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Interactive Experiment: Partial Pressure and Molecular Diffusion

Welcome to the FIZZICS Molecular Simulator. This growing suite of interactive experiments explores the science of carbonation from a molecular point of view, using the kinetic theory of gases.

Equilibrium of Gases: Dalton's Law and Partial Pressure

Question: Can a different gas keep CO₂ from escaping?

Let's find out.

Experiment 1 – CO₂ by itself

1. Add 40 red CO₂ molecules to the left chamber.

2. Open the door between the two chambers.

3. Watch the CO₂ molecules diffuse from left to right.

The number of CO₂ molecules on the left decreases while the number on the right increases. Likewise, the CO₂ partial pressure on the left falls while the pressure on the right rises.

When the CO₂ pressure becomes approximately equal on both sides, the system has reached equilibrium. The simulator will play a sound when this happens.

Experiment 2 – Does Argon keep CO₂ in?

You might reasonably conclude that any pressurized gas on the right should keep the CO₂ on the left.

Let's test that idea.

1. Reset the simulator.

2. Add 40 CO₂ molecules to the left chamber.

3. Add 40 Argon molecules to the right chamber.

4. Open the door.

What do you notice?

  • CO₂ molecules continue moving from left to right.
  • Argon molecules move from right to left.
  • Neither gas seems to care about the other.

CO₂ once again reaches equilibrium at approximately the same pressure on both sides—about half the pressure where it started.

Experiment 3 – What actually keeps CO₂ in equilibrium?

1. Reset the simulator.

2. Add 40 CO₂ molecules to the left chamber.

3. Add 40 CO₂ molecules to the right chamber.

4. Open the door.

What happens?

Nothing changes.

The molecules continue crossing in both directions, but just as many CO₂ molecules cross from left to right as from right to left.

The system was already in equilibrium.

The Key Result

Pressure does not keep CO₂ in place.

Partial pressure balances partial pressure.

The pressure of CO₂ on the right doesn't "keep" the gas on the left. It simply balances the rate at which CO₂ molecules cross in each direction.

Even 1000 PSI of Argon could not keep 60 PSI of CO₂ in equilibrium on the left.

The only thing that can balance 60 PSI of CO₂ is 60 PSI of CO₂.

In other words, if you want to keep CO₂ anywhere—in the left chamber, inside a bottle, or dissolved in a beverage—you need more CO₂ to do it.

That simple idea explains almost everything about preserving carbonation.