If a new substance suddenly appears that wasn't there originally,
then a chemical change has occurred.
Like for example, (this is the only example I can think of right now):
-- You leave your bicycle outside in the rain, and it gets wet, and
a few days later there's some rust on it.
-- You scrape off some of the rust, take it to school, give it to the
Chemistry teacher, and ask her to analyze it and tell you what it
is. Later that day, she tells you it's a substance called "Iron oxide".
-- Where did that come from ? There was no iron oxide there.
There was only iron in the bicycle, and air, and water.
-- The iron oxide formed from a chemical change when the iron
on the surface of the bike combined with some of the oxygen in
the air, and molecules of a new substance were created. (For
some reason, the presence of water makes this chemical reaction
go faster.)
