Easy Ways to Remember Basic Chemistry Solubility Rules

A basic topic in introductory chemistry is chemical reactions in water. This article is a study guide on solubility and precipitate; how to tell if a reaction will dissolve completely or form a solid.

Product formation is one of the driving forces for a reaction, so if two mixtures can form a solid then most likely there will be a precipitate. A precipitate is the same as a solid. Therefore, in order to tell what will happen in a reaction, it's important to know what compounds will dissolve in water and which will form solids.

In introductory chemistry classes, study guides often include so-called solubility rules. These rules state which compounds will form solids, and which will simply dissolve. The rules have to be memorized so they can be applied to real-life problems, but luckily there are several ways of making them easy to remember.

By dividing compounds into soluble and insoluble, there are only about four compounds on each side to remember, so to memorize which chemical compounds will form precipitate (solid) becomes easy.

As a general solubility rule in chemistry, alkalis (Na, K) and ammoniums are soluble. This rule trumps other solubility rules, so whenever the metal in a compound is an alkali or ammonium, it will be soluble. Halogens (F, Cl) are also generally soluble, but with the exceptions of when they are combined with silver, lead, and mercury. Silver, lead and mercury will form a solid, so AgCl for example is not soluble, but will leave a precipitate of silver chloride. Other metals mixed with a halogen will dissolve in water. Finally, the last soluble compound is sulfate salt. Like with the halogens there are some exceptions to the sulfates as well, and they are barium, lead and calcium. An easy rule to remember here is that sulfate mixed with alkaline earth metal is not a solid, and take the solubility rule of lead from the rule of halogens.

The compounds that will leave a solid, the insoluble compounds, are easier to remember. The rules here are hydroxides, with the exceptions of group 1 and 2 metals, sulfides, carbonates, and phosphates. It might be faster to memorize the chemical products that will leave a precipitate, and then argue that if a compound is not on the list, then it must be soluble.

These are only the common solubility rules for basic chemistry, and they are of course simplified to make it possible for new chemistry students to remember. Hence there are exceptions to these, but as a study guide or tutorial for basic chemistry they more than sufficient to cover the material.