Floating Soap Bubbles

Age

Elementary School, Middle School

Format

Hands-on

Materials

   Soap bubble solution (available commercially)
   Wand for blowing soap bubbles
   A large transparent container with an open top (an empty 10 gallon aquarium works nicely)
   1/2 cup of baking soda
   1 cup of vinegar
   Shallow glass dish to fit inside large container (a glass baking dish works well) 

Safety Precautions

Science Theatre demonstrators must keep the safety of themselves and their audience in mind at all times. All Science Theatre demonstrators must have read through the Safety Training page. The ST Safety Box with first aid kit, fire extinguisher, etc. should always be available to demonstrators. Always wear safety gloves, glasses, and a labcoat if handling chemicals; always perform potentially dangerous demonstrations at a safe distance from the audience; and always keep a very close eye on any volunteers you call from the audience.

Preparation

Minimal! See #Demonstration section.

Demonstration

Set the large container on a table away from drafts and where you can easily look through its sides. Place the glass dish inside on the bottom of the large transparent container. Put 1/2 cup of baking soda in the glass dish. Pour 1 cup of vinegar into the dish with the baking soda. They react to form carbon dioxide. After the fizzing in the dish has subsided (~1min), gently blow several soap bubbles over the opening in the large container, so that they settle in the container. This may take some practice. Do not blow directly into the container, you will blow the carbon dioxide out. When a soap bubble settles into the container, it will not (should not, if things go right) sink to the bottom, as it would in air. Instead, it will float on the surface of the invisible carbon dioxide in the container.

What to Say

How many of you have mixed baking soda and vinegar? What happens when you mix them? Hopefully someone will say that it fizzes or reacts. This is the same reaction you might have used to make a model of a volcano! Do you guys know what gas is formed when baking soda and vinegar are mixed? The answer you are looking for is carbon dioxide. Now I want you all to imagine a beach ball floating on the water. Does anyone know why it floats? The beach ball is filled with air. The reason the beach ball filled with air floats on water is because air is less dense than water. That means that a certain amount of air (gesture with your arms to indicate that amount=volume) will weigh a lot less than that same amount of water. Things that are less dense can float on things that are more dense. So, because of this difference in density between carbon dioxide and air, I can make these bubbles float inside this aquarium. The bubbles are filled with air, so they are less dense than the carbon dioxide sitting in the bottom of this aquarium. This allows them to float.