Chemical Change
A chemical change produces one or more new substances with different properties, as when wood burns, iron rusts, or milk sours.
Definition
A chemical change, or chemical reaction, converts one or more substances into new substances with different chemical compositions and properties by breaking and forming bonds, so the resulting stuff is genuinely not the same as what you started with. It is accompanied by an enthalpy change reflecting the difference in bond energies, an entropy change reflecting the shift in disorder, and a free energy change that determines whether the process happens spontaneously, and chemical changes are classified as synthesis, decomposition, single and double displacement, combustion, acid-base, and redox reactions. Evidence includes an unexpected color change, gas production, formation of a precipitate, release or absorption of heat or light, and a difficulty reversing the change, though mass is conserved throughout every case.
Example
Toast browning, a bike rusting, and a cake baking are all chemical changes that cannot be undone by cooling or waiting. Iron rusting follows $4Fe + 3O_2 \rightarrow 2Fe_2O_3$, turning the gray, magnetic, strong metal into an orange, crumbly, non-magnetic oxide, while burning magnesium gives off a brilliant white light and leaves behind white $MgO$ powder. Cooking an egg denatures its proteins by breaking hydrogen bonds and disulfide bridges and forming new cross-links, a chemical change that cooling cannot reverse, and caramelizing sugar above about $160^\circ\text{C}$ breaks sucrose into hundreds of new compounds that create its brown color and new flavors.
Key Insight
Bubbles alone are not proof of a chemical change, since boiling water bubbles physically and so does an opened soda; what signals a real chemical change is a gas that was not there before, like the fizz from baking soda meeting vinegar. The physical-versus-chemical boundary gets blurriest with solids, since melting ice is physical but heating limestone to $840^\circ\text{C}$ decomposes it chemically into lime and $CO_2$, showing the very same input, heat, can trigger either kind of change depending entirely on the substance involved.