Re: Briefly, what exactly is a physical change?

A physical change is one in which the chemical identity of the material(s)
involved is not altered.

For example, if you throw ice on a hot griddle, there is no chemical change; 
you go from solid water to liquid water to gaseous water. If you pump air into 
your bike tire, that's a physical change, too.

But if you mix up banana-bread batter and bake it, you've induced a chemical 
change because not only does the heat alter the chemical nature of the batter,
but some of the ingredients (the baking soda) react with other ingredients
(the bananas and the sour milk).

                                        Dan Berger