SAFETY

This activity involves matches. For any activity involving fire have an adult present, water, a fire extinguisher and any other safety equipment needed.

You will need

• jar

• rubber glove or balloon

• rubber bands

• water

• tea towel

• scissors

• matches.

What to do

1. You will need a piece of rubber to seal the jar. If you are using a rubber glove, cut a square of rubber from it large enough to stretch over the top of the jar. If you are using a balloon, cut the neck off.

2. Place about 1 cm of water in the jar. Warm water works best, but cold is fine.

3. Dry the outside of the jar, so the rubber won't slip on it.

4. The next few steps need to be done fairly quickly:

a. Have your adult helper light a match and let it burn for a few seconds.

b. Have your adult blow out the match and immediately drop it into the jar.

c. The moment the match is dropped, stretch the rubber over the top of the jar to seal it.

d. Place some rubber bands around the top of the jar to hold the rubber in place.

e. Pinch a bit of the rubber and pull it up. You should find a white fog forms in the jar. When you release the rubber, the fog will vanish.

What's happening

There is invisible water in the air all around us, called water vapour. How much water vapour there can be in the air depends on the temperature of the air. The hotter it is, the more water vapour there can be in the air.

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Clouds form when air containing lots of water vapour is cooled down, or if there is a drop in pressure, the water vapour condenses out to form a cloud.

In your jar, some of the water evaporated. When you pulled up on the rubber, you lowered the pressure in the jar. This in turn lowers the temperature of the air. With a lower temperature, some of the water condenses out of the air to make tiny droplets of water. These droplets are the fog that you see in the jar. When you let go of the rubber, the pressure and temperature go back to normal and the droplets evaporate again.

The smoke from the match is important, because it gives the fog somewhere to form. The smoke contains tiny particles of soot, so small you probably couldn't even see them in jar. The droplets of fog actually form around the soot particles. If you try it again, without using the match, you won't get the same fog, because the water will condense on the sides of the jar.

Real clouds and fog form in a similar way. The air around us contains water vapour. When air containing lots of water vapour is cooled down, or if there is a drop in pressure, the water vapour condenses out to form a cloud. In nature, the droplets often form around dust, pollen, smoke or any other tiny particles in the air. Fog is just a cloud that forms at ground level.