Understanding chemical and physical changes is a strange part of science. Once you learn to use chemical equations, it is so simple that no one even thinks about it. Before you learn to use chemical equations, it is so complex that most people don't really understand it. For a better view of that, you might also read Changing How We Look at Changing.
A catalyst is a substance that takes part in a chemical reaction, causing it to happen faster, but the catalyst is not used up by the reaction. There is just as much left afterwards as there was before the reaction began.
I got the idea for this experiment while working on the rocks we collected recently. When we got home last weekend, we unloaded the 750 pounds of quartz crystals from our Arkansas trip onto the back patio. Then I left for Philadelphia to present some electricity shows. Today, I went out to unpack some of the boxes. We had wrapped the best crystals in newspaper, and all the newspaper that was exposed to sunlight had turned yellow. Cool! A chemical color change that happens fairly quickly, and is caused by exposure to light!
This is another of those fun bits of science that many of us think we understand until we really start to look at it, or even better, try to explain it to someone else. Then we reach a point where it becomes obvious to ourselves, and to our audience, that we don't understand it as well as we thought we did.
One way that crystals form is from chemicals dissolved in water. If the water gets cooler, dissolves other chemicals, or evaporates, some of the dissolved chemicals can be deposited as crystals. Often, growing crystals can be a fairly long, involved process, but with this activity, we will grow some nice crystals quickly and easily.
I love it when teachers ask questions! I alway enjoy a good science question because they make me really think about the subject to be sure I give the right answer, but when a teacher asks a question, I feel obligated to give an answer (if I can) that will let them explain the subject to their students. Recently, I have been talking with a teacher friend, about chemical and physical changes, and the more we talked, the more I thought the rest of you might find this useful.