Solubility

Ever put a rock into a glass of water? It doesn’t dissolve instead it just sits there at the bottom of the glass. Why is that? After all, if used a cube of sugar instead it would have slowly disappeared into the water. The answer solubility, or the degree to which something dissolves in a particular solvent.

As we can see from this basic example different substances have different solubilities. While the rock is insoluble, meaning it doesn’t dissolve, the sugar is soluble because it does. However, if we kept adding more sugar to the glass it would eventually stop dissolving and instead, sit in its solid form, called a precipitate, just as the rock did. It isn’t that the sugar suddenly became insoluble only that the glass of water ran out of room for more dissolved sugar.

When this occurs we say that the solution is saturated meaning it can’t dissolve any additional solute molecules. Essentially, it has reached its maximum dissolving capacity. Prior to this point, the solution was unsaturated meaning added sugar would continue to dissolve until it reached saturation. We will look into these concepts and the details involved in more depth later when we discuss solution equilibria.

All of this solubility stuff occurs for a whole host of reasons including the intermolecular forces at play, the chemical reactions involved, and the temperature of the solution among others. Ultimately whether or not something dissolves comes down to the same thermodynamic factors, energy, enthalpy, and entropy, that govern all reactions.

Solubility Rules Sorta

Thankfully, we don’t have to understand all of the intricacies involved but we do need to understand some of the basics. First, let’s set the record straight on our solvent front. When discussing solubility from here on out our solvent will be water unless stated otherwise. It is the most common solvent on the MCAT and forms aqueous solutions with a wide array of different molecules. Here aqueous refers to a solution of water and whatever solutes molecule is dissolved.

Organic Compounds

First, let’s take a look at the solubility of the organic compounds since they have fewer rules to keep straight.

• Rule 1: Like dissolves like or polar dissolves polar thus polar functional groups increase the solubility of an organic compound with H-bonding functional groups being the most soluble

• Rule 2: Having a charge increases the solubility of an organic compound (e.g carboxylate ions are more soluble than carboxylic acids)

• Rule 3: Increasing the size of the hydrocarbon chain decreases the solubility of the organic compound

General Chemistry Compounds

From there we move into the more standard solubility rules. While I will list most of them here we won’t focus on memorizing all of them since the MCAT rarely if ever expects you to rotely recall them.

1. All ammonium (NH₄⁺) and alkali metal salts are water-soluble
2. All nitrate (NO₃^–) and acetate (CH3COO^–) salts are water-soluble
3. Chlorides (Cl^–), Bromides (Br^–), and Iodides (I^–) are water-soluble excluding Ag⁺, Pb²⁺, and Hg₂²⁺
4. Sulfates are water-soluble excluding Ca²⁺, Sr²⁺, Ba²⁺, and Pb²⁺
5. Metal oxides are insoluble excluding CaO, SrO, and BaO
6. Hydroxides are insoluble excluding Ca²⁺, Sr²⁺, and Ba²⁺
7. Carbonates(CO₃^2-), phosphates(PO₄^3-), sulfides(S^2-), and sulfates(SO₃^2-) are insoluble

First off it might seem that some of these rules contradict each other. For example, is NaNO₃ soluble or not? Rule 1 would say that it is while rule 6 would say it isn’t. Sodium nitrate is in fact quite soluble and the rule with the smallest number always takes precedence over the other. In this case, we would disregard rule 6 and instead follow rule 1.

Of all of the rules, the only two you should have memorized cold are rule 1 and rule 2. From there it is worth knowing that Pb²⁺ and Ag⁺ make the halides insoluble and that Ca²⁺, Sr²⁺, and Ba²⁺ plus oxygen are water-soluble unless it is mixed with a sulfate. Other than that the remaining rules are trivia, which you are welcome to memorize if you wish but it really isn’t necessary.