Reason to Use Leak Proof Bags

Polymer chains work together to prevent water from leaking out of the bag. Who would have thought that a plastic bag, some water, and some pencils would be screaming in fear of adults? Learn how to make a hole in a plastic bag filled with water. Well, that's the theory you're going to test. . . And it's wise to follow your liquid trick on the sink. It's a great way to learn about the chemistry of polymers.

Start by sharpening the pencils. Make sure the points are sharpened to a point.

Fill the bag half full with water, and then seal the bag. Ask your dining guests this question: "What if I try to push one of these pencils into a bag of water? 

Will the water run out, and there will be a big mess?

They will answer yes until they know the scientific secret, but you are not telling. Yet!

Helpful Hints:

Make sure the pencil points are sharpened to a point. Be careful not to push the pencils through all the holes, or your "spear" experience will turn into a big "clean" activity.

Here comes the terrible part. Hold the pencil in one hand and the top of the reusable leak proof bags in the other. Believe it or not, you can put the pencil on one side of the bag to the right and take it out halfway without spraying a drop. The long chains of molecules that form on the bag magically seal around the pencil and prevent water from escaping. Now, this is the "spare eight" of science! Sounds impossible? Try it first on the sink and then on your friend's head just for fun.

It's just like the Las Vegas-style magic process where the magician draws swords through the cabinet, yet the beautiful assistant comes out without hiding. Well, maybe not, but you have to agree that it's amazing that there's water in the bag. Be careful not to push the pencils through all the holes, or your magic trick will quickly turn into a "clean" activity.

When you're done, place the bag on top of the sink or bucket and take out the pencils. Toss the bag in the recycling bin and dry the pencils.

How Does It Work?

The zipper-lock plastic bag you used was most likely made of low-density polyethylene (LDPE) polymer. It is the most widely used packaging material in the world. LDPE is low-cost, lightweight, durable, moisture resistant, and very flexible.

Think of polyethylene molecules as long pieces of freshly baked spaghetti. The sharp pencil's tip can easily slip and separate the elastic wires of the spaghetti, but the elastic properties of the wires help create a temporary seal against the edge of the pencil.

When the pencil is removed, a hole remains in the plastic bag because the polybag molecules were permanently pushed aside, and water leaked out.

As you may have discovered, it is much easier for a stretched plastic to seal around a pencil's smooth sides than the straight edges found on other pencils. Hopefully, you discovered this sign during practice, not when the bags were on someone's head.