A chemical reaction occurs without adding heat under the condition of spontaneity. This means that the reaction can proceed on its own based on the thermodynamic properties such as enthalpy and entropy. Spontaneous reactions are typically characterized by a negative Gibbs free energy change (ΔG < 0), indicating that the products are more stable than the reactants. Examples include exothermic reactions, where energy is released, making these processes naturally favorable without external energy input.