Chemical Change
A change in which the atoms of one or more substances are rearranged into new substances with different properties.
Unit
Dimensionless
In depth
Bonds are broken and formed, so energy is always absorbed or released. Tell-tale signs include colour change, gas evolution, precipitate formation, a temperature change and a light or odour change. The total mass is unchanged, in line with the law of conservation of mass.
Examples in the real world
Methane burning to CO₂ and water, milk souring, and a strip of zinc displacing copper from copper sulfate solution.