= Solution
\b[A <combustible substance> can sustain a sufficiently rapid, heat-producing <chemical reaction> with a suitable <oxidizing agent>.] In ordinary <air> this usually means combination with <molecular oxygen>, producing enough <heat> to maintain the reaction after ignition. A visible flame is not essential: <carbon> can glow as it undergoes <combustion>.
<Combustibility> depends on the surroundings and experimental conditions. A cold block of <wax> does not ignite merely because it meets <air>: heating must first provide combustible vapour and overcome the kinetic barrier to reaction. Finely divided <iron> burns much more readily than a massive bar because more surface is exposed per unit <mass>. Nor is <oxygen> universally required: <molecular hydrogen> can undergo <combustion> in <chlorine>. The expression “supports <combustion>” identifies the surrounding reactant in a particular experiment; it does not establish an absolute division between two kinds of <chemical substance>.
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