Well, there's a few trends to keep in mind when you're comparing physical properties of two substances. For molecules, intermolecular forces ultimately determine what the values of those properties will be. I won't go into details as to what all those are (since you're likely familiar with them), but for your question, the main thing you should realize is for two compounds with similar molecular weights, branching increasing melting point but decreases boiling point -- this effect (sometimes dubbed "sandwiching") has a lot to do with the surface-to-volume ratio -- fewer van der waals (london dispersion) forces exist when compounds branch.
That aside, if you're simply comparing two compounds of different molecular weights, then usually the one with the larger molecular weight has a higher melting and boiling point (greater quantity of london dispersion forces).
As for impurities (ie. adding solutes to a solution), that aspect has to do with colligative properties. If you add solutes to a solution, the freezing point decreases (freezing-point depression), but the boiling point increases. If it helps any, think of making spaghetti. Usually, you add salt when boiling the water and this increases the time it takes for the water to boil. Why? Because the boiling point increased in the presence of adding solute (salt in this case). There's obviously more to it than that on an atomic scale, but generally, that's what you expect to happen.
An example of freezing point depression is ethylene glycol (antifreeze) that is added to water for car's cooling systems, particularly where temperatures are near zero degrees celcius (freezing point of water). Hydrogen bonding allows the solute (antifreeze) to dissolve in water, and the net effect of adding a solute acts to decrease the freezing point of water to something below zero. This is a good thing because the coolant can remain a liquid in areas where the temperature is extremely cold.
