Question

physical vs. chemical changes
in a physical change, the original substance still exists, it has only changed in form. in a chemical change, a new substance is produced. energy changes always accompany chemical changes.
there are four good signs for a chemical reaction (although these do not guarantee a reaction has occurred):

• energy change
• color change
• gas released (sometimes with an odor)
• precipitate formed (a precipitate is a solid formed during the reaction of two aqueous solutions)

classify the following as being a physical or chemical change.

1. sodium hydroxide dissolves in water.
2. hydrochloric acid reacts with potassium hydroxide to produce a salt, water, and heat.
3. a pellet of sodium is sliced in two.
4. water is heated and changed into steam.
5. potassium chlorate decomposes to form solid potassium chloride and oxygen gas.
6. iron rusts.
7. when place in h₂o, a piece of sodium catches on fire as hydrogen gas is liberated and sodium hydroxide forms.
8. evaporation.
9. ice melting.
10. milk sours.
11. sugar dissolves in water.
12. wood rotting.
13. pancakes cooking on a griddle.
14. grass growing in a lawn.
15. a tire is inflated with air.
16. food is digested in the stomach.
17. water is absorbed by a paper towel.
18. water freezing into ice.

Explanation:

• Physical change: Only form changes, original substance exists. Examples: sodium hydroxide dissolving (no new substance), slicing sodium (shape change), water heating to steam (state change), evaporation (state change), ice melting (state change), sugar dissolving (no new substance), tire inflating (volume change), water absorption by towel (no new substance), water freezing (state change).
• Chemical change: New substance formed. Examples: hydrochloric acid reacting (new substances - salt, water, heat), potassium chlorate decomposing (new substances - potassium chloride, oxygen), iron rusting (new substance - rust), sodium in water (new substances - hydrogen gas, sodium hydroxide), milk souring (new substances - lactic acid etc.), wood rotting (new substances - decomposed organic matter), pancakes cooking (new substances - cooked batter), grass growing (new substances - plant tissues), food digestion (new substances - digested nutrients).

Answer:

1. Physical change
2. Chemical change
3. Physical change
4. Physical change
5. Chemical change
6. Chemical change
7. Chemical change
8. Physical change
9. Physical change
10. Chemical change
11. Physical change
12. Chemical change
13. Chemical change
14. Chemical change
15. Physical change
16. Chemical change
17. Physical change
18. Physical change