or mix, rather than that the substance "disappears" or "dissolves", unless a solid dissolving is part of the question.
Self review
  • Define diffusion.
  • Explain why diffusion happens in liquids and gases but not noticeably in solids.
  • Explain why there is no net movement once particles are evenly spread, even though they are still moving.
  • Explain why diffusion is faster in a gas than in a liquid.
  • State the effect of raising the temperature on the rate of diffusion.
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1.2.1 Diffusion Revision Guide

  1. IGCSE
  2. /Chemistry
  3. /1.2.1 Diffusion

Revision notes for CIE IGCSE Chemistry 1.2.1 Diffusion: explanations and worked examples.

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