Chemical and Physical Changes — Answer Key
Part A: Fix the Sentence
Each sentence has an error. Rewrite it correctly on the line.
1. Fix the sentence:
Crushing a can into a flat shape are a chemical change.
Corrected: Crushing a can into a flat shape is a physical change.
Crushing changes only the shape of the metal, so the substance stays the same. Subject-verb agreement requires 'is' for the singular gerund subject.
2. Fix the sentence:
When water freezes into ice, it becomes a brand new substance.
Corrected: When water freezes into ice, it stays the same substance in a new state.
Freezing is a physical change because the molecules are still water; only their arrangement and state changed from liquid to solid.
3. Fix the sentence:
Mixing sugar into water make a chemical change happen.
Corrected: Mixing sugar into water makes a physical change happen.
Dissolving is a physical change because evaporating the water leaves the sugar behind unchanged. The verb must agree with the singular gerund subject.
Part B: Fill in the Blank
Write the missing word or number on each line.
1. Melting butter on a pan is a physical change because the butter is still butter, only softer.
Melting is a physical change because no new substance forms; the butter molecules stay the same.
2. Tearing paper into strips changes its shape but not its substance, which stays the same.
Tearing is a physical change because the torn pieces are still paper, made of the same substance.
3. When a puddle dries up after the rain, the water evaporates into vapor without changing identity.
Evaporation is a physical change because liquid water becomes water vapor; both are still H2O molecules.
4. Crushing chalk into powder is a physical change since the powder is still chalk.
Crushing is a physical change because the chalk pieces have the same composition as the original stick.
Part C: Short Answer
Answer each question in one or two complete sentences.
1. Explain why mixing sand and gravel together is a physical change, not a chemical change.
Sample answer: Mixing sand and gravel is a physical change because no new substance forms. The sand is still sand, and the gravel is still gravel. You can sift them apart and get back exactly what you started with, which proves their chemical identities never changed.
A physical change does not create new substances. Since sand and gravel can be separated and remain unchanged, only their arrangement was altered, not their composition.
2. Describe how freezing juice into a popsicle is a physical change at the molecular level.
Sample answer: Freezing juice into a popsicle is a physical change because the juice molecules slow down and lock into a solid pattern, but they are still juice molecules. No new substances form. If the popsicle melts, you get the same juice back, showing the change is only in state, not identity.
Freezing rearranges molecules into a solid structure without changing their chemical makeup. Melting reverses the change, proving the same substance remains throughout.