As I zip in and out of classes testing lab activities, I sometimes get asked questions by teachers and kids that I think I can answer rather confidently on the spot . However, when I get back to the office and think about it for a minute it sometimes bugs me enough that I have to look up the current thinking on the topic and sometimes find out that I was wrong. The most recent example of this is a question about eggs undergoing a physical change or a chemical change when they are heated. My thinking was the heat caused the proteins to coagulate and form a solid. Change of state, therefore a physical change. As it turns out, egg whites are made up of long chains of proteins called amino acids. In a raw egg these amino acids are raveled up in an organized tangle. When you heat the egg white it denatures, or breaks these amino acid bonds and they reconnect, forming new bonds with a new texture and a new color. Definitely a chemical change. The other big indicator that I forgot about is that physical changes can be reversed and separated. It is really hard to unscramble an egg.
So, in a nutshell, the heat causes the proteins in the egg white to rearrange, creating new bonds and a new BUT CHEMICALLY DIFFERENT kind of protein. CHEMICAL CHANGE.