SimuLab 15: Sublimation, Deposition, and Triple Point.


Your objective is to:

Recognize sublimation, deposition and triple point from a microscopic point of view.

You will be able to:

Describe the process of deposition and explain it in terms of potential energy.

Describe the process of sublimation and explain it in terms of energy.

Compare and explain the potential energies of particles that exist at the triple point.

Describe molecular motion at the triple point.

    1. Open SMD, select Experiment 1a in the Energy folder, set Iterations between Displays to 500 and press Start.

    The experiment starts at the stage where almost all the particles have coalesced into large droplets. The Heat Bath is on. You see 144 particles in contact with a thermal bath whose temperature is T=0.4.

    2. Switch Display Particles by to Potential Energy. Pause the simulation. Select Take a Snapshot - Screen. When the dialog box with the Title of the Picture appears type in: Liquid'' and press Ok.

    3. Now drop the temperature to T=0.3.

    This temperature is below the freezing temperature of the substance.

    4. Press Start.
    After 100 time units, pause the simulation.

    Watch the changes in the colour representing the potential energy of the particles.
    The particles in the gas start to form little snowflakes'' that eventually merge with the large crystal. This phenomenon is called deposition.


Q3.20: What happens to the droplet?

    5. Select Take a Snapshot - Screen. When the dialog box with the Title of the Picture appears type in: Solid'' and press Ok.

Q3.21: Now compare the snapshots in the two phases. What differences do you see? Explain.


Q3.22: What happens to the potential energy of the system when you drop the temperature to 0.3?


Figure 3.8: The system at temperature T=0.3. The droplet is frozen into a triangular'' crystal. The time on the graph indicates the time elapsed since the beginning of crystallization, i.e., since the temperature was lowered from T=0.4 to T=0.3. Next to the crystal is a little snowflake'' that will eventually coalesce with the crystal. The few gaseous particles are in equilibrium with the crystal.


Q3.23: Are there any particles not in the solid state? If yes describe them.

    6. Deposition requires a significant amount of computer time to simulate. We recommend opening the Experiment1b file in order to skip the earlier stages of the process and press Start. Switch Display Particles by to Potential Energies .

    You see some gas molecules surrounding the crystal. The gas has much lower density than before. The gas particles separate from and rejoin the crystal. This happens to snow in very cold weather-it gradually disappears, moving directly from the crystalline snowflake state into the water vapor state without ever passing through the liquid water state. The same thing happens to solid carbon dioxide (CO2, dry ice'') when it turn into a gas. This phenomenon is called sublimation.

    7. Set the temperature to T=0.33 which is the temperature of the triple point. At this temperature all three phases-solid, liquid, and gas-coexist. Press Start.

Q3.24: What happens to the crystal?

    8. Switch Display Particles by to Trajectories in order to see the movement of particles in the three states of matter.

Q3.25: Watching the trajectories of the particles identify the three states of matter. How can you distinguish gas, liquid and solid in terms of the particle trajectories?

    9. Set temperature to T=0.4. Watch the changes in the crystal. Set temperature to T=1.0. Describe the changes in the system.
    10. Switch Display Particles by to Absolute Kinetic Energy. Watch the color of the particles.

Q3.26: Do all the particles move at the same speed? Explain what you see.