Diving Ketchup

1. First, you'll need to perform a "float or sink" test to see how the ketchup packet works. Fill a bowl with water and drop the packet into it. If it floats, great! If it sinks to the bottom, no sweat. This shows that atmospheric pressure in the packet is pressing hard enough on the air bubble inside the packet to sink it. If this happens, you get to make more trips to your favorite fast-food restaurant to find a ketchup packet that just barely floats!
2. Scrunch the packet in half lengthwise and carefully push it into the soda bottle. Do not open the packet. Just carefully push it into the bottle without tearing the edges.
3. Fill the bottle full to the brim with water and screw on the cap.
4. Squeeze the sides of the bottle and hold the squeeze to make the packet sink. Let go and the packet rises. The packet of ketchup has learned to dive!

How does it work?

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.

Additional Info

Learn how to make a classic Eye-Dropper Cartesian Diver.