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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 evaporation, in particular: that the number of particles does not change, that the size and shape of particles do not change, and that the particles are in constant motion.
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Description

[stextbox id = “info”] Show the states of matter simulation from PhET Interactive Simulations: https://phet.colorado.edu/en/simulation/states-of-matter-basics. Starting with a substance in the liquid phase, raise the temperature in small increments. See Implementation Tips for additional guidance on using the simulation.
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Questions to Ask Students

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• What do you think each of the dots represents?
• What are all of the dots doing?
• What do you notice happening at the surface of the liquid as the temperature increases?
• How does the simulation help explain what happened with your disappearing handprint in Student Experience 2?

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Student Thinking

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• The demonstration confronts the following misconceptions:

• Matter is continuous (it’s actually made of particles)
• Particles do not move (the particles are actually always moving)

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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 you share it with your students. It allows you to adjust various settings, and being familiar with them will help.
• Recognize that the temperature shown in the simulation is in Kelvin. You can change to Celsius by clicking on the Teacher menu in the upper left-hand corner of the simulation. Or, to convert from Kelvin to degrees Celsius, subtract 273.
• Consider doing the simulation first with Argon or Neon instead of water. Argon and Neon have only one atom in their natural state, which avoids the potential confusion that a water molecule (which has three atoms) presents.
• Begin in the liquid phase. Focus students on the motion of the particles and their arrangement (in particular that the particles are, for the most part, staying at the bottom of the container. Add heat and ask students to focus on what happens to the particles at the surface (some begin to leave the group and float around the container), and at the bottom of the container (they hit the bottom of the container and do not soak through the container). Also help students to see that the particles that are still in the liquid phase (those still clumped together toward the bottom of the container) are moving and hitting against one another.
• 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 liquid in the puddle disappeared. Students can engage with other evidence for the model in other Driving Question Pathways.

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