Halloween Science: Magic Bag of Blood

 - by KitchenPantryScientist

You’ll be amazed when you fill a plastic zip-lock bag with “blood” and poke sharp skewers through, only to find that the bag doesn’t leak!  All you need is a ziplock bag, water, food coloring and wooden skewers.   Heavy-duty ziplocks work best!  Fill a quart-sized ziplock bag with water, add a few drops of red food-coloring, and seal it.  Slowly poke several wooden skewers completely through the bag, from one side to the other, avoiding the part with air in it.  See how many you can push through!  (Remember to be careful with the sharp points and I’d recommend putting a bowl underneath to collect drops.)

Why doesn’t the bag leak? Plastic is a polymer, made up of long, elastic molecules that form a seal around the spot where the skewer is poking through.  In addition, the bag is sealed and contains very little air, so there isn’t much air pressure pushing on the water. If you make a hole in the part of the bag with air in it, surrounding air pressure (atmospheric pressure) can push on the liquid and make the bag leakier!

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