Purpose
[stextbox id = “info”] To illustrate for students what they cannot see—that is, what happens when particles of different substances are combined together.[/stextbox]
Description
[stextbox id = “info”] Fill a clear measuring cup (capable of holding at least 16 oz.) with one cup of marbles (i.e., add marbles to the 8 oz. line). Fill a second measuring cup with one cup of sand (again, add sand to the 8 oz. line). Ask students to predict the total volume of the two substances when the sand is poured into the measuring cup with the marbles. Next, pour the sand into the cup with the marbles and tap on the cup holding the marbles a few times. The students will see that the volume is less than two cups, even though one cup of marbles was mixed with one cup of sand.[/stextbox]
Questions to Ask Students
[stextbox id = “info”]- Why is the total volume of the sand and marbles mixed together less than two cups?
- How does this demonstration help you think about what happened when we mixed water and alcohol together?
- [/stextbox]
Student Thinking
[stextbox id = “info”] This demonstration provides students with an opportunity to see a model of what they cannot see, namely how particles of different substances can fit in between one another. However, the space between the marbles (and between the sand) is filled with air, while the space between the particles in a substance is empty, so it’s not a perfect model. Students should be aware of this discrepancy. The activity can provide an opportunity to talk about models more generally, especially the fact that they never match reality perfectly.[/stextbox]
Implementation Tips
[stextbox id = “info”] You can use materials besides marbles and sand, but we recommend selecting items that are mostly uniform in size and shape (e.g., do not use something like pebbles). Using a uniform material will help avoid the misconception that particles of the same substance are different sizes and shapes.[/stextbox]
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