When I taught fifth grade my absolute favorite unit to teach was physical and chemical changes! While chemistry was never natural to get students to understand well, I loved the reactions students had when we had… well, reactions in the classroom.
When I taught physical and chemical changes, we always taught students to look for certain signs that would indicate if it was a physical change or a chemical change. For instance, a physical change could be reversible and did not create a new substance. If I took an ice cube and melted it, I could reverse the process and turn my water back into ice. Even further, no matter what state the water was in, it was always just water – Good Ole’ H2O!
With chemical changes though, it always produced a new substance. Students would watch for key signs like bubbles (or gas being released), or an increase in temperature (See the chart below for more signs. You can find these charts and experiments in my Physical and Chemical Changes Unit on TpT or here in my personal store).
One experiment that appears to be a chemical change but is, in fact, a physical change is dissolving a Styrofoam cup! (Anyone else remember all that from the ’90s with McDonald’s removing all their Styrofoam containers?). I dissolve it with chemicals; safe chemicals! (It is not harmful to touch. At most, it will dry the hands out. However, be aware of the strong smell.)
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To start, I break my students up into groups and provide each group with an aluminum pan and a plastic cup with a small amount of acetone in it. Acetone is the same thing that we use on our nails to remove nail polish. However, I like to use the straight product, so I go over to the paint section (or in some stores the hardware section) and grab it there. Then, give each group one Styrofoam cup.
Together we review all the signs to look for with physical and chemical changes. Then, I ask the students to predict what they think will occur when they pour this liquid on the Styrofoam cup. After discussing it together, I have the students pour it onto the cup. The reaction is somewhat fast, so they have to be ready!
As you pour the acetone onto the cup, you see it start to bubble and dissolve very quickly. While it appears to be a chemical change because it produces bubbles, it’s actually the trapped air being released out of the Styrofoam. Theoretically, if you were to blow air back into the cup, it could be reversible. I do allow my students to touch it. They squeeze the acetone out of the Styrofoam, and then I let them mold it into whatever shape they desire. Over time the rest of the acetone evaporates out of it, and it becomes a bit more firm and solid. The kids always enjoy this activity and always want to do it over and over!
This is not the only activity that we explore when talking about physical and chemical changes. We have also explored whether changes in states and mixtures and solutions are physical or chemical changes. You can also discuss physical and chemical weathering.
I hope this lesson not only excites you but also “changes” your thoughts about physical science! Save time and get your Physical and Chemical Changes Unit today!