Which of the following is true about the heat of combustion?

The heat of combustion is a specific type of thermodynamic measurement that quantifies the amount of energy released when a substance is burned in the presence of oxygen. It is expressed in units of energy per amount of substance, typically in kilojoules per mole.

Choosing the correct answer highlights the important fact that this measurement is standardized according to the amount of substance that is being combusted. This allows for consistent comparisons between different materials, as energy release is directly linked to moles of substance involved in the reaction.

The other options do not accurately reflect the nature of the heat of combustion. For instance, while heat of formation and heat of combustion do relate to the energy changes during chemical reactions, they are not directly comparable in terms of being "higher" or "lower." Additionally, the heat of combustion can vary with environmental conditions, such as temperature and pressure, and it is also influenced by the form of the substance being combusted (e.g., solid, liquid, or gas) as well as the specific structural characteristics of the molecules involved. Therefore, the statement regarding measurement in kilojoules per mole of substance is the only one that accurately represents a fundamental principle of the heat of combustion.