Concept preview

What you will be able to do

1. 01Distinguish a physical change from a chemical change
2. 02Interpret the heating curve of a pure substance
3. 03Explain why temperature is constant during melting and boiling

A chemical change is one in which new substances are formed — the set of chemicals afterward is different from before. A physical change is a change in the phase of a substance in which no new substance forms; the set of chemicals is unchanged. Melting ice and boiling water are physical changes: liquid water and steam are still water. Chemical changes are often accompanied by physical changes — hydrogen and oxygen gases react to form liquid water — but the primary change there is chemical.