Kitchen Science Experiments

Mixing two fluids of different densities

Sun, 2nd Apr 2006

Part of the show Brainwashing and the Science of Pain

What you Need

A bowl

A bowl with fairly smooth sides

Some coloured squash

Some brightly coloured squash - preferably with sugar.

What to do

Fill the bowl with water

Gently pour squash down the side of the bowl

watch what happens.

What may happen

As you pour the squash down the side of the bowl it will run down to the bottom, slosh around and end up settling at the bottom. While it is moving it does tend to swirl and mix the squash and water slightly. 

 

Once the fluid has stopped moving there is hardly any mixing at all. <img src='uploads/RTEmagicC_bowlend.jpg.jpg' style='width: 300px; height: 218px;' title='The squash ends up under the water (c) Dave Ansell' alt='The Bowl at the End' >

Why does it happen?

Squash has a lot of dissolved sugar in it so it is considerably more dense than water, so it will sink in the same way that a dense stone will sink. Once it has sunk there is nothing but molecular vibrations to mix the two liquids, it would probably take literally years for the two liquids to mix together. This is why it never works very well if you add the squash after the water - there is nothing to make them mix.

However when the squash is moving you can see it swirling as in moves next to the water.

The Flow

This is because the squash next to the water is slowed down which tends to twist the water in the same way that a car will twist if you drive one side into sand because one side will be slowed down.

A car in sand

 This random swirling is known as turbulence very good at mixing fluids and is actually what you are creating when you stir your coffee or when you pour water on top of your squash.

Dave Ansell

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