Here’s another really simple science magic trick that you can make with easy to find objects. You will need:
- Plastic drinks bottle.
- Vinegar sachet (From my experience the Heinz Malt Vinegar sachets work best. They can be bought in bulk from catering suppliers if you wish to do this as a class activity.)
The first thing you need to do is adjust the weight of the vinegar sachet so that it only just floats on the surface of the water. Some sachets will sink with no added weights, unfortunately you will have to find another one. Now add paper clips until the sachet is weighed down enough so with a light push it will sink. It’s best to do this in a sink or bucket of water, as if you do it in the bottle you’ll spend ages trying to fish the sachet out of the bottle each time you make an adjustment.
Now add the sachet to the drinks bottle and fill the bottle with water until it spills over the top. Screw the lid on tightly. If there are any air bubbles on the side of the vinegar sachet, hit the bottle hard to dislodge the bubbles.
Now for the magic. If you squeeze the side of the bottle the vinegar sachet should sink like a submarine. Release your tight grip and the sachet will float. If you disguise the squeezing you can give the illusion of telekinetic powers.
So what’s the science? Well… liquids are hard to compress unlike gases. When you squeeze the bottle it causes the small amount of gas at the top of the vinegar packet to compress. The volume of the vinegar sachet decreases. As the mass doesn’t change, the result is that the sachet’s density increases. (Density = Mass/Volume) The density is now greater than the surrounding water and so the sachet sinks. Releasing the squeezing pressure allows the gas to expand and the sachet’s density to become less than the surrounding water and so it floats again.