 We've now looked at a number of types of organic molecules. Think of the alkanes, alkenes, and alkynes, or the alcohols, or the carboxylic acids. The members of each of these groups have the same basic structure, and their formulae follow a pattern. For instance, the general formula for an alkane is CNH2N2. If you draw the alcohol series out, you can see that you could write a general formula for those, too. Can you figure out how to do it? Each of these series of molecules is known as a homologous series. So the alkanes are a homologous series. But if you were to mix up the alkanes and the cycloalkanes, for instance, they would not be a homologous series, since they have different general formulae and a different basic structure. Because the members of a homologous series contain the same sorts of atoms and bonding, they have similar chemical properties. And because their size gets steadily larger as you go through the series, their physical properties tend to follow trends. Generally, as the size of the molecule increases, melting and boiling points increase, along with viscosity. Remember that viscosity is the thickness of a liquid, or how easily it flows. These properties are affected by intermolecular forces, and as molecules get bigger, their van der Waals forces increase. This results in an increased attraction between molecules, which has an effect on how easy it is to melt or boil them, and also on how easily they flow past each other. Density also increases with the size of the molecule. This is not so much to do with intermolecular forces, but it's because atoms that are chemically bonded are pulled in closer together, taking up less room. Since density is mass over volume, if you can reduce the volume taken up by a certain number of atoms, that will increase their density. So compounds made of large molecules with many atoms bonded together will tend to have a higher density. Going in the other direction, volatility, which is how easily something evaporates, decreases as organic molecules get bigger. This also has to do with intermolecular forces. If the molecules are bigger, then they have greater van der Waals forces, which means they're more attracted to one another. This means they're less likely to evaporate, so their volatility is lower. And this also links with a higher boiling point. Working through a homologous series, larger molecules are less soluble in water. When a molecule dissolves in water, the larger the non-polar component of the molecule, the more it will have to break up the strong hydrogen bonding that water makes with itself. This is unfavorable, so larger non-polar molecules are less soluble in water. We'll look at some of these properties in more detail for various series of organic molecules in following videos.