Chemistry

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Video

Materials

• 1 drinking glass
• 1 kitchen towel
• 1 rubber band
• Water

Short explanation

The pores in the towel are too small for water to penetrate. Air, on the other hand, can get through, which becomes noticeable when you tighten the towel. Then the space inside the glass expands and the new void is filled with air that enters through the towel.

Long explanation

Water consists of water molecules that attract each other quite strongly. This means that water tend to stay together, like when water droplets hold together when they fall through the air and how the surface of the water forms a barrier that requires some force to break.

When water is "broken apart" into small parts, each water molecule doesn't "experience" as much pull from other water molecules anymore. They can now move relatively freely. Therefore, the pores in a kitchen towel can quite effectively absorb water you have on your fingers (the fabric also attracts water molecules to some degree). However, when water is in a large mass such as in a glass, the water spans across the pores of the kitchen towel and no water flows through. You can think of the water molecules as small figures holding each other's hands, where some hang over the pores but are still held back by their friends.

But why is the water bubbling? When you stretch the kitchen towel, you increase the volume in the glass. The water in the glass cannot magically expand to fill the new volume. Instead, air enters trough the towel. Air is a mixture consisting mainly of nitrogen molecules and oxygen molecules. Since these molecules don't attract each other, they are free to easily pass through the pores of the towel and fill the new space in the glass.

To get plenty of "boiling", let the towel bend down a good bit into the glass before you secure it with the rubber band.

Experiment

You can turn this demonstration into an experiment. This will make it a better science project. To do that, try answering one of the following questions. The answer to the question will be your hypothesis. Then test the hypothesis by doing the experiment.

• What happens if you change the kitchen towel to something else?
• What happens if you change the water to something else?
• What's the biggest container of water you can do this with?

Variations

To make the demonstration a bit more fun, you can add some food coloring to the water. However, this stains the kitchen towel.

You can turn this into a magic trick. When the glass is upside down, ask a friend to rub her fingers against her pants to "warm them up". Then ask her to put her fingers on the kitchen towel. At the same time, tighten the kitchen towel so it starts to "boil".

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