A Watched Pot

This experiment came from researching an experiment on cooked vegetables. As I was boiling each vegetable sample, I had quite a bit of time to watch the water boil. If you have ever watched a pot of water boil, did you notice all the strange things the bubbles did? No? Then it is time to go back to the stove and boil some water.

You will need:

- a pot with a lid
- a stove
- water
- patience

Fill the pot about half full of cold water. Place the pot on the stove burner and turn on the heat. Now get comfortable and watch. Keep watching.

Watch some more.

As the pot and water heat up, you will notice tiny bubbles forming on the bottom and sides of the pot. These first bubbles are not the start of boiling. Instead, they are from dissolved gases being driven out of the water. Still, these bubbles are important, as they will serve as starting points for the bubbles produced by boiling.

As the water heats more, the process of boiling begins. Heat causes the molecules of water to move faster and faster. When they are moving fast enough, they break away from the other molecules and change from a liquid to a gas. The molecules of water leap into the tiny bubbles we saw earlier. This forms a bubble of water vapor. When the bubble gets large enough, it breaks free and begins to rise. Watch closely and you will see an amazing thing. The bubble vanishes before it reaches the top of the water!

Where did it go? Heat from the pan caused the water molecules to move fast enough to become a gas. When the bubble begins to rise, it moves through the cooler water in the upper part of the pot. This cooler water condenses the water vapor back into liquid water and the bubble disappears.

As the water in the pot gets hotter and hotter, it finally reaches the point where the bubbles reach the surface. Now you need to watch very closely to see the next amazing thing. Some of the bubbles pop, but most do not. Instead they shrink and disappear. The same thing is happening, but this time it is the cooler air that causes them to condense back into liquid.

The next step is to put the lid on the pot. Very quickly you will probably find that the pot is boiling over. Bubbles and liquid begin to come out from under the lid. Carefully lift the lid and you will see that the pot is full of bubbles. Almost instantly, the bubbles go away and the pot is once again just boiling water. Why does putting the lid on make the pot boil over? Think for a minute. With the lid off, the cool air caused the bubbles to collapse. With the lid on, the air inside the pot gets hot enough to keep the water vapor in its gaseous form. More and more bubbles develop and the pot quickly fills with foam and boils over. When you lift the lid, cooler air comes in contact with the bubbles and they go away.

All of this boils down to the fact that while watching a pot will not keep it from boiling, keeping the lid off to watch it will help keep it from boiling over.