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Hot Air Bottle

This experiment is a simple one that I came across while washing dishes. Even thought it was an accidental thing, it really is a great way to show how much air expands when it gets hot. To try it, you will need:

  • a sink or bathtub of hot water
  • a plastic soft drink bottle or similar container

The first thing we need is a lot of hot water. You could fill the sink with hot water if it is time to wash the dishes, or you could fill the tub with water if it is time to take a bath. You need the water to be fairly deep, so you will have room to get most of the bottle under water. The water should be hot, but not hot enough to burn you.

Turn the bottle so that the opening is at the bottom and push it into the water. Hold the bottle there and watch the opening carefully. What happens? Air begins to bubble out of the bottle. As you watch, more and more air continues to come out. You may be quite surprised at how much air leaves the bottle.

Once the air stops coming out of the bottle, put the cap back on. Screw it on tightly. Then remove the bottle from the water. Place it on the table and watch it carefully. What happened? Slowly, the bottle collapses. Why?

When you put the bottle into the hot water, the air in the bottle gets warmer. As it warms up, the molecules move around more energetically. The molecules bounce against the bottle and the water with more force. This forces some of the air out of the bottle. As the air gets warmer and warmer, the air in the bottle pushes outwards more and more, causing more and more bubbles to come from the bottle.

When the air in the bottle had expanded all it is going to, you put the cap on the bottle and removed it from the water. At that point, the air in the bottle began to cool. The molecules moved less energetically, causing the air to exert less force on the sides of the bottle. The stronger pressure on the outside of the bottle crushes it inwards.

You can take this experiment a step farther by using a balloon. Put the balloon over the mouth of the bottle before you put it into the hot water. As the air heats up, it will inflate the balloon. Then remove the balloon to equalize the pressure. Put it back on the bottle and then remove it from the water. Now as the air in the bottle cools, it will exert less force and the stronger pressure on the outside will force the balloon into the bottle. Keep playing and you can discover all sorts of things, as well as having some fun.