 Welcome back. I hope you had a go at this question on hydrogen bonding. The answer in this case was C, methanol and water. This is because both methanol and water have bonds between oxygen and hydrogen, and so both have hydrogen bonding available, which means that there can be hydrogen bonding between the two different kinds of molecules. And now on to the conclusion of our exploration of intermolecular forces. Finally, we have the dispersion forces. This is the force that holds non-polar molecules together. At any one time, the electrons in a molecule or atom may spend more time on one side of the molecule than the other. But at another time, they might be skewed the other way. This means that on average, the electrons will be close to evenly distributed around the molecule or atom. But when the electron cloud is skewed one way, this creates a temporary dipole in the molecule. The positive part of this temporary dipole can then pull the electrons in another molecule towards it, which in turn induces a temporary dipole in the neighbouring molecule. This kind of bonding and the skewing of the electron cloud can be seen in action if we consider the bonding in one of the noble gases, like argon. It's also the force responsible for holding liquid helium together. Now, how do these forces compare in strength? To get an idea of bond strength, we can examine the energy we need to put into the system to break the bond. This is called the dissociation energy. We can see that our intramolecular bonds, that is our covalent bonds, require the greatest amount of energy to break. And so these are our strongest bonds. The next strongest are our hydrogen bonds, followed by the dipole-dipole bonds. The dispersion forces are generally the weakest of the intramolecular bonds. It's also important to realise, though, that each of these types of bonding has a range of strengths, because the strength of each particular bond is different on a case-by-case basis. I think it's pretty amazing how different in strength these forces can be, even though they all have the same physical origin, which is the attraction of dipoles, whether they're induced or permanent, between different molecules. These inter-molecular bonds are vital in determining some of the most important properties of materials. We saw an example of how hydrogen bonding dramatically changes the boiling point of water compared to what we would expect. It's also responsible for holding together the nucleic acids in our DNA. While dispersion forces are important for understanding the different boiling points of different hydrocarbons, that is, our fuels. Now you should be able to identify the three main inter-molecular forces between molecules. By knowing which elements make up a molecule and their electronegativities, and understanding that molecule's polarity, you should be able to understand and explain what kind of inter-molecular force will hold a substance together. Understanding inter-molecular forces also enables us to use those forces to our advantage. In our next video, we're going to learn about a technique called chromatography, which is based on the interaction between a substance and different solvents or surfaces. See you next time.