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STATES OF MATTER - properties of gases and liquids (fluids) and solids

6. Using the particle model to explain diffusion in liquids - experiments illustrated, described and explained

[Author © Dr Phil Brown GRIC, PhD: Doc Brown's chemistry exam revision notes on states of matter - physical properties of gases, liquids and solids, suitable for students of UK GCSE level and international IGCSE/O level chemistry courses, ~US grades 9-10 chemistry notes [page updated RE-EDIT]

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INDEX of all my notes on the states of matter

GCSE (~US grades 8-10) level multiple choice QUIZ on the states of matter: gases, liquids & solids

  • DIFFUSION: The natural rapid and random movement of the particles means that substances dissolving in liquids will spontaneously ‘spread’ - diffuse.
    • Diffusion is much slower in liquids compared to gases because there is less space for the particles to move in and more ‘blocking’ collisions happen.
    • Just dropping lumps/granules/powder of a soluble solid (preferably coloured!) will resulting in a dissolving followed by an observable diffusion effect.
    • Again, the net flow of dissolved particles will be from a higher concentration to a lower concentration until the concentration is uniform throughout the liquid.
  • Diffusion in liquids – evidence for random particle movement in liquids:
    • If coloured crystals of e.g. the highly coloured salt crystals of potassium manganate(VII) are dropped into a beaker of water and covered at room temperature.
    • Despite the lack of mixing due to shaking or convection currents from a heat source etc. the bright purple colour of the dissolving salt slowly spreads throughout all of the liquid but it is much slower than the gas experiments described in section 5 because of the much greater density of particles slowing the spreading due to close proximity collisions.
    • The same thing happens with dropping copper sulphate crystals (blue, so observable) or coffee granules into water and just leaving the mixture to stand.
    • Experiment to show the slower diffusion in liquids eg water
    • You start with a beaker of still pure colourless water and drop a few crystals of ANY highly coloured soluble crystals into it and put on a lid cover to prevent any air disturbance.
    • The beaker is left to stand, preferably at a constant temperature to prevent mixing due to convention. Immediately the crystals are added they will begin to dissolve and due to natural random particle motion the coloured molecules will begin to spread from an area of high concentration to one of low concentration and in all directions. You could take a series of photographs to record the spreading. The spreading is self-evident and direct experimental evidence for the natural constant random movement of particles (molecules or ions).
    • After many hours all of the crystals will have dissolved AND due to the random movement of ALL the particles, everything dissolved becomes evenly distributed giving an evenly coloured solution. Note that although the colour doesn't seem to spread anymore, ALL the particles are still moving with a random motion, nothing stops!
      •

A particle model of diffusion in liquids

Imagine the diffusion gradient from left to right for the green particles added to the blue particles on the left. So, for the green particles, net migration is from left to right (from a higher to a lower concentration) and will continue, in a sealed container, until all the particles are evenly distributed (as pictured), then there is no net migration or change in concentration throughout the mixture.

Diffusion is slower in liquids because there is less space between the particles for other particles to move into and random collisions will occur more frequently slowing down the particle spreading effect down a diffusion gradient.
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Diffusion in a gel

This biology experiment demonstrates slow diffusion in liquids, but note, although even slower, the permeability of the gel towards molecules (water or ions) is always present.

The gel cubes contain an alkali (dilute sodium hydroxide) and phenolphthalein indicator that turns pink.

The gel cubes are placed in dilute hydrochloric acid, which slowly diffuses into the gel cubes, neutralises the alkali and turns the indicator colourless/

LINKS WITH BIOLOGY

The importance of diffusion and gas exchange in living organisms

For plant gas exchanges and photosynthesis see

Part 2. What is the chemical process of photosynthesis?

