This experiment reminds me of the story of Hans Brinker or The Silver Skates. In the tale, Hans saves his town by plugging the hole in a dike with his finger, preventing a disastrous flood. In our demonstration, we will create our own “hole in the dam” using a pencil and a plastic bag.
Materials:
- Ziplock bag (quart size)
- Water
- Sharpened Pencil (several might be useful)
Experiment:
Fill the baggie about ¾ full with water and seal the bag tightly. You may need help from someone holding the bottom of the bag while you hold the top. Over the sink, quickly pierce the bag with the pencil. Push the pencil through the front and rear of the baggie. Does the pencil keep the water from flowing out? Can you put more than one pencil through the baggie without causing a leak?
A pencil pokes through both sides of a plastic bag filled with water, and somehow it is not leaking!
What’s happening?
These sandwich bags are made from low-density polyethylene (LDPE) polymers. The molecules that make up this polymer are very long chains with many branches. Imagine many of these chains stacked on top of each other and then piercing through them with the pencil. The molecules can move out of the way as the pencil pushes through, but then they can tightly surround that pencil.
Diagram showing how polymer molecules form ranched chains and tangle
Additionally, as we’ve seen in many previous ChemShort articles, water is quite amazing, and the water molecules like to stick to each other. Here, the water in the bag holds onto the other water molecules that are next to the hole and are on the verge of leaving.
Unless the hole gets too big, the water will not be able to squeeze through that small opening. This also explains why the Capri-Sun bag doesn’t leak when stabbed with the straw. Of course, plugging a hole in a plastic bag is much easier than stopping a leak in a concrete dam — but the basic idea of sealing off a potential flood remains the same.
Extension:
What happens if you wiggle the pencil around a lot? What happens if you squeeze the bag? Will this work with other plastic bags? Can you use a latex balloon filled with air? You may need skewers and oil for this to work well because the air molecules in the balloon can escape more easily than water molecules.
References:
https://en.wikipedia.org/wiki/Low-density_polyethylene
To view past “ChemShorts for Kids” activities, go to:
https://chicagoacs.org/ChemShorts.
