Summer of Science: Sink or Swim with Ketchup Packets

Simple lesson in how density, bouyancy and pressure work.

Summer of Science: Magic Ketchup Packets

MYRTLE BEACH, SC (WMBF) - Here’s a fun way to demonstrate how objects float and sink, with common items you likely have laying around the house.


A large clear water bottle (1 liter size works great)

Ketchup Packets (You can also try mustard, soy sauce and other condiments. Have a few different types)

Salt (Optional)

Ketchup Packet and Water Bottle
Ketchup Packet and Water Bottle

Step 1:

Fill the bottle to the top with water

Step 2:

Put the packet in and close the lid.

IF YOUR PACKET FLOATS: You are in good shape so far.

IF YOU PACKET SINKS: Put salt in the water to help the water become less dense, and the packet to float to the top.

Step 3:

Squeeze the bottle and you should notice the packet become vertical and start to sink. When you let go of the bottle, the packet should rise back up to the top. A fun game is to try and see if you can squeeze just enough to keep the packet floating in the middle of the bottle.

There is a bit of trial and error. If your packet does NOT sink when you squeeze, you’ll have to try a different packet.

Science Perspective:

This is a lesson in how density and pressure play a role in whether objects sink or float, also known as buoyancy.

The air bubble in the packet is what causes it to float. When you squeeze the bottle, you are adding pressure on that bubble. When that pressure increases on the bubble, it gives the air molecules less room to move, causing the bubble to become smaller and the packet to be more dense. When the packet is more dense than the water, it sinks. When you let go of the bottle, the packet’s bubble inflates, and it rises.

Other ways to make this experiment different is to try the water at different temperatures, try different types of packets, or try different size bottles.

Squeezing the side of the bottle causes the packet to sink.
Squeezing the side of the bottle causes the packet to sink.

Copyright 2019 WMBF. All rights reserved.