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

The term "fixed carbon" refers to the carbon present in materials like coal and other solid fossils that is not in gaseous or volatile form. It represents the carbon that is incorporated into the structure of the organic matter and is not combined with hydrogen. Fixed carbon is a significant concept in areas such as combustion and energy generation, where it is used to understand the carbon content available for burning.

In contrast, organic carbon generally refers to carbon found in compounds that contain carbon and hydrogen, such as carbohydrates, proteins, and lipids. Elemental carbon would describe the pure form of carbon, which can exist in allotropes like graphite or diamond, but it doesn’t specify its combination with other elements. Carbon monoxide is a toxic gas formed when carbon combusts with insufficient oxygen, providing no insight regarding its relationship with hydrogen or other compounds.

Therefore, the definition and context of "fixed carbon" ensure that it is the most appropriate answer to the question, highlighting the specific nature of carbon that is not combined with hydrogen.