Chemical Reaction
A chemical reaction is a process in which substances are changed into different substances by breaking and forming chemical bonds.
Definition
A chemical reaction is when substances change into new substances as the atoms get rearranged into different combinations. Reactants are transformed into products with different chemical identities as bonds break and new bonds form; atoms are neither created nor destroyed, so mass is conserved, and common types include synthesis, decomposition, single replacement, double replacement, and combustion, all described by chemical equations. At the electron level a reaction redistributes electrons among atoms through one or more elementary steps that make up a mechanism, passing through a transition state whose energy above the reactants is the activation energy. The overall enthalpy change equals the difference between bond energies broken and formed, and whether a reaction proceeds and how far depends on thermodynamics ($\Delta G$), while how fast it proceeds depends on kinetics.
Example
When wood burns it turns into ash, smoke, and gases, and when iron rusts shiny metal becomes flaky orange rust; both are chemical reactions. Hydrogen and oxygen react to form water, $2H_2 + O_2 \rightarrow 2H_2O$, and baking soda and vinegar react to release carbon dioxide gas: $NaHCO_3 + CH_3COOH \rightarrow CH_3COONa + H_2O + CO_2$. The combustion of methane, $CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O$, releases $890\text{ kJ}$ per mole of methane, yet natural gas sits in a pipe indefinitely without reacting because its activation energy is high; a spark supplies enough energy to start the chain, and the heat released then sustains it.
Key Insight
Clues that a reaction happened include a color change, bubbles, heat or light given off, or a new solid forming, while melting or dissolving are not reactions. The products can look nothing like the reactants: two explosive or flammable gases, hydrogen and oxygen, combine to make water, the substance we use to put fires out. Every reaction is in principle reversible, and many reach an equilibrium where forward and reverse rates match, so "goes to completion" means the equilibrium lies so far toward products that the reactants are effectively gone, not that the reverse reaction is impossible.