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Purpose

[stextbox id = “info”] To illustrate for students what they cannot see—that is, what happens at a particle level during dissolving, in particular: that the salt does not disappear, that the number of particles does not change, and that the size and shape of the particles does not change.
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Description

[stextbox id = “info”] Have students observe simulations on dissolving (see Implementation Tips for information about using the software):

• PhET Interactive Simulations, Sugar and Salt Solutions: https://phet.colorado.edu/en/simulation/sugar-and-salt-solutions
• Molecular Workbench, Dissolving Salt in Water: http://mw.concord.org/modeler/
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Questions to Ask Students

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• What happens to the salt particles when they are shaken into the water? How do they move in the water?
• What happens to the salt particles when the water is evaporated? Describe their movement. Are there more salt particles? Are there fewer particles?
• What do you notice about the size and shape of the salt particles, both when dissolved in the water and when the water was evaporated?
• How do the simulations help explain what happened in the glass of saltwater (from Student Experience 1)?

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Implementation Tips

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• Tell students that the simulations will allow them to see a model of what they cannot see, even with the strongest microscope.
• Spend some time with the simulations before you share them with your students; it helps to be familiar with their settings.
• Molecular Workbench requires a quick download of the program. Once downloaded, navigate to the simulation library by clicking “Browse entire library.” Under Chemistry, click on “Water molecules and solution.” Finally, click “Dissolving salt in water.”
• In the PhET simulation, the “micro” tab in conjunction with the “water” tab will likely be most helpful for showing what happens to the salt particles in water. The “micro” tab does not show water particles; the “water” tab does show water particles.
• 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 weight of the salt and water mixed together was the same as the sum of them individually.  Students can engage with other evidence for the model in other Driving Question Pathways.

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