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showing convection currents with soup...urgent advice needed please

Discussion in 'Science' started by Kamikaze-Chemist, Jul 5, 2011.

  1. Hi CrazyChemist,
    I haven't seen this demo done with soup myself; I do however rather like the plate tectonics demo that uses the same idea -golden syrup in a beaker, digestive biscuit broken in half placed on the top & pushed back together, will float apart when heated directly from the centre underneath.
    Assuming you're using the same kind of idea, yes you will need to heat it from a single point - it's advisable to use a gauze without a heatproof bit on it, unless you can get a beaker large enough to sit on a tripod stably.. And anything that floats on the surface should work to show the convection currents. If you're worried about it cracking the beaker, try just turning the gas tap down a little to start with (depends on the height of the bunsen flame, try to get the blue cone lower than you would otherwise) then turn it up again, just to heat the glass through more gently first.
    If the school you're interviewing at has a good tech team (good luck!) they've probably provided this sort of equipment before and will sort you out with what they know works best.
    I guess, if you can't get hold of a gauze without a heatproof bit, you can probably hold the bunsen to the bottom of the beaker at the edge and make the croutons float the other way?
    Best of luck with the interview!
  2. blazer

    blazer Star commenter

    I've never seen it done with soup.

    I do it using potassium permanganate.

    Get the biggest beaker in the department and place on a tripod. Fill it with cold water and leave to stand for at least 20 minutes (set it up before the lesson)

    Then you need a length of glass tubing long enough to reach the bottom of the beaker.

    Put the tube into the beaker vertically near the side. Drop a crystal of permanganate down the tube so that it sinks to the bottom of the beaker.
    Place your finger over the end of the tube and withdraw it so that the purple coloured water stays in the tube.
    Then light a bunsen and use the smallest flame possible and place it beneath the crystal. You could even use a candle flame instead just put the flame immediatly under the gauze under the crystal.

    After a minute you will get a purple 'mist' rise off the crystal and travel to the top of the beaker, go across the top and down the other side.

    You can demonstrate convection in gases by making a paper spiral and suspending it on cotton thread above a candle flame.
  3. blazer

    blazer Star commenter

    Tea bag balloons are also good.

    Buy some tea bags,(not pyramids or round)
    Cut off one end and remove the tea
    Push you finger up inside the bag to open it up a bit and place open end down on a bench mat.
    With a splint gently light the top of the bag.
    It burns down leaving behind a 'bag' made of soot. Eventually the mass of this bag is so small that the hot air inside will lift it to the ceiling.

    You could also blue tack a thermometer to the ceiling and take the temp near the floor and by the ceiling. Or blue tack one on the wall with the bulb at floor level and a second one immediatly above but with the bulb at the top and at ceiling level. (just make sure that they read the same temp when they are on the table as thermometers can differ by about 4 degrees!
  4. A gauze, being metal, is a good conductor. It is designed to spread the heat across the bottom of the glassware.
    This is not what you want, so NO, you do this without a gauze. However, as others have stated, you use a small flame and allow time for the effect to occur.
    Soup sounds interesting - I can imagine it working well but how far away are the pupils going to be?
    What else have you planned - this sounds like a 5 minute job at the most?
    I often used data-loggers to get a real-time graph of temperature rise in different places (works well with a copper rod, for conduction, but could work in air for convection I think). It is possible to switch the energy source on and off and see how the changes propogate through the medium. It can all be projected on a screen for everyone to see, describe and explain.
    Good luck.
  5. Thanks for all the advice, only just finished getting myself sorted. Sorry if my post is a bit rambled I'm in a rush to get to bed so will reply with more detail tomorrow.
    I found out about the soup idea on here and thought is sounded good, I hope it works! Don't worry I have got plenty of other stuff planned so hopefully will use all the time I've got.
    I was given less than 24 hours notice about this interview so today has been all rush rush rush, hence me only just finishing now. Up super early in the morning too as the school is quite a drive away. If I had more time I would probably have planned something more 'complicated' but with having to decide what to do and also get my equipment request in I went with the simplest idea.
    Thanks again, ill reply properly tomorrow. [​IMG]
  6. I'm back and firstly I would like to say...I GOT THE JOB woooo.
    Secondly, the experiment works brilliantly if anyone else wants to try it. I originally got the idea off here but will explain it for anyone who comes across this in the future.
    Tinned tomato soup (I used tesco value) and put about an inch into a 500ml beaker. I bought small croutons (approx 1cm cubed) and put 4/5 directly in the middle. Heated from below with a blue flamed bunsen, through a gauze without a heatproof part. Took about 3 minutes for the croutons to separate. I had students wear safety goggles just incase beaker smashed, they stood around the desk so they could see.
    Also used scotch eggs cut in half to model the earths structure. Both worked really well and the students were great.
    So exhausted now! Thanks for all the responses. Off to relax now.
  7. blazer

    blazer Star commenter

    Congratulations, your lesson is making me hungry!
  8. phlogiston

    phlogiston Star commenter

    Well done
  9. Well done on getting the job.
    However, you asked about convection and that is why people mentioned potassium permanganate etc. You seem to have dealt with convection INSIDE THE EARTH, which is a slightly different topic. It is always sensible to give as much detail as possible because then folk can provide the most suitable help.
    I get really cross when people post titles such as "Help" or "Cells" which doesn't make it clear whether this refers to biology, chemistry or physics. I suspect many people don't even bother reading these.
    Once again - well done. Good luck for the future.
  10. Sorry, yes I did mean convection inside the earth. Apologies for ny confusion, but I have never taught this aspect of science and can't even remember doing it at school so my knowledge on this subject is minimal. I probably should have given more detail but I was in a blind panic and rushing to get everything sorted.

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