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.
scenario

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 is 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.

physical or chemical change?
evidence

Explanation:

1. Physical change. No new substances formed, just cutting and spreading.
2. Chemical change. Burning produces new substances like carbon - containing compounds and char, and there is smoke.
3. Chemical change. Rust (iron oxide) forms due to reaction of iron in knife with oxygen and water.
4. Physical change. Blowing air just moves water droplets, no new substances.
5. Chemical change. Baking powder reacts to produce carbon dioxide gas, a new substance.
6. Chemical change. Oxidation of silver to form silver sulfide (black spots).
7. Physical change. Only shape of wire changes, no new substance.
8. Physical change. Food color disperses in water, no chemical reaction.
9. First is physical change (chewing), second is chemical change (enzymatic breakdown of starch).
10. Chemical change. Fireworks contain chemicals that react to produce heat, light and new substances.

Answer:

1. Physical change. Evidence: No new substances formed, just change in shape and state of bread.
2. Chemical change. Evidence: Burning produces char and smoke, new substances are formed.
3. Chemical change. Evidence: Reddish - brown rust (iron oxide) is a new substance formed.
4. Physical change. Evidence: Only movement of water droplets, no new substances.
5. Chemical change. Evidence: Carbon dioxide gas is produced as a new substance.
6. Chemical change. Evidence: Black silver sulfide spots are new substances formed.
7. Physical change. Evidence: Only shape of wire is changed, no new substance.
8. Physical change. Evidence: Food color disperses, no chemical reaction.
9. Physical change (chewing); Chemical change (enzymatic breakdown). Evidence: Chewing is just mechanical breakdown, enzymatic breakdown forms new substances (sugars).
10. Chemical change. Evidence: Heat, light and new substances are produced during the explosion.