A quick and colourful demonstration to inject a little science into otherwise grey and rainy spring days.

Materials needed:

• a large, clear jar
• a smaller glass container (doesn’t have to be clear)
• food colouring
• hot and cold tap water

Method:

Fill the large jar with the coldest water you can get from your tap. Don’t fill it all the way to the top – leave room to accommodate the volume of the smaller container. For really cold water, put the water in your fridge or freezer for a few minutes.

Put ten drops of food colouring into the small container and fill it to the top with hot water.  For the most dramatic effect, boiling hot water is best, so that there is a greater difference in temperature between the hot and cold water. However, if you are working with small children, you can still get satisfactory results with the hot water from your tap.

Ready to drop!! Carefully drop the small (hot) container into the larger (cold) one as gently as you can.  Try not to add any extra disturbance or motion with your hand.

Watch the convection current develop, as the coloured hot water mixes with the cold water.

What’s going on here?!?!

Convection.

The molecules of the warmer, coloured water are less dense than the cool clear water molecules and “float” to the top of the container. As they rise to the top heat is exchanged and the coloured fluid cools, and is replaced by hotter water still rising from the heat reservoir.