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What is in washing up liquid and water that makes bubbles?

What is in washing up liquid and water that makes bubbles?
Ashleigh Ham from Swansea (Age 5-14)

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One Response

  1. Hi Young Scientist :).
    Water molecules have a natural attraction towards each other which creates a kind of ‘stickiness’. When detergent, is added to water the ‘soapy’ solution which results has less ‘surface tension’. Surface tension is the skin-like top area of a liquid which is normally quite strong and even elastic in nature.
    It is the combined action of these two characteristics which enables air to become trapped – where the pressure of this gas then pushes outwards and the thin
    ‘soap-film’ surface layer stretches into bubbles. These bubbles are spherical in shape, when on their own. Because of the elastic nature of it’s surface, each bubble resists expansion and as the molecules of gas that are within exert an outward thrust in all directions equally, the outcome is a regular shape of minimum surface area – the sphere. For comparison, the volume contained in a sphere would need, if fitted into a cube, a structure with nearly 25% more surface area!
    Check out surface tension in action – It is surprising and worth the wait I promise…
    You will need
    • A drinking glass or a similarly-sized beaker.
    • Ordinary ‘mains’ tap water.
    • Tissue (the rougher sort, not ‘kitchen roll’).
    • Washing-up liquid (a tiny amount only).
    • Scissors – ( please take special care with these ).
    • Patience… This could take about an hour!
    • A paper-clip.
    • Note paper and a pencil.
    • A nearby clock.
    First take the paper-clip and break it in two pieces.
    Next, use the scissors to carefully cut a small piece of the tissue.
    (About 25mm x 15mm will be enough)
     This will be a temporary raft
    Fill the container you have chosen with water.
     Rinse it out a few times first. Can you guess why?
    Carefully float the tissue raft on the surface of the water and equally carefully place a piece of the paper-clip onto it.- Rub the metal with your fingers first (again… Why?).
    Check the time and write it down. We need to measure the delay till the result occurs.
    Wait and watch the raft- this will take several minutes to produce a result so sit down.
    Eventually, the ‘raft’ will sink… BUT, what happens to the paper-clip?
    Look at the clock again and write down the time.
    Now, take the washing up liquid.
    Carefully add one drop of the ‘detergent’ to the surface of the water at the edge.
    Wait and watch the paper-clip for a minute. – Or while you count to 60.
    If no change occurs, carefully add one more drop of the detergent as you did earlier.
    Wait and watch for another minute, if necessary.
    If still no change occurs, carefully add one more drop of detergent in the same way as you did previously. I expect that you will, by now have a result so write down the time, exactly, that the change occurred and think about what may have caused this change. The delay times measured can enable comparisons with other combinations.
    You could also think of other, similar experiments which may reveal more, interesting
    ‘Facts about Detergency’. How ‘soapy water’ cleans greasy surfaces for instance.
    I hope that you will enjoy science. I think that science is GREAT!!
    Good Luck

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