1. Section 3.4—Changes in Matter
Objective:
Distinguish between chemical and physical changes
What type of changes can produce a gas for an airbag?

2. Physical Changes
Physical Change – a change in which the chemical structure of the substances is not changed.
Physical changes do not produce new substances -breaking, dissolving, distilling, cutting, etc.
Changes in state are physical changes (boiling, condensing, melting and freezing)

3. Chemical Changes
Chemical Change – a change in which the chemical structures of the substances are changed.
Chemical changes do produce new
substances
rusting, burning, digesting food, oxidation or reduction, reacting with oxygen, etc.
Chemical changes ARE chemical reactions.

4. Physical & Chemical Changes
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, so many people think this is a physical change.
But if the water is evaporated (a physical change), the salt is left in the container.
Since the original change was un-done with a physical change, then the original change (the dissolving) was a physical change as well.

5. Confusing changes People often use the following terms incorrectly.
Definition
Type of Change
Melting
Changes a solid into a liquid
Physical
Burning
Reacting with oxygen
Chemical
Dissolving
Adding one substance to another to form a homogeneous mixture
Physical
Drying
Heating a sample to evaporate the water
Physical

6. Practice Problems
Determine whether the following are physical or chemical changes:
Grinding a piece of meat
Adding Kool Aid mix to water
Rotting fruit
Melting butter
Getting a hair cut
Baking a cake
Toasting a marshmallow
Denting the car

7. Possible Signs of Chemical Changes
Gas production (bubbling, fumes, change in odor)
Energy change (temperature change)
Color change
Formation of a precipitate (making an insoluble substance from two soluble substances)

8. They’re “Possible” signs
Sometimes these “signs” accompany physical changes as well!
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)
However, some of these signs also accompany physical changes, so you must take into account many observations to determine if the change was in fact chemical.

9. 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!