Matchstick boats

Build matchstick boats that will zip across a water surface - powered only by soap.

You will need:

• A matchstick
• A bowl or tray of clean water, which isn't soapy
• A knife
• A drop of washing-up liquid

What to do:

First, use the knife to make a split in the wooden end of the matchstick, making it into a Y shape.
Fill the bowl with water (just a cm or so is fine).
Gently float the matchstick on the water.
Place a tiny drop of washing-up liquid into the split on the matchstick.
The matchstick zips away from the washing-up liquid as soon as you drop the washing-up liquid on the water.

What’s going on?

Water molecules have an attractive force between them, so if you imagine that you're a water molecule right in the centre of a tank of water, you'd be pulled in every direction by all the water molecules around you - and all these forces would cancel out so you wouldn't move. But if you’re a water molecule at the surface then you won’t be pulled from the top at all because there is just air above you. So, you end up with a more dense film of molecules on top of the water. The result of this is surface tension – the molecules are more attracted to each other then to the air above. Water molecules are also attracted to other substances, so a matchstick, for example, will be pulled by the water wherever it is touching it.
Washing-up liquid is a surfactant, which is something that breaks down surface tension. So by adding washing up liquid, one side of the matchstick has the attraction between the water molecules broken, whereas on the other side of the matchstick, they're still attracting each other and the matchstick.
What the matchstick feels is a pulling force from all the molecules on the clean water side, but on the other side there is virtually no surface tension. So rather than being repelled by the washing-up liquid, it's actually being pulled from the other side, across the bowl of water. Clever eh?

Cheesy Waves

Microwaves in a microwave oven are the kind of waves that don't travel anywhere but just stay in one place. These are called standing waves. You can make standing waves with a skipping rope or by banging a drum. In this experiment you can see standing waves by melting cheese in a microwave oven. Plus of course afterwards there is the added bonus of eating it!

You will need:

• A microwave oven


• Some slices of processed cheese


• A (microwaveable) plastic tray

What to do:

Remove the turntable from the microwave.
Lay strips of cheese on the tray and put it into the oven.
Either give it a quick blast (about 10 sec) on high power or try leaving it in there for 1 min on a low setting like defrost. (This is better if your microwave timer doesn't have seconds on it).
Take the tray out of the oven and look to see where the cheese has melted. The places where the cheese has melted show where the microwaves inside the oven are biggest (where the waves have maximum amplitude).

What's going on?

In some parts of the oven, the waves have a high amplitude and the cheese gets hot and melts. In other parts, the amplitude is small, or zero, and the cheese doesn't melt.
This is why you need a turntable in a microwave oven. If the food isn't turned round it doesn't get cooked evenly all over.
(I did this experiment in the microwave in the staff room and saw parts where the cheese didn't melt!)