Many organic molecules can undergo combustion, forming carbon dioxide and water in an exothermic reaction. The heat released in the combustion reaction (the enthalpy change) can be used as an indicator of the relative stability of isomers. Combustion is more exothermic for unbranched alkanes, for example, than for their branched isomers, and we can infer that the branched isomer is the more stable. Such comparisons are often used in organic chemistry. For example, ketones are pointed out as more stable than their aldehyde isomers. The more stable the isomer, the lower the heat of combustion.