3 minutes reading time (604 words)

    Light Scattering by a Colloid (The Tyndall Effect) - "The Thiosulfate Sunset"

    A beam of white light is shone through a solution of sodium thiosulfate and onto a screen. Dilute hydrochloric acid is added to the thiosulfate and colloidal sulfur is formed. The light beam becomes visible in the solution as a bluish 'Tyndall cone' due to light scattered from the colloidal particles. Blue light is scattered more effectively than red so the transmitted light that reaches the screen becomes red and then fades as the colloidal particles become numerous enough to block all the transmitted light. This gives an effect rather like a sunset.


    Slide projector with slide holder. (The demonstration can be done using an overhead projector).
    Projection screen. (A white wall will do, or a large sheet of white paper can be used).

    • Beaker, 1L
    • Access to fume cupboard (optional).
    • Light meter (optional)


    • Sodium thiosulfate-5-water (Na2S2O3.5H2O), 20g
    • Deionised water, 1L
    • Hydrochloric acid, 1M, 1mL

    Before the demonstration

    Cut a piece of card to the size of a 35mm slide and make two holes in this each about the diameter of a pencil and as far apart horizontally as possible. Place the card in the slide holder of the projector. This is to produce two beams of light; one will shine through the solution and the other will act as a reference.
    If the demonstration is to be done on an OHP, make two holes in a sheet of card, the size of a slide, that covers the OHP stage. These should be positioned about 15 cm apart so that one beam of light shines through a 1L beaker standing on the stage and the other acts as a reference.

    Make up a solution of 20g of sodium thiosulfate in 1L of deionised water and place it in the beaker.

    The demonstration

    Darken the room. Arrange the projector, beaker and screen so that one beam of light shines through the solution in the beaker and onto the screen and the other shines directly onto the screen. Add about 1mL of 1M hydrochloric acid to the thiosulfate solution and stir thoroughly. After about a minute, the solution becomes visibly cloudy and a cone of blue scattered light (the Tyndall cone) becomes visible in the beaker. The transmitted light begins to show on the screen as red. This fades gradually as more sulfur is produced and virtually no light is visible after about five minutes. More concentrated acid will produce the same changes more quickly.
    After the experiment, filter the remaining cloudy liquid in a fume cupboard (because of the sulfur dioxide which is dissolved in the solution) to show that the filtrate is still cloudy and that the sulfur is colloidal.

    Visual tips

    At least a partial blackout is desirable. The screen will probably need to be angled towards the audience so that both the transmitted light and the Tyndall cone can be seen.

    Teaching tips

    The blue of the sky is caused by blue light scattered by colloidal particles in the atmosphere. The redness of the Sun near sunset is also caused by the scattering of blue light. As the Sun sets, we observe it through an increasing thickness of atmosphere and more blue light is scattered leaving a higher proportion of red transmitted light.


    The reaction that takes place is:
    Na2S2O3(aq) + 2HCl(aq) → 2NaCl(aq) + SO2(g) + S(s) + H2O(l)


    Repeat the experiment holding a light meter in the beam of transmitted light. The reading drops gradually.


    Wear eye protection.

    Dispose of the liquid by flushing it down the sink in a fume cupboard with plenty of water.
    It is the responsibility of teachers doing this demonstration to carry out an appropriate risk assessment.

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