The simplest example of a molecular solution is

Options

A. one gas dissolved in another gas
B. one liquid dissolved in another liquid
C. one solid dissolved in another solid
D. all of these

Answer

1

Explanation

Air, essentially a solution of `O_(2)` and `N_(2)`, is an example. The intermolecular forces in gases are weak. The only solubility factor of importance is the netural tendency for molecules to mix (the inclination toward disorder). Therefore gases are always miscible.
Substances may be miscible even when the intermolecular force are not negligible. Consider the solution of the two similar liquid hydrocarbons heptane, `C_(7)H_(16)`, and octane, `C_(8)^(18)`, which are components of gasoline. The intermolecular attractions are due to London forces, and those between heptane and octane molecules are nearly equal to those between octane and octane molecules and heptane and heptane molecules. The different intermolecular attractions are about the same strength, so there are no favoured attractions. Octane and heptanes molecules tend to move freely through one another. Therefore, the tendency of molecules to mix results in miscibility of the substances.
As a counter example, consider the mixing of octane with water. There are strong hydrogen bonding forces between water molecules. For octane to mix with water, hydrogen bonds must be broken and replaced by much weaker London forces between water and octane. In this case, the maximum forces of attraction among molecules (and therefore the lower energy) result if the octane and water remain unmixed. Therefore octane and water are nearly immiscible.
Let's consider the immiscibility of liquid in general: Suppose an A molecule moves from liquid A into B. If the intermolecular attraction between two A molecules is much stronger than the intermolecular attraction between an A molecule and a B molecule, the net force of attraction tends to pull the A molecule back into liquid A. Thus liquid A will be immiscible with liquid B.
The statement "like dissolves like" concisely expresses these observations. That is, substances with similar intermolecular attractions are usually soluble in one another. The two similar hydrocarcons heptane and octane are completely miscible, whereas octane and water (with dissimlar intermolecular attractions) are immiscible.