Diving Ketchup – Clever Cartesian Diver

Is it mind control or just a clever science trick?

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Diving Ketchup

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!

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Here's What You'll Need

  • Clear plastic soda bottle with cap (1-liter size works great)
  • Ketchup packets
  • Bowl
  • Water

Let's Try It

  1. 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!

  2. Diving Ketchup - Step 2

    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.

  3. Fill the bottle 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, on command!

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.

Science Fair Connection

Here’s how to turn this demonstration into a real science experiment. Ask yourself these questions and remember the 3 C’s… change, create and compare. Does the size of the bottle affect how much you have to squeeze to get the packet to sink? Do different food packs (ketchup, mustard, soy sauce) have the same density? Does the temperature of the water affect the density of the ketchup packet? Give one (or more) of these variations a try and compare your results and draw some conclusions.

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