Boiling Point, Melting Point, Density
12 February 2008
The primary objective of this experiment was to study the properties of alkanes and
alcohols in terms of their boiling points and melting points. Using this information, the
composition of an unknown compound can be determined by calculating its melting point,
boiling point, and density.
Melting point is the temperature at which a crystal starts to melt until the temperature at
which the crystal disappears entirely. It is one chemical property that can be used to determine
unknowns. If the melting range is less than 2°C, then the substance being tested is considered to
be pure. The more impure a substance is, the lower the boiling point is and the wider the melting
range is. There are several different factors which can determine the melting point of a
compound: symmetry, molecular weight, and intermolecular forces of attraction.
The more symmetrical a compound is, the higher its boiling point is. For example, 2,2,3-
trimethylpentane has a higher melting point than nonane. The branching of the methyl groups on
the pentane make it more symmetrical than a straight chain of carbons and hydrogens.
The greater the molecular weight of a compound, the higher the melting point is. For
has a higher melting point than C
. The molecular weight of C
is 282 g/
mol, whereas the molecular weight of C
is only 268 g/mole/
Intermolecular forces of attraction can be broken up into three different categories:
hydrogen bonding, dipole-dipole attractions, and London dispersion forces. Of the three forces,
hydrogen bonding is the strongest due to the differences in electronegativity. Hydrogen bonding
occurs when hydrogen atoms bond with fluorine, nitrogen, or oxygen atoms, such as H
Dipole-dipole bonds are the next strongest and it occurs between polar molecules, such as HCl.
London dispersion forces are the weakest of the three forces of attraction. They occur between
nonpolar molecules, such as CH
. As the strength of the forces of attraction increases, so does
the melting point. Therefore, H
O has the highest melting point, followed by HCl, with CH
the lowest melting point.
Boiling point is the temperature at which the vapor pressure of the liquid equals the
environmental pressure surrounding the liquid. This chemical property is also affected by several
factors: molecular weight, intermolecular forces of attraction, and elongated symmetry.
The greater the molecular weight of a compound, the higher its boiling point will be, in