Physical Changes versus Chemical Changes
Introduced| English |
|---|
| physical change/ˈfɪzɪkl tʃeɪndʒ/ |
| chemical change/ˈkemɪkl tʃeɪndʒ/ |
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.
Ice melting into water is a...
It is still water, just a different state -- physical.
A chemical change
- A chemical change 化学变化 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...
New substances (ash, gases) form, so it is chemical.
Select all chemical changes.
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...
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.
Evaporating the water returns the original salt.
A change of state such as boiling is a ____ change.
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.