Section 3.4—Changes in Matter

What type of changes can produce a gas for an airbag?

Objective: Distinguish between chemical and physical changes

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)

Chemical Changes

Chemical Change – a change in which the chemical structures of the substances are changed.

Chemical changes do produce newsubstancesrusting, burning, digesting food, oxidation or reduction, reacting with oxygen, etc.

Chemical changes ARE chemical reactions.

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.

Definition

Changes a solid into a liquid

Reacting with oxygen

Confusing changes

People often use the following terms incorrectly.

Term Type 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

Practice Problems

Determine whether the following are physical or chemical changes:Grinding a piece of meat Adding Kool Aid mix to waterRotting fruitMelting butterGetting a hair cutBaking a cakeToasting a marshmallowDenting the car

Possible Signs of Chemical Changes

Gas production (bubbling, fumes, change in odor)

Energy change (temperature change)Color changeFormation of a precipitate (making an

insoluble substance from two soluble substances)

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!

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!