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. 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.

Explanation:

Step1: Analyze scenario 1

Cutting bread and spreading butter only changes its physical form. No new substances are formed.
Physical change; No new substances formed.

Step2: Analyze scenario 2

Bread turning black and smoke formation indicate new substances are created due to burning.
Chemical change; New substances (charred bread, smoke) formed.

Step3: Analyze scenario 3

Reddish - brown spots on the knife are rust, a new substance formed by the reaction of iron in the knife with oxygen and water.
Chemical change; Rust (new substance) formed.

Step4: Analyze scenario 4

Blowing dry wet hair only changes the state of water from liquid to gas. No new substances are formed.
Physical change; Water changes state from liquid to gas.

Step5: Analyze scenario 5

Baking powder reacting to release carbon - dioxide bubbles and make dough rise involves chemical reactions creating new substances.
Chemical change; Carbon dioxide (new substance) formed.

Step6: Analyze scenario 6

Dullness and black spots on silver spoons are due to silver reacting with sulfur - containing substances in air to form silver sulfide.
Chemical change; Silver sulfide (new substance) formed.

Step7: Analyze scenario 7

Coiling a wire to form a spring only changes its shape. No new substances are formed.
Physical change; Only shape is changed.

Step8: Analyze scenario 8

Food color dissolving in water is a physical mixing process. No new substances are formed.
Physical change; No new substances formed, just mixing.

Step9: Analyze scenario 9

Chewing food is a physical change as it only breaks food into smaller pieces. Enzymes changing starch to sugars is a chemical change as new substances are formed.
Physical change (chewing); Chemical change (starch - to - sugar conversion); Physical: Only size of food changes. Chemical: New substances (sugars) formed.

Step10: Analyze scenario 10

Fireworks exploding and giving off heat and light involve chemical reactions creating new substances.
Chemical change; New substances formed, heat and light released.

Answer:

1. Physical change; No new substances formed.
2. Chemical change; New substances (charred bread, smoke) formed.
3. Chemical change; Rust (new substance) formed.
4. Physical change; Water changes state from liquid to gas.
5. Chemical change; Carbon dioxide (new substance) formed.
6. Chemical change; Silver sulfide (new substance) formed.
7. Physical change; Only shape is changed.
8. Physical change; No new substances formed, just mixing.
9. Physical change (chewing); Chemical change (starch - to - sugar conversion); Physical: Only size of food changes. Chemical: New substances (sugars) formed.
10. Chemical change; New substances formed, heat and light released.