D I V I N G   K E T C H U P

Did you know that you can cause a packet of ketchup to rise and fall on command in a bottle of water? People might even think that you have the ability to move objects with your mind! The truth is that you don’t have the gift of telekinesis. You actually just know a really cool science trick!

First, gather a ketchup packet that floats. To ensure the ketchup packet floats, perform a “float or sink” test. Fill a bowl with water and drop the packet into it. If it floats, great! If it sinks to the bottom, no sweat. This just shows that the atmospheric pressure in the packet is pressing hard enough on the air bubble inside the packet to cause it to sink. If this happens, you get to make more trips to your favorite fast-food restaurant to find a ketchup packet that floats!

Once you have a floating ketchup packet, fold the packet in half lengthwise (like a hotdog bun) and carefully push it into the soda bottle. Do not open the packet. Just carefully push it into the bottle without tearing the edges.

Fill the bottle to the brim with water and screw on the cap.

Squeeze the sides of the bottle and hold the squeeze to make the packet sink. Let go and the packet rises. Now, the packet of ketchup has learned to dive on command!

The packet floats because an air bubble gets trapped inside the packet when it’s sealed at the factory. If the packet sinks when you test-float it, then the air bubble is too small to make it float.

As you squeeze the bottle and push the water against the floating packet, you compress the air bubble into a smaller space. This happens because gases are more “squishable” than liquids, so the air compresses before the water. According to the density equation (Density = Mass divided by Volume), when you decrease the volume or make the bubble of air smaller, you increase the density and the ketchup packet sinks. When you release the pressure on the bottle, the compressed air expands inside the packet (increasing the volume), the density decreases, and the diving ketchup floats to the top of the bottle.

This is a well known scientific ‘trick’ that has been around for decades. But moving beyond simple water bottles, there are actually real uses to the science behind this – we use it to control the depth of submarines! Submarines have big air tanks, which keep it afloat on the water’s surface. When it wants to sink, it squashes the air in the tanks (the scientific word for squashing air is compressing). All this compressed air make the submarine tanks more dense and heavier, so the submarine sinks in the water. The same as our ketchup sachet, but on a much bigger scale!