# General rules for deciding volatility

Given an organic compound, is there any way to decide if it is volatile (or compare volatility--everything is volatile in the end)?

Volatility is due to the tendency to evaporate. The conclusion I can draw from this is that a more volatile compound will be lighter and/or will have less intermolecular forces. But this doesn't seem to work--I recall that methanol is less volatile than ethanol ([citation needed]).

So are there any general guidelines for this or is it another experimental property?

This answer to this question applies to your query. Many physical properties depend on the intermolecular forces experienced in the liquid or solid. These intermolecular forces are hydrogen bonding (strongest), dipole-dipole attractions (in polar molecules), and London dispersion forces (weakest of the forces when comparing molecules of the same size, but dependent on the number of electrons in the substance). Stronger intermolecular forces would make the substance less volatile.

As noted in the first response, methanol is more volatile than ethanol. Methanol and ethanol would both have both hydrogen bonding (a relatively strong type of dipole-dipole attraction) and London dispersion forces. Because ethanol has more electrons (because it is a bigger molecule, but not necessarily because it is a heavier molecule) it would have more London dispersion forces in comparison to methanol, so, with stronger intermolecular forces, it would be less volatile. So, this example follows the general guidelines--figure out the intermolecular forces present in the molecules you want to compare and the molecule with the weakest cumulative intermolecular forces will be the most volatile.

Intramolecular bonding (covalent bonds, when you're talking about molecules) does not affect physical properties.

In chemistry and physics, volatility is the tendency of a substance to vaporize. Volatility is directly related to a substance's vapor pressure. At a given temperature, a substance with higher vapor pressure vaporizes more readily than a substance with a lower vapor pressure.(Taken from Wikipedia)

But this doesn't seem to work--I recall that methanol is less volatile than ethanol
I think you have got this wrong. According to the wikipedia page of Methanol and Ethanol the vapour pressure of Methanol > Ethanol thus Methanol is more volatile than Ethanol

Volatility can also be predicted on the type of intermolecular and intra molecular bonding and of course on the BP of the compound.

Volatility depends on four factors for organic compounds:

1) Branched chained hydrocarbons are more volatile than straight chained hydrocarbons

2) The more branched an hydrocarbon is the more volatile it is

3) Depending on the bonds i.e hydrogen bond, dipole-dipole intermolecular bond and van der Waals' forces. Volatility increases from hydrogen bond to van der Waals' forces. Therefore compounds like ethanol would be less volatile in comparison to ethane.

4) Volatility decreases with increase in relative molecular mass. That is, it decreases down an homologous series. E.g methane would be more volatile than ethane.

Volatilty is the tendency of a substance to vapourize. It is directly related to vapoure pressure of a substance(pressure exerted by the vapourse in equilibrium with the liquid at a given temperature). more voletile substance have low BP because of weake intermolecular forces.