The Science of Polymers — The Leak Proof Bag

Here’s an easy and inexpensive way to get kids excited about science. All you need is a few pencils or cooking skewers, water and some plastic bags. This experiment is perfect for early childhood learners, preschoolers or even older children with a little adult supervision. Sharp pencils and skewers can also poke holes in your skin.

This is a good experiment to do outside, over a sink or tub to catch the drips. Fill the plastic zipper-lock bag with water more than half full. Don’t use sandwich bags or you will get wet. Seal the bag and then take your pencils or skewers and carefully poke them through the bag and through the water, out to the other side.

Are you getting soaked? You shouldn’t be. There may be a few leaks and drips, but the bag should seal itself around the pencil and keep the water from spilling.

How does this work? The plastic bag is made of polymers. Polymers are long chains of molecules. When you poke through the plastic, you are stabbing between the long strands of polymers and the bag seals itself around the pencil keeping the water inside. Just watch out when you remove the pencils. Shower time!

4 thoughts on “The Science of Polymers — The Leak Proof Bag”

  1. it worked when i did it but i think i put in to much water but it was because i put in a lot of pencils

  2. Hello Steve,

    I really like the Leak-Proof Bag experiment for my upcoming year of middle school physical science. I understand the polymer structure and why the bag does not leak when the sharp objects are pierced through, but why do the holes in the bag not attempt (excuse the personification) to seal up when the pencils are removed. If the polymers were not broken, but only separated, why can they not return to their original orientation to be right next to each other? Why would the polymers along each hole not attempt to “squeeze shut” over the hole?

    Also, there is more pressure inside the bag than out and a higher concentration of water inside the bag compared to outside. I think of this like patching a bike tube; when you apply the patch to the tube you are supposed to inflate the tube to help the patch seal itself completely over the tube. The pressure from the inflated tube forces the seal to be made. Why would the pressure from the water inside the bag not force the “flaps” of the hole in the plastic bag back together? I understand there is still a hole in the bag so the pressure from the water inside the bag would then force the flaps apart so they are open from inside out.

    So, I can understand from a common sense point of view why the bag leaks when the pencils are removed, but not from a scientific/polymer-understanding one. What is preventing the polymer molecules from coming back together and forming a seal? Is something like high heat necessary to make polymers “stick” together?

    Thank you for taking the time to read this long inquiry and I appreciate any feedback. I have enjoyed all of your experiments and videos and am planning on doing them in my own classroom this upcoming year.

    – LK

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