Air is always pressing around everything, but we can’t see it. This week we did some experiments that helped us see air pressure in action.

Experiment 1 – Rising water

What You Need

• shallow dish
• tall glass
• candle
• clay
• food colouring
• matches/lighter
• water

What you do

1. Place the candle on top of the clay in the shallow dish.

2. Pour in some water and add a few drops of food colouring.

3. Light the candle and quickly place the inverted glass over it.

What happens

When the candle burns out, the water level inside the glass rises.

The scientific explanation

When the candle has used up the oxygen in the glass, the flame goes out. The air inside the glass cools and contracts, and water rises up into the glass to fill the gap.

Experiment 2 – Ice Water Can Crusher

What you need

• empty drink can
• shallow tray or dish
• tongs
• ice
• water
• stove or hot plate (and an old saucepan if, like me, you have an induction hob)

What you do

1. Prepare a tray filled with enough ice to cover its base, and water to cover the ice.

2. Put a small amount of water into the empty can and set it on the stove until the water inside boils. (If you have an induction hob, place the can on the base of an old saucepan.)

3. Quickly pick up the can with the tongs and put it upside down into the tray of icy water so that the opening is under water.

What happens

Very soon after it enters the icy water, the can is suddenly and noisily crushed!

The scientific explanation

When the can is placed in the cold water, the air inside it cools and contracts. The greater air pressure from outside presses on the can and crushes it.

My top tip

We had to do this twice as it didn’t work the first time.  I had put too much water in the can and didn’t heat it enough before putting it in the icy water. The second time I used less water and made sure steam was coming out before I put it in the water.

Experiment 3 – Only do this if you have a wide-necked glass bottle

We tried this one but our bottle didn’t have a wide enough opening for it to work properly.

Try to insert a (peeled) hard-boiled egg into the neck of a glass bottle. Observe that the egg is too big to go into the bottle.

Then drop a couple of burning matches into the bottle and try again. If your bottle neck is wide enough, after a few seconds the egg should be sucked into the bottle. This happens because when the air cools the pressure drops and the egg is sucked in to fill the gap.

Our bottle wasnt wide enough to suck in the egg, but the kids noticed that the egg changed shape because of the suction from inside the bottle. We gave it a bit of a push and the egg broke!

To get the egg back out of the bottle, turn the bottle upside down and blow hard into the bottle. This increases the air pressure and the egg pops out. (Or do what I did when I forgot these instructions, and instead light a match under the inverted bottle, which also works.)

Resources

All these experiments came from Science Experiments: Loads Of Explosively Fun Activities You Can Do.

Join me at Adventures in Mommydom’s Science Sunday for more homeschool science fun.