Chemistry

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Video

Materials

• 1 plastic bag with zip lock (or other transparent and strong plastic bag)
• A couple of sharpened pencils
• Water

Short explanation

When you make a hole in the bag, you separate some of the molecules that make up the plastic bag. However, the remaining molecules still hold together, which prevents the plastic bag from tearing completely. The reason why water doesn't come out is that water drops are too large to get through the small spaces between the pencils and the plastic.

Long explanation

The plastic bag consists of long molecules called polymers. They are like long spaghettis. These polymers bond together with chemical bonds called cross-links, just like spaghettis stick together.

It's these cross-links that hold the plastic bag together, even when it's stretched. However, only to a certain point. If the polymers are pulled apart too much, the cross-links will break.

When you insert the pen through the plastic, some cross-links will break, but since the strain is not too much, the remaining cross-links are numerous enough to hold the plastic bag together.

So that's the reason why the plastic bag doesn't burst. But why water leak, since there are small gaps between the pen and the plastic if you look closely? Well, water consists of water molecules that attract each other quite strongly. This results in the surface tension of the water, i.e. how 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. Thus, in normal cases, water can only exist in larger clumps (like drops) and these clumps are too large to get through the gaps. You can think of the water molecules as small figures holding each other's hands, where some hang over the gaps but are still held back by their friends.

However, water may in some cases start to seep out through the gaps. If you squeeze the bag, you increase the pressure inside and water is forced out through the holes. Now the pressure becomes too strong for all the water molecules to be able to hold on to each other - they tear apart. Water will also leak out if you wiggle the pencils and make the gaps bigger.

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.

• If you tilt the bag, will this make the water leak?
• If you squeeze the bag, will this make the water leak?
• If you wiggle the pencils, will this make the water leak?
• If you wait for a long time, will water eventually start to leak?
• What is the thickest object you can stick through the bag without water leaking?
• Does it matter if the pencil is hexagonal or round in shape?
• What is the biggest plastic bag you can use and still make the demonstration work?

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