Consider the following substances and predict which member of the pair the should have the higher boiling point?
2. Octane (l) (gasoline - `C_8H_18` - nonpolar) or `CH_4` ,methane - natural gas
3. `I_2` or `F_2`
For each of these substances, you simply need to consider the intermolecular forces involved. Just remember the following order of bond strength:
Dipole-Dipole > Dipole-Induced Dipole > Induced Dipole-Induced Dipole
In the first situation, we have a very polar molecule, water, which makes very strong dipole interactions with each of its atoms. These strong "hydrogen bonds" that water can generate are very strong. Methanol has the capacity to form hydrogen bonds, but the carbon atom actually interrupts its capacity to form them. Thus, methanol's intermolecular interactions are weaker than water. In this case, then, water has the higher boiling point.
In the second situation, we are considering the size of the molecule. Generally, the bigger the molecule, the higher the boiling point. This has to do with the fact that there is "more molecule" for each other molecule to interact with. So, we can immediately say that octane will have a higher boiling poing than methane.
Similarly, boiling point increases as we go down the periodic table. Thus, iodine should have a higher boiling point than flourine.