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.

1. 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

1. you forgot to dry the bread knife when you

washed it and reddish brown spots
appeared on it.

1. you blow dry your wet hair.
2. 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.

1. you take out your best silver spoons and

notice that they are very dull and have
some black spots.

1. a straight piece of wire is coiled to form a

spring.

1. food color is dropped into water to give it

color.

1. 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.

1. in a fireworks show, the fireworks explode

giving off heat and light.

Explanation:

• Scenario 1: Cutting bread and spreading butter are physical changes as they only alter the form, not the chemical composition.
• Scenario 2: Burning bread (toasting too long) is a chemical change as new substances (like carbon from charring) are formed and there is smoke (a sign of chemical reaction).
• Scenario 3: Rust (reddish - brown spots on the knife) is a chemical change. Iron reacts with oxygen and moisture to form iron oxide (rust), a new substance.
• Scenario 4: Blowing dry wet hair is a physical change. It is just the removal of water (a state change of water from liquid to gas) from the hair, no new substances are formed.
• Scenario 5: Baking powder reacting to release carbon dioxide is a chemical change. A chemical reaction occurs to produce a new gas (carbon dioxide) which causes the dough to rise.
• Scenario 6: Silver spoons getting dull and having black spots (tarnishing) is a chemical change. Silver reacts with sulfur compounds in the air to form silver sulfide (a new substance).
• Scenario 7: Coiling a wire is a physical change. It only changes the shape of the wire, not its chemical composition.
• Scenario 8: Dropping food color into water is a physical change. It is a process of mixing, and no new chemical substances are formed.
• Scenario 9: Chewing food is a physical change (mechanical breakdown). Changing starch into sugars by enzymes is a chemical change as a new substance (sugar) is formed from starch.
• Scenario 10: Fireworks exploding and giving off heat and light is a chemical change. Chemical reactions occur in the fireworks to produce light, heat, and new substances.

Answer:

1. Physical change. Just cutting and spreading (form change).
2. Chemical change. Burning (new substances like carbon, smoke).
3. Chemical change. Rust (iron oxide - new substance).
4. Physical change. Water evaporation (state change).
5. Chemical change. Gas (carbon dioxide) production (new substance).
6. Chemical change. Tarnishing (silver sulfide - new substance).
7. Physical change. Shape alteration (no chemical change).
8. Physical change. Mixing (no new chemical substances).
9. Physical (chewing), Chemical (starch to sugar).
10. Chemical change. Light, heat, new substances (from reactions).