Imagine you have two cups full right to the brim with drinks. Now someone comes in and tells you that you’ve put them in the wrong cups! That drink was meant to be in this cup and this cup was meant to be in that cup…

Can you swap the liquids over straight from one cup to another?

What is the trick?

What you will need

  • Two identical cups, bottles, jars or any other container
  • Hot and cold water
  • Food colouring
  • Oil
  • A plastic card – anything flat and waterproof

How to do it

Oil and water

Here’s the easiest one. Fill one cup with water and one cup with oil. Place the card on the top of the water cup so that it totally covers the top. Hold the card in place with your whole hand and turn the cup upside down. If you’re lucky and you’ve got a good seal you will be able to let go of the card now and it will stay in place! Best to practise over a bucket… just in case…

Place the water cup with the card lid on top of the other cup and line all the edges up. Gently slide the card out from between the two cups so that there is a little gap.

The oil will start to rise and squeeze through the gap and swap with the water – the bigger the gap, the faster they will swap over. Sooner or later the top cup will be totally full of oil and they will have swapped!

I’ve got them the wrong way round and nothing is happening – help!

Don’t worry – if you’re careful you can just flip the whole thing over

Can we do this trick with just water?

Oil and water definitely don’t mix and that makes this trick easier. To make things a bit more difficult we can try and do this trick with two liquids that definitely do mix – what if both liquids were water?

There is one way that we can make just pure water rise and fall without adding anything – heat. This doesn’t work quite as well as fully swapping over but we can do a cool trick with it.

Fill one cup with hot water and one cup with cold water. Place a card on the cold cup and flip it upside down as before – slide the card out of the way. See what happens…

…bit disapointing? You just end up with the two mixing together. But what happens if we try it the other way around? Try the hot water on the top this time and see what happens now.

This time the liquids don’t mix at all! The hot water stays on top!

What’s happening?

Simple really – especially if you’ve read the other X-Peri-Mas posts so far. This is all about density – we are placing the most dense liquid on the top so that when the gap is open the most dense liquid wants to sink through the other liquid and take all the space at the bottom – pushing the less dense liquid upwards. Oil is less dense than water so if you put the water on top, they will swap places.

Slightly differnet with the water since it’s the same liquid in both cups! This time the difference between the two is temperature. Heat rises so the hot water on the bottom moves up into the cold water but since it is all just water they mix together really easily so you just end up with slightly warm water. If we do it the other way around though and have the hot water on top then the hot water rises and stays at the top of the cup and the cold water sinks and stays at the bottom. They will mix eventually as the hot water cools down and starts to sink a bit. About half an hour later they will be just as mixed as if you’d shaken the whole thing.

This is an easy one and doesn’t need much kit or practice so no excuses! Try it out and share your pictures and videos on twitter with #XPeriMas!

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