What is the main characteristic of heat as a form of energy?

The main characteristic of heat as a form of energy is its ability to initiate chemical changes. Heat plays a critical role in various physical and chemical processes, including those that occur during combustion reactions. When heat is absorbed by a substance, it can lead to changes in molecular structures, enabling reactions to occur that transform materials.

In the context of fire investigations, understanding how heat interacts with materials can help determine fire origins and the nature of the burning process. For example, the application of heat can break molecular bonds, thus initiating reactions like combustion or decomposition, which are essential in understanding how materials behave under thermal exposure.

The other options do not accurately define heat as a form of energy. Heat is not always visible, as it can be sensed through temperature changes rather than sight. Heat is capable of transferring between materials, which is fundamental to heating processes and the conduction of thermal energy. Additionally, while combustion produces heat, heat can also be generated through other means, such as friction or electrical energy, highlighting that it is not solely produced by combustion.