How do you find the relative melting point of a group of substance?

Answer 1

You have to look at the actual structures of each substance in the group. For each individual substance, think of it being multiplied many times, until you have a little "sea" of all one type of molecule. There are three forces that are possibly at work between these identical molecules: Van Der Waals, dipole-dipole, and hydrogen bonding. (You can find definitions for these terms with an online search if you're not sure what they are.) If a substance can participate in hydrogen bonding between two of its molecules (draw a picture of the structure, and then a copy of that structure oriented in such a way that the H of an H-F, H-O, or H-N bond can line up with a non-bonding electron pair on the second molecule) then it will have strong intermolecular forces, needing a lot of energy to break, and by extension a higher melting point. Dipole-Diploe forces, in the absence of hydrogen bonding, are next in line. The lowest melting points of substances occur when there are only Van Der Waals forces at work between the molecules of a substance.

Answer 2

To find the relative melting point of a group of substances, you would compare their individual melting points and determine which substance has the highest and lowest melting points. The substance with the highest melting point is considered to have the highest relative melting point within the group, whereas the substance with the lowest melting point has the lowest relative melting point.