 Like all chemicals, organic compounds can be characterized by their properties. A property is a characteristic of a substance that you can observe or measure. We can classify properties according to whether they're physical or chemical. Physical properties are characteristics that don't involve any chemical reactions. That is, the molecules themselves are not altered. So these are things like melting and boiling points, volatility, viscosity, density, solubility, inner solvent, and also effectiveness as a solvent. The melting point of a substance, for instance, it's the temperature at which the substance changes from a solid to a liquid. So it is changing state, but the molecules themselves are not altered in their shape or form or formula. In contrast, chemical properties are characteristics that have to do with how the substance reacts with other substances. These are things such as pH, toxicity, how does the substance react in your body, and characteristic chemical reactions of all kinds. We're going to look at the reactions of different functional groups later on. In this set of videos, we'll just look at physical properties. So what is it that makes one substance have a different boiling point to another? It is the structure of the molecule that defines all its properties. The types of atoms and the way they're joined together determines how the molecule behaves. Why is this? Well, it all comes down to intermolecular forces.