Amaze Kids with Science Magic.
1. Fill a quart zip-lock plastic bag about half-full with tap water.
2. Zip the bag closed.
3. Support the bag with one hand as you piece it with the sharpened end of a round pencil.
4. I suggest placing a collecting pan just in case —-the bag might be dropped.
Note: The bag doesn’t leak as long as the pencil remains in place.
Why doesn’t the bag leak?
The plastic bag is made out of very long stretchy entangled molecules. When the pencil pierces the plastic, it pushes the molecules in the plastic aside. When pushed aside, the molecules in the plastic, much like molecules in a rubber band, are stretched out of position. Like any elastic material, when stretched the plastic bag molecules try to return to their original shape, but the pencil is in the way. Thus, the molecules fit tightly around the pencil forming a water proof seal. If you remove the pencil water will pour out the tear made by the pencil.
More to Explore
1. What effect does the shape of the pencil have on the results? Would a pencil with hexagonal shaped pencil have the same results?
2. Are all plastic bags leak-proof? Test different types of flexible plastics, such as the plastic bags used to sack up purchases at the store, or how about garbage bags.
For Inquiring Minds
Plastics are made of polymers.
poly– means “many”
mer– means “part” or “unit”
Thus the word polymer means many parts or many units. Polymers are megamolecules made up of many units called monomers. Mono-means “one. The monomer that makes up the mega-molecule in the plastic bag is ethlene. The name for the polymer is polyethylene.
Compare and contrast Low Density Polyethylene (LDPE) and High Density Polyethlene (HDPE).
Did you notice how the water in the plastic bag refracted light?
While light refr
action was not part of the activity, if asked why the pencil looks broken when it enters the water, you can explain that light changes direction when it goes from air into water, thus light coming to your eye from inside the bag makes the part of the pencil inside the bag appear to be separated from the pencil.
More about polymers and other fun science activities can be found in 204 Sticky Gloppy, wacky & Wonderful Experiments.
