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mixing chemicals
In chemistry, how can you tell the difference between a chemical change and a physical change?

Answer:

While some elementary textbooks say you can identify physical changes because they are reversible, or that you can identify chemical changes because they cause color changes, produce bubbles, give off heat, or absorb heat, none of those is a reliable test. There are physical changes, such as mixing copper and zinc to make brass, that are not reversible. Brass is a solution, not a chemical compound. The copper and brass are not changed chemically. There are also chemical changes that are easily reversed, and some (called clock reactions) reverse themselves. There are physical changes that cause a change in color, produce bubbles, give off heat, or absorb heat. The ONLY reliable way to identify chemical and physical changes is to look at the chemical formulas of the materials involved. If the chemical formulas change, it is a chemical reaction. If they don't, then it is a physical change.

For more information on this, read Changing How We Think About Changing