1. Section 3.4—Changes in Matter What type of changes can produce a gas for an airbag?

2. Definition Physical Change - n. Change in which the chemical structure of the substances is not changed. Chemical Change - n. Change in which the chemical structures of the substances are changed.

3. Physical & Chemical Changes Physical changes do not produce new substances  breaking, dissolving, distilling, cutting, etc.  Changes in state are physical changes (boiling, condensing, melting and freezing) Chemical changes do produce new substances  rusting, burning, metabolizing food, oxidation or reduction, reacting with oxygen, etc.

4. Possible Signs of Chemical Changes Gas production (bubbling) Energy change (getting hot or cold) Color change Light given off Formation of a precipitate (making an insoluble substance from two soluble substances)

5. They’re “Possible” signs Gas production (bubbling). Bubbles are formed during boiling (a physical change) Energy change (getting hot or cold). Energy changes accompany changes in state (physical changes) Color change. Color change can occur due to dissolving a substance (a physical change) Sometimes these “signs” accompany physical changes as well!

6. How do you know for sure? Measure and observe chemical and physical properties before the change in question. Measure and observe the properties after the change. If the properties are the same, then it was a physical change!

7. Physical & Chemical Changes Also…if a change can be un-done by a physical change, then the original change was physical as well. If salt is dissolved in water, it seems to disappear… But if the water is evaporated (a physical change), the salt is left in the container. many people think this is a chemical change. Since the original change was un-done with a physical change, then the original change (the dissolving) was a physical change as well.

8. Definition Changes a solid into a liquid Reacting with oxygen to produce CO 2 and H 2 O Confusing changes People often use the following terms incorrectly. TermType of Change Melting Physical Burning Chemical Adding one substance to another to form a homogeneous mixture Dissolving Physical Heating a sample to evaporate the water Drying Physical