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When a candle burns, it undergoes a chemical process known as a combustion reaction. This reaction involves the fuel—in this case, the wax, which is a hydrocarbon—and oxygen from the air. During combustion, the wax molecules react with oxygen to produce carbon dioxide and water vapor, liberating energy. The chemical equation representing this combustion can be written as:
candle wax hydrocarbon + oxygen → carbon dioxide + water vapor + energy
The chemical structure of the wax, represented as \( C_{n}H_{2n+2} \), reacts with oxygen (\( O_2 \) molecules to transform into new substances: \( nCO_2 \) and \( (n+1) H_2O \). This type of reaction is fundamental in many everyday processes, not just in burning candles but also in engines and power plants.

Energy transformation is a key concept in understanding how a candle provides light and heat. In a candle, the chemical potential energy stored within the wax's molecular bonds is converted into other forms of energy when the wax burns. This energy is not lost but transformed into heat, which is felt as warmth, and light, which is seen as the candle's flame.
Chemical potential energy is the energy held within the substances' bonds and is released during a chemical reaction. In the context of a candle, as the wax, comprised of hydrocarbons, undergoes combustion, the bonds break apart and reform into other molecules, a process which involves shifting energy from a stored state (chemical potential) to kinetic and thermal states (heat and light). This illustrates one of the fundamental laws of physics — the conservation of energy — which states that energy can neither be created nor destroyed, only transformed.

The term 'exothermic reaction' refers to a type of chemical reaction that releases energy through light or heat. The combustion reaction of a candle is a classic example of an exothermic reaction. As the combustion takes place, it liberates energy that was previously stored as chemical potential energy within the chemical bonds of the wax.
This released energy is perceived in two forms: as the warmth felt near a burning candle and the light emitted from the flame. The heat produced not only maintains the candle's flame by melting more wax but also warms the surrounding environment. Similar exothermic reactions occur in other contexts, such as hand warmers, campfires, and in larger scales in industrial power generation where fuel combustion is used to produce electricity.