Activity Title: The Leakproof Bag
How Do I Do It?
1. Sharpen all pencils to a fine, sharp point. Ideally, the pencils should be about the same length.
2. Fill up the ziploc bag so it is about 2/3rds full of water and seal it closed.
3. Now for some hypothesis formulation! Ask your pre-schooler some questions
a. “What would happen if I tried to push one of these pencils through the bag of water?”
b. “Would the water leak out and make a giant mess?”
4. Hold the bag up (probably over a sink) and slowly insert pencils piercing the plastic, going through the water, and out the other side of the bag. Make sure you don't poke the pencil all the way through. Why stop with one? Keep putting pencils in!
5. Be amazed that the bag isn't leaking.
6. Now move the bag to the sink to remove the pencils and watch water squirt out everywhere!
Extend the experiment:
7. Try experimenting with plastic bags of different sizes and thicknesses. The thicker the bag, the harder it is to get the pencil to pass through.
For a really thin bag, use a plastic bag from the produce section of the grocery store.
Experiment with different sizes and shapes of pencils. Some pencils have flat edges while others have perfectly round, smooth edges. Which type of pencil works best?
What's the Science Behind It?
The polymers in the plastic are chains of molecules that separate but don't break when poked with the pencil. They squeeze in so snugly around the pencil, it forms a tight waterproof seal. The zipper-lock plastic bag you used was most likely made out of a polymer called low-density polyethylene (LDPE). It’s one of the most widely used packaging materials in the world. LDPE is low in cost, lightweight, durable, a barrier to moisture, and very flexible.
Think of the polyethylene molecules as long strands of freshly cooked spaghetti. The tip of the sharpened pencil can easily slip between and push apart the flexible strands of spaghetti, but the strands’ flexible property helps to form a temporary seal against the edge of the pencil. When the pencil is removed, the hole in the plastic bag remains because the polyethylene molecules were pushed aside permanently, and the water leaks out. As you might have discovered, it’s much easier for the stretched plastic to seal around the smooth sides of a round pencil than the straight edges found on other pencils.
STEM Category: Science