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Purposes

[stextbox id = “info”] 1. To have students apply their emerging understanding of dissolving to a new situation.

2. To help students understand that salt does not disappear when it is dissolved into water.
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Description

[stextbox id = “info”] Ask students if they think the salt can be separated from the water in the cup (from Student Experience 1). Select students with differing opinions to share their thinking with the class.

Place a piece of black construction paper in a shallow pan (like a jelly-roll pan or a cafeteria tray). Pour some of the water from the cup in Student Experience 1 onto the construction paper and spread it evenly on the paper. Place the pan outside in the sun for about 15 minutes.

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Questions to Ask Students

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• What is on the construction paper? How did it get there?
• If we left the cup of saltwater (from Student Experience 1) in the sun all day, what do you think would happen?
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Student Thinking

[stextbox id = “info”] Some students believe that dissolution is irreversible and will predict that there’s no way to get the salt back (Nakhleh et al., 2005). This is particularly true of students who believe the salt disappeared.
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Implementation Tips

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• If it is too cold or overcast to evaporate the water outdoors, you can do this demonstration using a lamp, but it will likely take longer.
• You can also show the students the following video on YouTube (~2:30 minutes in length).

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