What is carbon that is not combined with hydrogen referred to as?

The term "fixed carbon" refers to carbon that is not in a gaseous or free state but is instead part of a solid material, primarily in the context of coal and other carbonaceous materials. It represents the carbon content that is bonded in a stable form, distinct from volatile carbon forms that can be vaporized.

In the study of refrigeration and related fields, understanding the different forms of carbon is important, especially when considering emissions and combustion processes. Fixed carbon is associated with the structural integrity and energy content of materials, such as coal, and plays a significant role in the thermal properties of these substances.

Other terms like "mono-carbon," "separated carbon," and "free carbon" do not accurately represent the concept of carbon that is stable and not combined with hydrogen. Free carbon might suggest elemental carbon in a pure state, while separated carbon is not a recognized term in this context. Hence, fixed carbon is the appropriate term to denote the stable, uncombined form of carbon relevant to this question.