let's take a look at comparing the properties of molecules by looking at the forces of traction that they have available so we're going to start off by comparing molecule one let's clear that up a bit molecule number one with molecule number two now if you look at both of these they're both made up of only two elements hydrogen and carbon so these are hydrocarbons now we know something special about hydrocarbons especially since all of these hydrocarbons are made up of only single carbon-carbon bonds no double bonds just singles makes these hydrocarbons alkanes and since they're alkanes based on our notes alkanes are held together by dispersion forces now both of these molecules have dispersion force because they're just regular old hydrocarbons their alkanes and alkanes have dispersion dispersion is the weakest Force available to molecules to attract to one another but we have a special rule associated with dispersion force and that's that dispersion force is going to increase as molecular weight increases so whichever molecule is heavier is going to have the stronger dispersion forces between the two so looking at the two molecules one two three carbons on the left molecule one two three four carbons on the right molecule means that our right molecule is going to have stronger forces of attraction or stronger intermolecular forces which we labeled abbreviated as IMF or attractive forces the left molecule is going to have weaker forces of attraction or weaker IMF so if we were to look at a process like boiling well boiling is going to involve going from the liquid phase to the gaseous phase if we have particles or molecules in the liquid phase they're close together they're attracted to each other and if we want to get them to the gaseous phase we have to break those attractions and separate those molecules or particles out so the stronger the IMF or stronger the force of attraction we can say as IMF gets bigger we're going to see that the boiling point is going to grow as well it's going to increase we see the same Trend if we look at melting point melting point is also going to increase we could also look at volatility now volatility means how readily a substance is going to go to the gas phase so the stronger the force of attraction the less volatile a substance is going to be so volatility decreases as IMF increases so let's compare these which is going to have a higher boiling point or a higher BP pause the video and evaluate between molecules 1 and 2 which is going to have a higher boiling point all right going back to this the molecule 2 has a stronger force of attraction stronger IMF and as IMF increases boiling point increases so molecule 2 would have the higher boiling point if we looked at the same idea and asked which of the two molecules one or two is going to have a lower volatility so pause the video and evaluate which of the two molecules should be less volatile should have a lower volatility all right between molecule 1 and molecule 2 if we want lower volatility that means it'll less readily go to the gas phase so it resists going to the gas phase something that's strongly attracted to itself is going to resist going to the gaseous phase so we'd look for the stronger intermolecular force between the two for a lower volatility so again substance 2 is our winner it's going to have the lower volatility of the two molecules