Physical vs Chemical Change
Physical change
A physical change alters the form, state, or appearance of a substance without changing its chemical identity — the particles are the same before and after, just arranged or spaced differently. Melting, freezing, dissolving, and tearing are physical changes.
Example: Melting an ice cube: solid water becomes liquid water, but every particle is still H2O before and after.
Chemical change
A chemical change (a chemical reaction) rearranges atoms into one or more entirely new substances with different properties. Bonds break and new bonds form. Burning, rusting, cooking an egg, and digesting food are chemical changes.
Example: Rusting of an iron nail: iron reacts with oxygen and water to form iron oxide, a reddish-brown substance nothing like shiny iron.
Key differences
| Aspect | Physical change | Chemical change |
|---|---|---|
| Substances produced | No new substance — the same chemical species exist before and after. | One or more new substances with different properties are produced. |
| Particle-level view | Particles only move apart, come together, or rearrange in space; bonds within molecules are untouched. | Atoms are reconnected: old bonds break and new bonds form between different atoms. |
| Reversibility | Usually easy to reverse (refreeze melted water, evaporate salt water). | Usually hard or impossible to reverse directly (you cannot un-burn paper). |
| Energy changes | Small energy changes tied to state or spacing (melting absorbs modest heat). | Often larger energy changes tied to bond breaking and forming (combustion releases lots of heat). |
| Clues it happened | State, shape, or size changes only; properties like density per state stay familiar. | Color change, gas bubbles, precipitate, odor, temperature change, or light emission. |
Dissolving looks chemical but is usually physical — salt disappears into water, yet evaporating the water recovers solid salt unchanged. Reversibility is a helpful clue but not a perfect rule: some physical changes are hard to reverse (shredding paper), and a few chemical changes are reversible (some equilibrium reactions). When in doubt, ask the particle question: are the same molecules or atoms present at the end, just rearranged? If yes, it is physical.