Part 3. Plant structure and photosynthesis - leaf adaptations

For animal organ gas/nutrient exchanges see

Part 3. Gas exchange in the human lungs by diffusion, comments on breathing, COPD and ventilators

Part 4. Gas exchange and the structure of fish gills

Part 5. The function of villi in the exchange surface of the small intestine

Part 6. Exchanges surface structure adaptations in other animals

More on transport systems in plants and animals

(2) A particle model and factors affecting the rate of diffusion and Fick's Law of diffusion

(3) The action of partially permeable cell membranes - selective diffusion and examples

(4) Osmosis - examples and explanation

(5) Some details of examples of osmotic action in individual animal or plant cell types

(6) Osmosis experiments - demonstrations of osmotic action

KEY POINTS about diffusion in liquids

Syllabus-aligned revision points on diffusion in liquids explained by the kinetic particle model, tailored for GCSE/IGCSE chemistry across the major exam boards.

It's structured for clarity, with examples, exam tips, and misconceptions highlighted.

Diffusion in Liquids: Kinetic Particle Model

Core Explanation

  • Arrangement of particles: Close together, no fixed positions, weaker forces than solids but stronger than gases.
  • Motion: Particles move randomly, sliding past each other.
  • Why diffusion occurs: Random motion causes particles to spread from high concentration to low concentration.
  • Rate of diffusion: Slower than in gases because particles are closer together and collide more often.
  • Temperature effect: Higher temperature → faster particle motion → quicker diffusion.

Examples

  • Potassium permanganate crystal in water: Purple colour slowly spreads throughout the liquid.
  • Food colouring in water: Dye disperses without stirring, showing diffusion.
  • Sugar dissolving in tea: Molecules spread out due to random motion (though dissolution also involves solubility).

Typical Exam Board Requirements

Specification Focus Example/Notes
Particle model applied to liquids; diffusion practicals KMnO₄ crystal in water
Evidence for particle theory; diffusion in liquids Food colouring spreading in water
Link particle motion to properties; diffusion explained KMnO₄ diffusion experiment
Particle diagrams; diffusion linked to Brownian motion KMnO₄ diffusion
Everyday applications of diffusion; particle motion evidence Tea or coffee diffusion

Student Exam Tips

  • Draw particle diagrams: Show close particles with arrows indicating random motion.
  • Compare diffusion in liquids versus gases: Liquids = slower, gases = faster.
  • Use precise terms: random motion, collisions, concentration gradient.
  • Always link to evidence: Mention KMnO₄ or food dye experiments.
  • Temperature effect: Higher temperature speeds diffusion – examiners often test this.
  • Practice past papers: Many boards ask students to explain diffusion using particle theory.

Typical Misconceptions

  •  “Diffusion only happens in gases” → Wrong. It also occurs in liquids, though slower.
  •  “Particles in liquids are fixed” → They move freely but remain close together.
  •  “Diffusion in liquids is fast” → It is slower than gases due to frequent collisions.
  •  “Stirring is required for diffusion” → Stirring speeds it up, but diffusion occurs naturally.
  •  “Particles stop moving at equilibrium” → They keep moving randomly; equilibrium means even distribution.

Quick Overlay Summary

  • Liquids: Close particles, random sliding motion, fixed volume, take container shape, slow diffusion.
  • Diffusion evidence: KMnO4 crystal in water, food dye spreading.
  • Exam Tip: Always connect diffusion to random motion and kinetic energy.

Learning objectives to do with diffusion in liquids

Be able to draw particle pictures to illustrate and explain diffusion in liquids.

Be able to describe and explain what diffusion is in liquids and solutions using the kinetic particle model.

Know that the net migration of liquid or dissolved particles due to their random motion is from a high concentration to a lower concentration.

Be able to interpret the simple experiment where potassium manganate crystals dissolve in water and the coloured particles are observed to diffuse and spread out throughout the liquid.

Be able to describe the importance of diffusion of dissolved substances in and out of cells of living organisms by migration through the cell membrane.

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INDEX of all my notes on the states of matter

GCSE (~US grades 8-10) level multiple choice QUIZ on the states of matter: gases, liquids & solids

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