Formation of Mixtures

Intermolecular interactions also determine whether two chemicals are miscible, that is, whether they can be mixed together to form a homogeneous solution. But here, the key is whether the two materials use IMFs of similar strengths.

If two chemicals have dissimilar IMF types, the energy required to break the forces between molecules that use the stronger IMF type will be more than the energy released when new IMFs form between the two different kinds of molecules. The net loss of energy is what prevents the two from mixing.

When two chemicals mix, the forces holding the molecules of each chemical together must break, and new bonds form between the two different kinds of molecules. For this to happen, the two chemicals must have compatible IMF types.

In the lab this week, you will observe a number of substances to see whether or not they form mixtures. We'll look at some of these examples now, though of course this is no substitute for visiting lab and looking yourself.

In these first three examples, we can see the principle of "like dissolves like" on display. Carbon tetrachloride is non-polar and does not mix well with highly polar water. When we try to dissolve sugar in both substances, it only dissolves in water (since sugar, like water, has hydrogen bonding). However, non-polar iodine, with its dispersion forces, happily dissolves in carbon tetrachloride, while remaining largely undissolved in water.

Ion-Dipole Forces

Normally, network materials do not mix with other chemicals because the network bonds are just too strong. The exception to this is that some ionic materials will dissolve in some polar liquids – most commonly water. Ordinary salt (NaCl), for example, will dissolve easily in water.

When an ionic compound dissolves in a very polar liquid like water, the ionic bonds are broken and the ions separate. This requires a lot of energy (the bonds are very strong), and with most liquids this bond strength is too much to overcome. For this reason, salt will not dissolve in liquids like acetone or carbon tetrachloride. But in water (and some other very polar liquids), a new, very strong IMF called an ion-dipole force is formed, which compensates for the energy required to break the ionic bonds.

Ion-dipole forces are not as strong on an individual basis as ionic bonds. But they are quite strong, stronger than hydrogen bonds. In addition, each ion dissolved in water can form a large number of ion-dipole forces. Many water molecules cluster around each ion in solution. This larger number of ion-dipole bonds helps make up for the fact that they are weaker than the ionic bonds they replace.