Physical Changes versus Chemical Changes
Introduced| English | Meaning |
|---|---|
| physical change/ˈfɪzɪkl tʃeɪndʒ/ | A change in form or physical state that does not change a substance's chemical identity. |
| chemical change/ˈkemɪkl tʃeɪndʒ/ | A change in which atoms are rearranged to form substances with different chemical identities. |
Two kinds of change
- Ice melting and paper burning both transform something.
- But only one makes a brand-new substance.
- Melt the ice and you can freeze it right back.
- Burn the paper and the ash will never be paper again.
A physical change
- A physical change 物理变化 alters form but not identity.
- Melting, boiling, dissolving, or cutting — the same substance throughout.
- It is often easy to reverse.
A physical change rearranges the same molecules; a chemical change 化学变化 makes new ones
Ice melting into water is a...
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It is still water, just a different state -- physical.
A chemical change
- A chemical change makes new substances with new properties.
- Burning, rusting, and cooking rearrange atoms into new compounds.
- It is usually hard to reverse.
Physical or chemical change?
Sort each change.
Burning wood into ash and smoke is a...
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New substances (ash, gases) form, so it is chemical.
Select all chemical changes.
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Rusting and souring form new substances; chopping is physical.
How to tell
- Ask whether the substances after the change differ from before.
- A new colour, a gas, or a solid appearing hints at a chemical change.
- If only the shape or state changed, it is physical.
The key test for a chemical change is whether...
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A genuinely new substance signals a chemical change.
Is dissolving salt in water physical or chemical?
- The salt separates into ions but is still salt.
- Evaporate the water and the salt returns, so it is physical.
Dissolving salt in water is usually a physical change because the salt can be recovered.
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Evaporating the water returns the original salt.
A change of state such as boiling is a ____ change.
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Boiling changes state but not the substance's identity.
A change of state (melting, boiling) is physical — the substance is unchanged, just rearranged. Dissolving is usually physical too, since you can recover the solute. The real test is: did a new substance with new properties form? If yes, it is chemical.
A physical change alters form but not identity (melting, dissolving) and is usually reversible. A chemical change makes new substances with new properties (burning, rusting) and is hard to reverse. The test is whether a genuinely new substance appeared.