Virtually all substances of interest in fuel chemistry consist of covalently bonded molecules. Many are hydrocarbons in the literal sense of the word – compounds containing only hydrogen and carbon atoms. Others contain one or more heteroatoms, i.e. atoms of oxygen, nitrogen, or sulfur. Physical properties of fuels have numerous important roles in fuel technology and utilization, e.g. boiling point, because distillation is commonly used for separations; density, because the amount of fuel that can be carried on vehicles or aircraft is limited by volume and not by mass; and viscosity, because we need fluids to flow, or to be pumped, from place to place. An understanding of how chemical composition and molecular structure influence physical properties shows that the properties of substances do not come about by some haphazard chance but rather because of fundamental links between composition, structure, and properties. Further, such links provide useful guidelines or rules of thumb for estimating expected properties from composition, or vice versa.
The most noticeable property of most substances is their physical state: solid, liquid, or gaseous. The first point of inquiry becomes that of why molecules form solids or liquids at all. Why isn't everything a gas? To exist in a condensed phase, i.e. as a liquid or solid, there must be attractive forces among molecules strong enough to hold them in proximity.