Homogeneous Mixtures

All mixtures, whether homogeneous or not, consist of two or more substances. Each substance has its own identity and properties, and can exist on its own outside the context of a mixture. We can imagine taking the pure substances and mixing them to form the solution, and similarly we can imagine separating them in some way.

To think about the properties of solutions, it will be useful not just to describe their structure, but also how they are formed from their components. In doing so, we will revisit many important ideas from the last two lessons, especially the effects of intermolecular forces and bonds.

Mixing Materials

As you know, some substances have a molecular structure, while others have a network structure. The former category includes most covalent compounds, and the latter includes ionic compounds, metals, and a couple oddball covalent substances (diamond, silicon dioxide, etc.). For purposes of this discussion, we will deal just with molecular compounds. The concepts we cover can be applied to covalent substances as well, though often a more subtle view is required.

With this caveat, lets talk about what happens when a solution forms.

The image above shows two molecular substances, A and B, mixing to form a solution. Each block represents one molecule. As you can see, forming the solution mixes the molecules in between each other, totally evenly. You may look at that image and go "hey, that's not evenly mixed: I see some clumps of A and B" - such as the molecules of A adjacent to each other in the two corners. These are not clumps, however because the molecules are not bonded to each other. When molecules are mixed in a solution, they do not form a perfectly regimented pattern ( like ABABABABABAB) ... their distribution is instead random, which will inevitably mean some molecules of A and B close to each other. However, the random distribution means it is very unlikely these stretches will be more than a few molecules long. Additionally. in a liquid or gas solution, the molecules are in constant motion, so they would be reshuffled in their positions from instant to instant. Thus, we can say the image above shows perfectly even mixing.

Intermolecular Forces

Let's think about the intermolecular forces that are broken in this process, as well as those that form. For the image above, we will say that an IMF exists between two molecules if those molecules border each other along an edge.

Under this definition, in the pure substances, there are 24 A-A interactions and 12 B-B interactions. In the solution, there are only 10 A-A interactions and 2 B-B interactions, so we have broken 14 A-A and 10 B-B IMFs. We have also formed 27 new A-B interactions. This illustrates a general principle: when a solution forms, some or all of the IMFs within each substance are broken, while new IMFs between the substances are formed.

The number of IMFs broken and formed are roughly equal in number, so it makes sense that they would potentially balance each other out. However, we need to think about a second factor: their strength.

When A and B are mixed, because B is non-polar, the A-B interactions will be weak dispersion interactions. That means that, if A and B form a solution, we would be breaking 14 strong hydrogen bonds and 10 weak dispersion forces, and forming 27 weak dispersion forces. This is what we in the chemistry game call a "bad deal" - each molecule of A is effectively "trading" some strong hydrogen bonds to other A molecules for weak dispersion force interactions with B molecules. For this reason, A and B would be unlikely to form a mixture under these circumstances.

Like Dissolves Like

This is an example of a general principle in solution formation: "like dissolves like." This is shorthand for "substances with weak IMFs mix well with other substances with weak IMFs, and likewise for substances with strong IMFs." Because IMF strength is mostly a function of molecular polarity, it can be stated as "polar things dissolve polar things and non-polar things dissolve non-polar things."

Next, let's extend our observations of iodine to include not only the solubility of iodine in water and carbon tetrachloride, but also in ethanol and acetone.

This illustrates that the "like dissolves like" rule of thumb is a crude approximation to things that actually happen. It is a valid rule, but not totally so. First of all, you cannot arbitrarily classify each and every material as being either nonpolar or polar. There are many degrees of polarity, all sorts of gradations in between the extremes of polar and nonpolar. You have seen that things such as ethanol and acetone are not only soluble in water, but also soluble in carbon tetrachloride. They are polar enough to dissolve in water, but not so polar that they won't dissolve in carbon tetrachloride. They are partly polar or slightly polar and they will dissolve in both.

You should remember the phrase, "Like dissolves like," but also remember that it is an over-simplification of the way that chemicals actually interact with one another.

Terminology

A quick refresher on some terminology that is hopefully familiar from CH 104.

Solvent: the substance present in the greatest amount in solution.

Solute: a substance present in small amount in solution. The solute(s) dissolve in the solvent.