Within this unit, we have examined several gas variables in relation to gas particle behavior including temperature, volume, and pressure. In this lesson, we will examine a fourth variable, number of particles.

We will specifically study the effect of two different variables on pressure; we will investigate how changing the number of particles affects pressure, and how changing volume affects pressure.

Feel free to play around with the simulation, but make sure to refresh your browser before working through the following steps in order to learn how to manipulate the simulation for this lesson. 

1. Click "Ideal"
2. Particles Menu
   • Click the green plus sign next to particles
   • To subtract or add 1 particle or 50, click the arrows to the left and right of the number of particles
3. Reset button
   • Click the white button to the left of the pump to reset the simulation. 
4. Particle Pump
   • After resetting the simulation, drag the handle of the pump up and down to add a more random number of particles. 
5. Pause button
   • After adding some particles to the chamber, click the pause button on the lower left side of the screen to pause the simulation.
6. Play simulation / one frame forward
   • After clicking pause, you'll notice that the pause button turns into a play button. Click the play button to restart the simulation.
   • Pressing pause also lights up the smaller button to the right of the pause button. This smaller button allows particles to shift one frame forward.

Let's look at a GIF of the simulation we just used so we can understand what causes pressure on a microscopic level.

The pressure gauge remains at 0.0 atm until the moment a particle collides with the wall of the container. The pressure gauge then reads a pressure, but goes down again until another particle collides with the wall.

Pressure can be defined as the frequency and force with which gas particles collide with the walls of their container.

Since we are examining pressure and number of particles in this simulation, we need to make sure that the other two variables (volume and temperature) are held constant. 

This is the set-up you will need to complete the data table below: 

1. Click "Ideal".
2. Under "Hold Constant", click "Volume (V)". The program does not allow you to hold two things constant at the same time, but as long as you do not manipulate the "hot-cold" switch the temperature will remain at 300 K.
3. Click the green plus sign next to particles.
4. Click the double rightward facing triangle below "Heavy" to add 50 heavy particles at once.
5. Wait until all particles from the pump have had a chance to hit a wall of the container, then record the highest pressure observed.
6. Press the reset button, then repeat steps #2-5 two more times.
7. Complete steps #2-6 for 100 heavy particles and 150 heavy particles.

If we graph the data we acquired on page 3 (your data is shown on this page for your reference), our graph would look something like this:

The graph pictured above shows a direct relationship between the two variables (number of gas particles and pressure), that is, as number of particles increases, pressure also increases.

Now we will examine how volume affects pressure. Again, we need to make sure that the other two variables (number of particles and temperature) are held constant. 

This is the set-up you will need to complete the data table below: 

1. Click "Ideal".
2. Click the green plus sign next to particles.
3. Click the double rightward facing triangle below "Heavy" to add 50 heavy particles at once.
4. Under "Hold Constant", click "Temperature (T)".
5. In order to adjust the container's volume, click "Width" and you will see a ruler appear under the container. Reduce the width to 5.0 nm, then record the highest pressure observed.
6. Press the reset button, then repeat steps #2-5 two more times.
7. Complete steps #2-6 for widths (volumes) of 10.0 nm and 15.0 nm.

If we graph the data we acquired on page 5 (your data is shown on this page for your reference), our graph would look something like this:

The graph pictured above shows an inverse relationship between the two variables (volume and pressure), that is, as volume increases, pressure decreases.