<P> In general, compounds with ionic bonds have high normal boiling points, if they do not decompose before reaching such high temperatures . Many metals have high boiling points, but not all . Very generally--with other factors being equal--in compounds with covalently bonded molecules, as the size of the molecule (or molecular mass) increases, the normal boiling point increases . When the molecular size becomes that of a macromolecule, polymer, or otherwise very large, the compound often decomposes at high temperature before the boiling point is reached . Another factor that affects the normal boiling point of a compound is the polarity of its molecules . As the polarity of a compound's molecules increases, its normal boiling point increases, other factors being equal . Closely related is the ability of a molecule to form hydrogen bonds (in the liquid state), which makes it harder for molecules to leave the liquid state and thus increases the normal boiling point of the compound . Simple carboxylic acids dimerize by forming hydrogen bonds between molecules . A minor factor affecting boiling points is the shape of a molecule . Making the shape of a molecule more compact tends to lower the normal boiling point slightly compared to an equivalent molecule with more surface area . </P> <Table> Comparison of butane isomer boiling points <Tr> <Th> Common name </Th> <Td> n - butane </Td> <Td> isobutane </Td> </Tr> <Tr> <Th> IUPAC name </Th> <Td> butane </Td> <Td> 2 - methylpropane </Td> </Tr> <Tr> <Th> Molecular form </Th> <Td> </Td> <Td> </Td> </Tr> <Tr> <Th> Boiling point (° C) </Th> <Td> − 0.5 </Td> <Td> − 11.7 </Td> </Tr> </Table> <Tr> <Th> Common name </Th> <Td> n - butane </Td> <Td> isobutane </Td> </Tr> <Tr> <Th> IUPAC name </Th> <Td> butane </Td> <Td> 2 - methylpropane </Td> </Tr>

When a pure liquid is boiling its temperature