What is the difference between a physical change and a chemical change?

Asked by: Tim Meyer

Answer

There are two possible definitions for Physical and Chemical changes which I use in school:

1. A physical change is reversible, a chemical change is not. For example, the freezing of water would be a physical change because it can be reversed, whereas the burning of wood is a chemical change - you can't 'unburn' it

2. A physical change is a change in which no new substance is formed; a chemical change results in the formation of one or more new substances. Again, consider the previous examples: Freezing water into ice just results in water molecules which are 'stuck' together - it's still H₂O. Whereas burning wood results in ash, carbon dioxide, etc, all new substances which weren't there when you started.
Answered by: Michael Treacy, M.S., Physics Teacher, Ireland

This is a good question! One would think that a change is a change is a change so why would there be a distinction between a physical change and a chemical change?

Well, there is a big difference! If you change something physically you still have what you started with. For example, if you tear a piece of paper you still have paper. If you change something chemically you end up with something very different than what you started with. For example if you burn a piece of paper you end up with carbon and no more paper.

But, these are obvious examples. Is it always that simple? What if you put sugar into water? Is that a physical or a chemical change? How about if you freeze water? Is that physical or chemical? For these examples we have to examine what it would take to get the original substances back after wo have made the change. With the sugar and the water we could evaporate the water and find that the sugar is left and there is no change, except in how it looks. It is still sweet and it can be remixed in water. The frozen water can be melted and we have the same water we started with.

If we take that same sugar and water and mix it with flour and eggs and vanilla and chocolate chips and cook it we will have performed a chemical change making cookies. No matter what you do to the cookies you will never get the sugar and the water out of them as before.

So, in summary, there are really only two criteria for whether a change is chemical or physical: 1) After the change does it look the same and 2) can you change it back? If the answer is yes, the change is physical every time. If the answer is no the change is chemical every time.
Answered by: Tom Young, B.S., Science Teacher, Whitehouse HS