Identification of Molecules by Physical Properities

Physical properties of chemicals: Melting point, and boiling point


Background
If you were asked to describe a friend, most likely you would start by identifying particular physical characteristics. You might begin by giving your friend’ s height, weight, hair color, eye color, or facial features. These characteristics would allow you to single out the individual from a group.

Chemicals also possess distinguishing physical properties which enable their identification. In many circumstances, a thorough determination of the physical properties of a given chemical can be used for its identification. If faced with an unknown sample, a chemist may compare the physical properties of the unknown to properties of known substances that are tabulated in the chemical literature; if a match can be made, an identification can be assumed (unless chemical evidence suggests otherwise).

The physical properties most commonly listed in handbooks of chemical data are color, crystal form (if a solid), refractive index (if a liquid), density , solubility in various solvents , melting point,sublimation characteristics, and boiling point. When a new compound is isolated or synthesized, these properties almost always accompany the report in the literature.

The transition of a substance from a solid to a liquid to a gas, and the reversal, represent physical changes. This means that there is a change in the form or the state of the substance without any alteration in the chemical composition. Water undergoes state changes from ice to liquid water to steam; however, the composition of molecules in all three states remains H2O.