The Science of the Leak-Proof Bag

This experiment seems like magic, but it is actually all about polymer chemistry! Most zip-lock bags are made from a plastic called low-density polyethylene (LDPE). This material is a polymer, which means it consists of long, flexible chains of molecules.

When you stab the pencils through the bag, you aren’t actually “breaking” the molecules; instead, the sharp point of the pencil slides between the long polymer chains, pushing them apart. Because these chains are flexible and “stretchy,” they immediately try to return to their original position.

Once the pencil is through, the polymer chains push back tightly against the sides of the pencil. This creates a temporary, watertight seal held in place by friction. As long as the pencil remains in the hole, the water stays inside. If you pull the pencil out, the chains cannot bridge the gap fast enough, and the water will pour out!

Variables to test

Find out more on variables here.

• Thermal Energy
  Does it make a difference with hot vs. cold water? Heat can sometimes make polymers more flexible—does this make the seal better or worse?
• Material Quality
  Try different brands of zip-lock bags. Are “heavy-duty” freezer bags more effective than thin sandwich bags?
• Object Diameter
  Can you use thin vs. thick pencils? At what thickness does the polymer finally fail to create a seal?
• Surface Texture
  What about other objects like bamboo skewers, sewing needles, or tomato stakes? Does a smoother surface create a better seal than a rough one?