Melting Points

The melting point of a substance is the temperature at which that substance starts changing from the solid phase to the liquid phase. At this temperature, the liquid and solid phases are in equilibrium. With additional heat, the substance will melt completely. But what determines a substance’s melting point? Let’s think about solids and liquids. A solid’s molecules hold each other in a rigid, ordered structure called a lattice, while a liquid’s molecules have weaker interactions and move around.

Heating a solid transfers energy to the molecules. With enough energy, the molecules overcome the forces keeping them in the lattice and start moving around. In other words, if we heat a solid enough, it melts into a liquid. So, the melting point depends on the energy it takes to overcome the forces between the molecules, or the intermolecular forces, holding them in the lattice. The stronger the intermolecular forces are, the more energy is required, so the higher the melting point is.

Many intermolecular forces depend on how strongly atoms in the molecule attract electrons — or their electronegativity. Nitrogen, oxygen, fluorine, and chlorine are highly electronegative, while carbon, hydrogen, and sulfur are only moderately electronegative. Bonds between atoms with significantly different electronegativities are polar. For instance, a typical carbon-oxygen bond is polar, but a typical carbon-hydrogen bond is not.

A molecule’s electrons spend more time around its most electronegative atoms, giving it a slight negative charge on that side and a slight positive charge on the other side. This is called a dipole. If the dipole isn’t canceled out by an equal and opposite dipole in the same molecule, the molecule has a permanent dipole and is polar.

Now, let’s discuss three important intermolecular forces: hydrogen bonding, dipole-dipole interactions, and London dispersion forces. Hydrogen bonding occurs between an electron-withdrawing atom with a lone pair of electrons and a hydrogen bound to a more electronegative atom. Hydrogen bonds are among the strongest intermolecular forces.

Dipole-dipole interactions occur between polar molecules. In an attractive dipole-dipole interaction, the negative side of one dipole aligns with the positive side of another dipole. Dipole-dipole interactions are generally weaker than hydrogen bonds.

London dispersion forces come from brief, random shifts in a molecule’s electron distribution, which cause corresponding shifts in nearby molecules. These random shifts happen in every molecule, so this is one of the few interactions available to nonpolar molecules. London dispersion forces are among the weakest intermolecular forces.

Earlier, we predicted that stronger intermolecular forces corresponded to higher melting points. We can see this in action with hexadecane, 2-hexadecanone, and hexadecanoic acid. As the strength of the intermolecular interactions available to each molecule increases, so does the melting point.

Intermolecular forces aren’t the only factor that determines the melting point of a substance. Its purity significantly affects its melting and freezing points in an effect called ‘freezing-point depression’. This effect means that a solution has a lower freezing point than the pure solvent does. That’s why streets are sprinkled with salt when it gets very cold. If any water collects on the street, the salt quickly dissolves to make a solution with a much lower freezing point than pure water.

In a solid, impurities are incorporated into the lattice structure. These areas often have weaker intermolecular interactions, making parts of the structure easier to disrupt. So, compared to a pure solid, melting starts at a lower temperature and occurs over a wider temperature range.

In this lab, you will measure the melting points of two known organic compounds and then analyze a mixture to explore how impurities affect the melting point range.