I found a little science experiment on sciencebob.com and thought that it might be fun to try it out with my 7 year old. No, it’s not science fair time. Just time for some good, clean fun.
Here is how you set up the experiment:
- Plastic soda bottle (1 liter or 2 liter)
- Ketchup pack from a fast food restaurant
- Rinse out an empty soda bottle and fill it up to the VERY top with water.
- Put an unopened ketchup packet into the bottle.
- If your ketchup packet floats, move to the next step. If it sinks to the bottom of the bottle, add about 3 Tbsp of salt to the bottle and shake til it is dissolved.
- With the cap tightened on the bottle, squeeze the bottle gently but firmly and watch the ketchup magically float down to the bottom of the bottle. Release your hold and the ketchup will float back up to the top.
How we made this a magic trick
I’ll be honest with you – when I showed this to my almost-7-year-old, he was not entertained past the first two dunks. So I got conspiratorial with him.
“Go get Reese. Let’s tell her that the ketchup falls when you blow on the cap. When you blow, Dad will squeeze the bottle and we’ll trick her!!”
Yes, I really said that.
Suddenly, it was a game. AJ had SO. MUCH. FUN. tricking Reese, and she was completely stumped, especially since none of her blows would move the ketchup at all. Dad sat calmly holding the soda bottle at the bottom, as if steadying it, and no one guessed that he was in on the trick.
We tried it on 11 year old Twizzler, as well, whose analytical brain was struggling to figure out the mystery. Nobody could determine how AJ and I could make the ketchup move up and down while they huffed and puffed with no results.
We did let them in on the secret eventually…
Here’s how it works:
It’s really quite simple. DENSITY. You know that bubbles float, right? Well, there’s a little bubble inside the ketchup packet which keeps the ketchup from sinking. When you squeeze the bottle, you put pressure on the packet, causing the bubble to get smaller and the packet to become MORE DENSE (or less buoyant). As the packet becomes more dense than the water around it, it will sink.
Release the pressure and the bubble again expands and the packet floats back up.
This is sometimes known as a CARTESIAN DIVER.
How to make this a science fair experiment
Ask some questions and try some variations: would a mustard packet work? What about soy sauce or mayo? Does water temperature make a difference? What happens with different sized bottles? Does it work with soda?
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