Dancing Sultanas

by Deb on February 18, 2011

Dancing Sultanas

Thanks to that recurring act of parental insanity known as the playdate, I had a house full of hyperactive girls after school.  Something was needed to keep them from bouncing while I finished chopping fruit and pouring drinks, so Dancing Sultanas it was.  It’s a very quick and simple activity, but the 2 year old was happy watching it and the 5 year olds worked together to come up with an explanation.

Materials

  • A clear glass or jar.
  • Sultanas/currants/raisins – something relatively small and light but not too light.
  • Bubbly liquid.  We used soda water (aka sparkly water) from our Soda Stream, but you can also use lemonade or a bicarb soda and vinegar mix.
    • Dissolve about half a teaspoon of bicarb in water in your glass, then add a good splash of vinegar.  If it isn’t working, add more vinegar.

Method

  1. Prepare your sparkly liquid in the jar.
  2. Add the sultanas and watch them dance up and down.
  3. We also did the same with normal water, so they could see the difference.
  4. Challenge older kids to work out why.

Explanation

This was a group effort by the 5 year olds and some leading questioning.  Bubbles!  Bubbles go up.  When there are enough bubbles they will take the sultana up with them, but when they get to the top some of the bubbles pop so it falls back down again.

Basically, a bubble in a liquid is more likely to form where there is an anchor of some sort.  Have a look the next time you are boiling water in a pot or pouring a glass of champagne, the bubbles generally start where there is a scratch or speck of dust.  Because they’re rough, sultanas provide lots of nice anchors so lots of bubbles will start to form on them.  The sultanas themselves are denser than water, so they sink.  But when you add the gas in the bubbles, the whole thing can become less dense or lighter than the water, so it rises to the top where indeed the bubbles do pop so it sinks again.

Bonuses

The girls noticed something else – a lot of the bubbles will stick together and turn into bigger bubbles, which are more likely to pop off the sultana and go up by themselves.  There are two things happening here, which I didn’t go into with this age group.  Firstly, bubbles are round unles they are forced into another shape.  This is because the gas is pushing out in all directions equally, which forms a sphere.  When two bubbles touch each other there is no ‘wall’ to keep them apart, so they join together and form a new, bigger sphere.  This is also the reason they are more likely to leave the sultana and float.  A sphere only allows a tiny area to touch the sultana and hold on.  When they were separate bubbles they each had a grip, but when they join the gripping area will get smaller.  It is a bit like hanging from both hands, then letting go with one – you are much more likely to fall.

We left them going, and the little girl checked them out again in the morning.  She noticed they were still working but going much slower.  They were also bigger and had less wrinkles.  What’s happened here is that the sultanas have absorbed some of the water overnight, plumping them up and making their density closer to water.  This means it only needs a small change to make them move, but the small change will only move them slowly.  She also discovered they don’t taste very good 🙂

Try it yourself with different liquids and let me know how it works!

 

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{ 10 comments… read them below or add one }

Trish February 18, 2011 at 7:26 am

This looks like fun for a rainy day ahead ! Thanks.
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Jayne February 18, 2011 at 9:27 am

I’m going to do this with my kids on the weekend! 🙂

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Glowless February 19, 2011 at 6:42 pm

I remember doing this as a kid! We experimented with all different sized pieces of fruit and would have to guess before hand if we thought the bubbles would make this one sink or float.
Thanks for joining me while I flog-sit 🙂
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Deb February 28, 2011 at 3:52 pm

That would be lots of fun, I was thinking of trying things like lentils and popcorn and comparing them.

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Melissa March 1, 2011 at 9:55 am

Informative and fun hunt! Thanks so much – ill be back to check out more of your page later *bookmarked* 😀

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Sara March 2, 2011 at 9:52 am

Thanks for the fun hunt, I’ve bookmarked your page too, looks like a great website – will definitely be checking it out for more fun science ideas 🙂

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ange March 2, 2011 at 6:03 pm

Thanks for a fun hunt.

I will come back and find some more stuff to do with my little boy.

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Louise March 2, 2011 at 10:51 pm

I really loved visiting your page, some great ideas and information that I know will help me with my teaching degree!

Thanks for the hunt

I tried to complete your survey but the link is not working, please email me when this is available 🙂

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Bec March 3, 2011 at 9:03 pm

Love your site, from a fellow geek mum (and Dad)!

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Jacinta March 21, 2011 at 12:37 pm

Hi, I have just done this with my 18-month old son and he was absolutely fascinated and loved it! I’ll have to keep a closer eye on your site for more ideas 🙂

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