Air: Kinetic Energy Demo – The Bubble Viewer

Key Takeaways:

• Air molecules move around and exert a force when they hit against each other and other objects. This is measured as pressure.
• Kinetic energy in air molecules is measured as Temperature.
• Higher Temperature means the air molecules are moving faster and exerting higher pressure
• Lower Temperature means the air molecules are moving slower and exerting lower pressure
• Therefore, more energy (higher temperature) makes the bubble expand as the air molecules press the soap bubble outward. And, lower energy (cold temperature) causes the air molecules to slow down and not have as much momentum force, the bubble shrinks.

Experiment:

Gather these items:

• Empty and clean water, soda, detergent bottle or an empty jar or can
• 2 Bowls Deep enough to mostly submerge the empty bottle, jar or can
• Very hot & Very Cold Ice Water
• Dish soap and water or bubble solution
• A shallow dish to pour the bubble solution into
• Towels to clean up spills
• A sink and work space

Steps:

1. Fill 1 bowl with very hot water
2. Fill 1 bowl with very cold ice water
3. Fill approx 1/2 inch of bubble solution in a shallow dish
4. Take the empty bottle and invert, open side facing down
5. Dip the opening of the bottle in the bubble solution to make a bubble film over the opening
6. Carefully turn the bottle upright, do not break the film
7. Dip the bottom 1/2 of the bottle in the hot water
8. Observe what happens. The bubble film should expand as the air molecules increase in temperature, speed up, and transfer more kinetic energy to the bubble film as they move faster and try harder to fly away faster, they press against the bubble film as the air expands.
9. Next, dip the bottom 1/2 of the bottle in the ice cold water.
10. Observe what happens. The bubble film should shrink and possibly disappear or break into the bottle as the air molecules decrease in temperature, slow down, and loose kinetic energy to the cold bath. The bubble film is not supported by the bouncing air molecules. The air molecules move slower, so they have less momentum and can’t exert as much force on the bubble film. The air volume shrinks.

Advanced Notes:

• Equations of state (EOS) are used to define the Pressure-Volume relation to the number of molecules and temperature.
• The basic ideal equation is Pressure*Volume = n*R*Temperature.
• Where n is the number of moles of the gas and R is a constant based on the units of Volume, Pressure and Temperature.
• In this demo, the bubble layer closes off a fixed number of gas molecules. So, “n” is a constant number.
• The constant R is also a constant number.
• The pressure is also fixed. Why? Because the soap bubble is not rigid and experiences the current “air pressure” in your region – typically about 1 atmosphere, or whatever the local weather forecast is indicating.
• Temperature changes with the hot or cold liquid bath. The heat or cold is conducted through the bottle and warms or cools the air molecules.
• The volume of the trapped gas changes as the temperature increases or decreases.
• Math: If nRV are all constant, re-arrange the EOS to show that (nR/Pressure) = Volume/Temperature.
• At the cold and hot conditions, the nR/Pressure term never changes. The Hot and Cold values are exactly the same, set them equal. Now it looks like this: V-hot/T-hot = [nR/P-hot = nR/P-cold] = V-cold/T-cold
• This math indicates: V-hot / T-hot = V-cold / T-cold
• Re-arrange math: V-hot = V-cold * (T-hot / T-cold)
• The final expanded volume equals the starting cold volume times the ratio of the hot temperature divided by the cold temperature.
• EVEN More Advanced Information at LibreText’s Chemistry Page