X-Peri-Mas #2 – A Tower of Water

Yesterday’s X-Peri-Mas was about density and stacking up layers of liquids that don’t mix too well with each other. We ended up with four layers where the density decreased as we went up the column. Hopefully you’ve all given it a go but follow the link above if you haven’t!

Today is a similar idea – but we are going to make it a little bit more complicated. This time we are going to make layers of liquids that definitely do mix together easily – it’s all going to be water. So we are aiming to pour water, onto more water, and somehow we are going to end up with layers?

Yup.

With a Christmas theme we have gone for red and white stripes. As before, to get one layer to sit on top of another they will need to have different densities – so the lower layer has to be more dense and the layer above has to be less dense. To change the density of the water we will add salt.

What you will need:

  • Water
  • Food colouring
  • An empty bottle
  • Some string
  • Sticky tack
  • Foil

As we go down the column, each layer is going to have more salt added to it. The top layer is going to be fresh water, the bottom layer is going to be totally saturated with as much salt as you can get in. In 100 mL of water (about half the size of a cup):

For a good starting point of 3 layers in your column…

LayerVolume of waterAmount of salt
Bottom100 mL6 teaspoons (saturated)
Middle100 mL3 teaspoons
Top100 mLNone – fresh water

For an ambitious 7 layers in your column…

LayerVolume of waterAmount of salt
Bottom (layer 1)100 mL6 teaspoons (saturated)
Layer 2100 mL5 teaspoons (should all dissolve)
Layer 3100 mL4 teaspoons
Layer 4100 mL3 teaspoons
Layer 5100 mL2 teaspoons
Layer 6100 mL1 teaspoon
Top (layer 7)100 mLNone – fresh water

There are a couple of ways to do this and we’ll have a look at both. The hard part of this experiment is pouring a layer of liquid so gently that it doesn’t disturb the layer you are pouring onto and cause the two to mix. The first way we are going to try and do that is to use some string – and pour the water down the string.

Here’s the set-up. I cut the top of a bottle away and turned that upside down. Then I drilled a small hole through the cap and fed a piece of string through it. I tied a big knot at the end of the string to spread the water out when it got to the end. To make sure there weren’t any big drips to ruin things I used a blob of sticky tack to seal around the string and then poured the liquid in!

Pull the string up slowly as the layers rise so that the knot is just in contact with the very top of the layers.

The faster the water is falling when it hits the layer below it, the deeper into that layer it will get. The deeper it gets, the more the two liquids mix and the less defined your layers will be. This is where the string comes in. It’s the same as going to the park and sliding down one of those poles – it slows you down on your descent and means that you touch down to the ground nice and softly. The water falling down the string does the same thing and gently touches down on that layer.

The second way to break the fall of the water is to have something floating on the top surface – I used rolled up balls of foil. Then you can gently drip the water onto those foil balls and it will stop the liquids mixing together and you can get nice clean layers.

This is playing back at 30x normal speed – so be patient. You can see the liquid falling through the layer and disturbing the one underneath a little bit – I tried to speed things up as the layer got thicker. You need to go slowly at first but you can get away with a bit more towards the end.

Try an even smaller version…

If you try this at home please do take pictures and share them with me! Find more on twitter with #XPeriMas!

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