Precipitation reactions: separating the soluble from the insoluble

What are precipitation reactions? The precipitation reaction is the reaction that occurs between two substances in an aqueous medium, where a product is formed that does not dissolve in water. This reaction can be easily identified by the formation of a solid that precipitates, therefore this solid is also called a precipitate.

Typically, precipitation reactions occur with ionic compounds, that is, compounds that break down into their constituent ions in water. For example, potassium chromate separates into CrO2-4 chromate ion and potassium ion, while barium nitrate separates into nitrate ion and barium ion.

Precipitate Formation Rules

To recognize a precipitation reaction, we must predict whether the product that forms is soluble or insoluble in water. For this, there are the following rules:

1. The solid compound must have zero charge. Therefore, two negatively charged ions or anions cannot combine, nor can two positively charged ions or cations.
2. For the compound to have no charge, a cation and an anion must combine to balance their charges with each other.
3. Most nitrate salts are soluble.
4. Most of the salts containing alkali metal ions and the ammonium ion are soluble.
5. Most salts with chlorine, bromine, or iodine are soluble, except when silver, lead, and mercury ions are present.
6. Most sulfate salts are soluble, except for barium sulfate, lead sulfate, mercury sulfate, and calcium sulfate.
7. The soluble hydroxides are sodium hydroxide and potassium hydroxide. The moderately soluble compounds are barium hydroxide, strontium hydroxide and calcium hydroxide.
8. Most sulfides, carbonates, chromates, and phosphates are slightly soluble, with the exception of salts with rule 4 ions.

Let’s take a look at the next experiment and if you can, let’s get down to business!