Chemical Change
Definition and meaning of Chemical Change in chemistry.
Chemical change is a process in which one or more substances are converted into different substances with new chemical compositions and properties, through the breaking and forming of chemical bonds.
In more detail
Unlike a physical change, which alters only a substance's form, state, or appearance while its molecules remain intact, a chemical change rearranges atoms into new molecules with distinct properties. Common signs include color change, gas evolution, precipitate formation, temperature change, or light emission, though none of these alone proves a chemical change occurred. Because atoms are only rearranged and not created or destroyed, chemical changes always obey the law of conservation of mass. Most chemical changes cannot be reversed by simple physical means, such as evaporation or freezing.
Key facts
| Field | General Chemistry |
|---|---|
| Also called | Chemical reaction |
| Reversibility | Generally not reversible by physical means alone |
| Governing law | Conservation of mass |
The rusting of iron is a chemical change: iron reacts with oxygen in the air to form iron(III) oxide, 4Fe + 3O2 → 2Fe2O3. The reddish-brown rust is a new substance with different physical and chemical properties than the original metal, and the process is not reversed by melting or dissolving it.
Frequently asked questions
How is a chemical change different from a physical change?
A physical change alters a substance's form, size, or state (like melting ice) without changing its molecular identity, while a chemical change produces one or more new substances with different chemical properties.
Does a color change always mean a chemical change occurred?
No. Color change is a common clue, but it can also result from physical processes like dissolving a colored solute, so it must be considered alongside other evidence.