Question

physical and chemical changes
part a
can you recognize the chemical and physical changes that happen all around us? if you change the way something looks, but havent made a new substance, a physical change (p) has occurred. if the substance has been changes into another substance, a chemical change (c) has occurred.

1. an ice cube is placed in the sun. later there is a puddle of water. later still the puddle is gone.
2. two chemical are mixed together and a gas is produce.
3. a bicycle changes color as it rusts.
4. a solid is crushed to a powder.
5. two substances are mixed and light is produced.
6. a piece of ice melts and reacts with sodium.
7. mixing salt and pepper.
8. chocolate syrup is dissolved in milk.
9. a marshmallow is toasted over a campfire.
10. a marshmallow is cut in half.

Explanation:

1. Melting of ice and evaporation of water are physical changes as no new substance is formed.
2. Production of gas from mixing chemicals indicates a chemical change (new substance - gas).
3. Rusting of bicycle (iron reacting with oxygen to form iron oxide) is a chemical change.
4. Crushing a solid to powder is a physical change (no new substance).
5. Production of light from mixing substances often indicates a chemical reaction (new substances with energy release).
6. Melting of ice is physical, but reaction with sodium is chemical (new substances formed).
7. Mixing salt and pepper is a physical change (no chemical reaction).
8. Dissolving chocolate syrup in milk is a physical change (no new substance).
9. Toasting marshmallow (carbonization, reaction with air) is a chemical change.
10. Cutting marshmallow is a physical change (no new substance).

Answer:

1. P
2. C
3. C
4. P
5. C
6. P (melting) and C (reaction)
7. P
8. P
9. C
10. P