Coloured sugar-coated chocolates are great for exploring changes in water density. As well as being fun and tasty, they can be used to understand many naturally occurring phenomena, including the ocean currents. This experiment can be performed by a wide range of age and ability groups, with the science behind it being explored at varying levels of depth.

## Method

1. Begin by filling one of your dishes close to the top with water, and placing it on your white surface.

2. Allow the water to settle for 30 seconds to a minute, and then add one chocolate to the middle of the dish.

You should begin to see the colour ‘bleed’ out of the chocolate into the water. Ask the class to observe this for a few minutes, and discuss what they are beginning to see happen to the colour.

3. After about five or six minutes, the colour should have migrated to the edge of the dish, leaving a clear ring around the chocolate itself.

4. Next, ask the class what is likely to happen if they place four chocolates of different colours (equally spaced) in a fresh dish. If you can, use highly contrasting and primary colours. Ensure to promote discussion and predictions of what the children think will happen, as this is a key aspect of understanding the scientific process.

5. Add the chocolates, and observe as they ‘bleed’. You should quickly be able to see that rather than the colours mixing together and creating new colours (as was possibly predicted); clear borders are created where they meet.

## What is happening?

This experiment is all about water density, and how it is changed by the dissolving of soluble substances.

As the shell dissolves, the sugar increases the density of the water it is in (in the centre of the dish). A density current is then created by the difference between the more dense water in the centre, and the less dense water on the edges, which has a lower concentration of sugar. The sugary water begins to flow towards the edges. This process continues once the colour has disappeared and the colourless inner shell dissolves. As the concentration of sugar (and therefore density) remains highest in the centre, the current keeps pushing outwards, forcing the colour to the edges.

The same density currents cause the results of the second experiment. The coloured sugary water is again pushed away from the chocolates, towards the less dense clear water. At the borders where the colours meet, the density of each section is equal. As a result, the currents abruptly stop. Given time, the borders will increase in intensity as more and more sugar is dissolved and pushed towards the borders.

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