Floating Soap Bubbles
| Physics, Chemistry, Earth Science: | Air Density |
| Grade Range: | Elementary School, Middle School, High School |
| Format: | Hands-on |
Younger students will love the chance to blow bubbles and see them floating on air, while older students will be interested in what allows the bubbles to float in the fish tank. This demonstration can be messy, so be sure to keep a towel nearby.
Materials
- Fish Tank
- Vinegar
- Baking Soda
- Bubble Solution with Wand
- Measuring Cup
Safety Precautions
Please read the Liquid Chemical section of the Demonstration Safety page before performing this demonstration.
Demonstration
- Set the fish tank on a table, and try to avoid having it in a drafty spot. Measure 1/2 cup of baking soda and spread it across the bottom of the tank.
- Measure 1 cup of vinegar and pour it into the tank across the bottom, and allow them to react. After they finish reacting, you can start blowing bubbles!
- When you blow a bubble, blow it above the tank and let it descend into it. Do NOT blow the bubbles into the tank, or you will blow the gas out of the tank!
- The bubbles will float on the gas that was released during the reaction inside the tank, making them seem to float in midair!
- When they stop floating as well, or if the tank gets air blown directly into it, you will likely have to add more baking soda and vinegar to continue the demonstration. When finished with this demonstration, be sure to rinse out the fish tank, and let it air-dry once you return.
Why This Works
Short Explanation
In this demonstration, we are looking at the difference in density of various gases. Density is when you divide the mass of something by the volume it takes up. For liquids and gases, a more dense liquid or gas will sink below a less dense one, letting the less dense gas or liquid "float" on top. This is the effect we are seeing with the bubbles in our fish tank!
We first had baking soda and vinegar react in the fish tank, which produces carbon dioxide gas (CO2). CO2 is a heavier gas in our atmosphere, and will sink below the other gases like oxygen and nitrogen in still air. This means that, when the reaction happens in the fish tank, the CO2 will stay inside the fish tank and push the other gases out. This is why we see the bubbles floating inside the tank when we blow them in as well! The bubbles have an air mix of CO2, oxygen, nitrogen, argon, and even some water vapor inside of them. This mix of gases is lighter than pure CO2, which allows the bubble to float inside the fish tank!
Full Explanation
The Density of a substance is equal to the mass of the substance divided by the volume. With fluids such as gases and liquids, the less dense fluids will float atop the more dense ones if left undisturbed. The density of a fluid correlates with the molar mass of the fluid. This means that if we know the molar mass of an element or compound, then we can predict if it will float above or sink below other elements and compounds that are in the same state of matter.
We had vinegar and baking soda react inside of this container to produce carbon dioxide. CO2 has a high molecular mass compared to the other gases that are in our atmosphere, so it will sink and stay within the fish tank after the reaction. When we start blowing bubbles above the container, they are filled with air from our lungs, which is a mixture of water vapor, nitrogen, oxygen, argon and carbon dioxide. This air mixture is about as dense as the air around, but the bubble it is inside of is slightly heavier than the air, so it will sink into the fish tank. Once inside, it will float atop the denser CO2.
| Gases | Molar Mass | Density |
| Water Vapor (H2O) | 16 g/mol | .804 g/L |
| Air (Sea Level) | ~28 g/mol | 1.23 g/L |
| Nitrogen (N2) | 28 g/mol | 1.25 g/L |
| Oxygen (O2) | 32 g/mol | 1.43 g/L |
| Argon (Ar2) | 36 g/mol | 1.78 g/L |
| Carbon Dioxide (CO2) | 44 g/mol | 1.98 g/L |
This table is provided so to look at how the density of a compound or element correlates with the molar mass. Notably, water vapor has the lowest molar mass, and with that it has a substantially lower density. This is part of the reason why we have clouds; humid air rises much faster than dry air, since the water vapor in the air is lowering the overall density, and therefore drier air will sink below it. This humid air will continue to rise until it reaches the Homosphere (Troposhere, Mesosphere and Stratosphere), where it will condense and freeze into clouds! It is also worth noting that the Air density is listed as being at sea level. This is because the density of the air around us can change based on where we are on the earth. Air density becomes lower as you go higher into the atmosphere, and at high altitudes it can become hard to breathe because the air is not dense enough. The opposite is also true; if you went into a cave that goes deep inside the earth, it will become harder to breathe because the air is too dense!
Additional Information
- This demonstration pairs well with the Density Rainbow, the Cloud in a Bottle, and the Cartesian Diver demonstrations.
- You can also try floating small, very light objects in the fish tank, such as Styrofoam peanuts and small feathers.