Where you have isomers, the more branched the chain, the lower the boiling point tends to be. Van der Waals dispersion forces are smaller for shorter molecules and only operate over very short distances between one molecule and its neighbors.

Why do isomers have different melting points?

The factor that affects melting point is, for the lack of a better term- packability. Molecules that are symmetrical can fit into each other more snugly and are difficult to melt. Therefore trans isomers, being symmetrical, always have a higher melting melting point.

Why do alcohol isomers have different boiling points?

Hydrogen bonding

In alkanes, the only intermolecular forces are van der Waals dispersion forces. Hydrogen bonds are much stronger than these; therefore, more energy is required to separate alcohol molecules than to separate alkane molecules. This is the main reason for higher boiling points in alcohols.