What defines a hydrocarbon?

A hydrocarbon is defined specifically as a molecule that contains only hydrogen and carbon atoms. This definition is crucial because hydrocarbons serve as the fundamental building blocks for many organic compounds and are a major class of compounds found in fossil fuels. They can exist in various structural forms, such as straight chains, branched chains, or rings, and can be either saturated (containing single bonds) or unsaturated (containing double or triple bonds).

The other options do not accurately represent what hydrocarbons are. For example, a molecule containing only hydrogen and oxygen is characteristic of substances like water or hydrogen peroxide, not hydrocarbons. Similarly, a compound that contains metals and non-metals cannot be classified as a hydrocarbon since hydrocarbons exclusively involve only carbon and hydrogen elements. Lastly, a reaction between hydrogen and carbon dioxide does not define a hydrocarbon; instead, it describes a type of chemical interaction that does not result in hydrocarbons, as carbon dioxide contains a carbon atom bonded to oxygen rather than hydrogen.

Therefore, the correct identification of hydrocarbons as compounds made solely of hydrogen and carbon is fundamental in understanding organic chemistry and the composition of many substances in nature.