Demonstrating a Chemical vs. Physical Change
Posted August 24, 2008on:
This is a standard demo, one I did with my 8th grade Physical Science class and it stuck with them. It uses sugar to show the difference between a physical change and a chemical change. The first step is to dissolve sugar in water and then evaporating the water over a low flame. I usually use a beaker over a burner. The sugar will crystallize out and can be dried and returned to its original form.
The second step involves heating sugar in a test tube until it carmelizes and turns to carbon. The kids smell the change and associate the smell with a property change. We try but can’t get the mess to turn back into sugar.
If you haven’t done this before, don’t go by the picture, it’s just a photo I found on the web. You want to gently heat the test tube with the sugar. You only need a small amount of sugar (1/2 at the bottom of the test tube) and if you do it slowly and carefully, you will first see the sugar melt and then start to change. Gently waft the odors to the students as it starts to change. If you go fast, you will stink up the place. I often hold the test tube in my hands as I heat the bottom. It doesn’t get hot if you go slowly.
I usually throw the test tube out, it’s just not worth cleaning it once the change takes place. If someone knows how to clean it easily, please comment. Thanks.