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

6. Particle model of liquids and diffusion experiments

Doc Brown's chemistry revision notes: basic school chemistry science GCSE chemistry, IGCSE  chemistry, O level and ~US grades 8, 9 and 10 school science courses or equivalent for ~14-16 year old science students for national examinations in chemistry and also helpful for UK advanced level chemistry students aged ~16-18 and US grades 11-12 K12 honors.


  • 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 container.
  • 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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(7) 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


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 level and advanced pre-university level revision notes)

Detailed notes on the states of matter and their properties

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