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

2. Definition
Physical Change – chemical structure remains the same. NO NEW substance forms!
breaking, dissolving, distilling, cutting, etc.
Changes in state are physical changes (boiling, condensing,
melting and freezing)
Chemical Change - chemical structures of the substances have changed. NEW Substances form!
rusting, burning, metabolizing food, oxidation or reduction, reacting with oxygen, etc.

3. 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)

4. 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.

5. 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!

6. 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.
Since the original change was un-done with a physical change, then the original change (the dissolving) was a physical change as well.

7. Using Changing Words Properly
Sometimes, students misuse the words that appear below:
Term
Definition
Type of Change
Melting
Changes a solid into a liquid
Physical
Burning
Reacting with oxygen to produce CO2 and H2O
Chemical
Dissolving
Adding one substance to another to form a homogeneous mixture
Physical
Drying
Heating a sample to evaporate the water
Physical