A compound consisting entirely of carbons and hydrogens is a hydrocarbon. As the maximum number of atoms available to bond with carbon is equal to the number of electrons that are attracted into the outer shell of carbon. The outer shell of carbon comprises 4 electrons, and thus has 4 electrons available for covalent Hydrocarbons are classified based on the presence of single, double or triple bonds. Saturated hydrocarbons possess only single bonds and the number of carbons and hydrogens exist in the following ratio: C_nH_2n+2(1-r), where r is the number of rings. Hydrocarbons possessing only one ring are cycloalkanes. Saturated hydrocarbons can be linear or branched. Hydrocarbons with the same molecular formula but different structural formula are called structural isomers. Branched hydrocarbons can be chiral. Chiral saturated hydrocarbons are often found in the side chains of biomolecules such as chlorophyll and tocopherol.

Unsaturated hydrocarbons have one or more double or triple bonds. Those possessing double bonds are called alkenes. Those with double bonds have the formula C_nH_2n (assuming non-cyclic structures). Those possessing triple bonds are called alkynes. Those with triple bonds have the formula C_nH_2n−2. Aromatic hydrocarbons have conjugated double bonds and are also known as arenes. Hydrocarbons can take the form of gases, liquids, or solids.

Methane (CH₄) and ethane (C₂H₆) are gaseous at ambient temperatures and cannot be readily liquefied by pressure alone. While propane (C₃H₈) is easily liquefied, and often exists in ‘propane bottles’ mostly as a liquid, it can also be a gas. Likewise, butane (C₄H₁₀) is also easily liquefied for lighters. Pentane (C₅H₁₂) is a clear liquid at room temperature, commonly used in chemistry and industry as a powerful nearly odorless solvent of waxes and high molecular weight organic compounds, including greases. As a component of gasoline, hexane (C₆H₁₄) is also a widely used non-polar, non-aromatic solvent. The C⁶ through C¹⁰ alkanes, alkenes and isomeric cycloalkanes are the top components of gasoline, naptha, and jet fuel as well as specialized industrial solvent mixtures. With the progressive addition of carbon units, the simple non-ring structured hydrocarbons have higher viscosities, lubricating indices, boiling points, solidification temperatures, and deeper color.