Question

part b
read each scenario. decide whether a physical or chemical change has occurred and give evidence for your decision. the first one has been done for you to use as an example.

1. umm! a student removes a loaf of bread hot from the oven. the student cuts a slice off the loaf and spreads butter on it.
2. your friend decides to toast a piece of bread, but leaves it in the toaster too long. the bread is black and the kitchen if full of smoke.
3. you forgot to dry the bread knife when you washed it and reddish brown spots appeared on it.
4. you blow dry your wet hair.
5. in baking biscuits and other quick breads, the baking powder reacts to release carbon dioxide bubbles. the carbon dioxide bubbles cause the dough to rise.
6. you take out your best silver spoons and notice that they are very dull and have some black spots.
7. a straight piece of wire is coiled to form a spring.
8. food color is dropped into water to give it color.
9. chewing food to break it down into smaller particles represents a ____ change, but the changing of starch into sugars by enzymes in the digestive system represents a ________ change.
10. in a fireworks show, the fireworks explode giving off heat and light.

Explanation:

• Scenario 1: Cutting bread and spreading butter only changes the form of the bread and butter, not their chemical composition.
• Scenario 2: Toasting bread too long causes a chemical reaction (burning), changing the bread's chemical structure.
• Scenario 3: Rust (reddish - brown spots) is a new substance formed by a chemical reaction between iron (in the knife) and oxygen/moisture.
• Scenario 4: Blow - drying hair only changes the state of water (from liquid to gas), not the chemical composition of hair or water.
• Scenario 5: Baking powder reacting to release carbon dioxide is a chemical reaction.
• Scenario 6: Silver spoons tarnishing (black spots) is a chemical reaction (oxidation).
• Scenario 7: Coiling a wire only changes its shape, not its chemical composition.
• Scenario 8: Food color dissolving in water is a physical mixing, no new chemical substances are formed.
• Scenario 9: Chewing food is a physical breakdown (changing size/shape), while starch - to - sugar conversion by enzymes is a chemical reaction.
• Scenario 10: Fireworks exploding involve chemical reactions (oxidation, decomposition etc.) that release heat and light.

Answer:

1. Physical change; Only form (cutting, spreading) is changed.
2. Chemical change; Burning (new substances like carbon, smoke are formed).
3. Chemical change; Rust (new substance) is formed.
4. Physical change; Water state changes (liquid to gas), no chemical change.
5. Chemical change; Baking powder reacts (new substance - carbon dioxide).
6. Chemical change; Tarnishing (new substance - silver sulfide etc.) occurs.
7. Physical change; Only shape (coiling) is changed.
8. Physical change; Mixing (no new chemical substances).
9. Physical; Chemical; Chewing (physical breakdown), starch - to - sugar (chemical).
10. Chemical change; Explosion involves chemical reactions (heat, light release).