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Purpose

[stextbox id = “info”] To illustrate what happens at the particle level when a liquid evaporates.
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Description

[stextbox id = “info”] Show the evaporation simulation from the Molecular Workbench http://mw.concord.org/modeler/. Click Run to start the simulation. You can add heat by clicking the Heat from the Sun button. See the Implementation Tips below for more information on using the simulation.
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Questions to Ask Students

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• What do you think each dot represents?
• Is the substance at the bottom of the container a solid, liquid, or gas?
• What happens to the number of particles in the gas phase over time compared to the number of particles in the liquid phase?
• What happens to the speed of the particles when you add heat from the Sun?
• Where do the dots that leave the container go?
• How could what you’re seeing explain how you’re able to smell a liquid like perfume from a distance?
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Implementation Tips

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• Tell students that the simulation will allow them to see a model of what they cannot see, even with the strongest microscope.
• Spend some time with the simulation before sharing it with the students. To run the simulation, you’ll first have to download the Molecular Workbench library by clicking Download MW. It should take only a minute. Once it has downloaded, click on the file to open it. Once you open it, click Browse Entire Library, and click on States of Matter under Chemistry. Then click on Evaporation (2D) to run the simulation.
• Focus students on what happens to a particle when it leaves the liquid with enough speed to escape the particles above it.
• Use Heat from the Sun to speed up the evaporation process.
• Stress to students that the particles that leave the container do not disappear. They keep moving in the same direction until they bump into something else, like an air particle.
• Encourage students to pick one particle and follow its movement for at least a minute, observing what happens to the particle as it collides with others.
• Students may ask how we know the model in the simulation is the right one. Models have to be supported by evidence, and one piece of evidence for the model in the simulation is that it explains how the odor moved from the container to their nose. Students can engage with other evidence for the model in other Driving Question Pathways.
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