Section 3 4Changes in Matter Objective Distinguish between
Section 3. 4—Changes in Matter Objective: Distinguish between chemical and physical changes What type of changes can produce a gas for an airbag?
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 new substances ¡rusting, 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.
Confusing changes People often use the following terms incorrectly. Term Melting Burning Dissolving Drying Definition Type of Changes a solid into a liquid Physical Reacting with oxygen Chemical Adding one substance to another to form a homogeneous mixture Heating a sample to evaporate the water Physical
Practice Problems l 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
Possible Signs of Chemical Changes ü Gas production (bubbling, fumes, change in odor) ü Energy change (temperature change) ü Color change ü Formation of a precipitate (making an insoluble substance from two soluble substances)
They’re “Possible” signs Sometimes these “signs” accompany physical changes as well! ü Gas production (bubbling). Bubbles are formed during boiling (a physical change) ü Energy change (getting hot or cold). Energy changes accompany changes in state (physical changes) ü Color change can occur due to dissolving a substance (a physical change)
How do you know for sure? ü Measure and observe chemical and physical properties before the change in question. ü Measure and observe the properties after the change. ü If the properties are the same, then it was a physical change!
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