Solubility Rules | Solubility of Common Ionic Compounds

Solubility Chemistry

It’s important to know how chemicals will interact with one another in aqueous solutions. Some compounds or solutes will dissolve, others will yield a precipitate or solid, and a few react with water.

You’ve probably run into solubility questions in your everyday life. Hot chocolate or flavored dry beverage mixes that don’t evenly dissolve in water would be examples of unwanted precipitates. Limescale or soap scum are precipitates left behind when water with higher mineral content evaporates and introduces previously dissolved metal cations to carbonates or soap anions.

Solubility is applicable to many laboratory processes and is also important in medicine. Some ions can be toxic when they separate in a solution but are helpful as part of a compound.

A saturated solution is one in which the maximum amount of solute has been dissolved. The opposite is a dilute solution; this solution can accept more solute.

Pressure and temperature affect solubility. This page discusses the solubility of compounds in water at room temperature and standard pressure. A compound that is soluble in water forms an aqueous solution.

Solubility Rules

Soluble

Ions Except
Alkali metals (Group I)
Na+, K+, etc.
 
Ammonium ions
NH4+
 
Nitrates, acetates, chlorates, and perchlorate
NO3-, C2H3O2-, ClO3-, ClO4-
 
Binary compounds of halogens (chloride, bromide, iodide, etc.) with metals
Cl-, Br-, I-, etc.

Fluoride
Silver, lead, and mercury
F-, Ag+, Pb2+*, and Hg2+

*Lead halides are soluble in hot water.


All sulfates
SO42-
Barium, strontium, calcium, lead, silver, and mercury
Ba2+, Sr2+, Ca2+, Pb2+, Ag+, and Hg2+

Slightly Soluble

Ions Except
Sulfates of lead, silver, and mercury
SO42- with Pb2+, Ag+, and Hg2+
*Lead sulfate is poorly soluble.
Hydroxides of alkaline earth metals (Group II)
OH- with Ca2+, Sr2+, etc.
Barium
Ba2+

Insoluble

Ions Except
Sulfides
S2-
Calcium, barium, strontium, magnesium, sodium, potassium, and ammonium
Ca2+, Ba2+, Sr2+,  Mg2+, Na+, K+, and NH4+
Hydroxides
OH-
Alkali metals (Group I), transition metals, aluminum, and ammonium
Na+, K+, etc.
Al3+, NH4+
Carbonates, oxalates, chromates, and phosphates
CO32-, C2O42-, CrO42-, and PO43-

Alkali metals (Group I) and ammonium
Na+, K+, etc.
NH4+

*Lithium phosphate is poorly soluble


How to Use Solubility Rules

  1. Identify the compound whose solubility you want to check. It can be helpful to write out the empirical formula so you can identify the ions that make up the compound.

  2. Look up each ion in the solubility rules. Check the left-hand column for the general rule, and look in the right-hand column to make sure you noted any exceptions.

Our solubility rules are not exhaustive. You may need to reference a periodic table if you’re looking up less common compounds.

Solubility Definitions

When discussing solubility, it’s useful to follow agreed standard definitions. The resources above present some general rules and loose definitions.

The United States Pharmacopeia (USP), a nonprofit organization committed to establishing standards for medicines, food ingredients, dietary supplement products, and ingredients, has established the definitions below. These definitions are used by other major pharmacopeia organizations throughout the world and are often paired with exact measurements for more precise application needs.

Descriptive terms Approximate volume of solvent in mL/g of substance

Very soluble

less than 1

Easily soluble

from 1 to 10

Soluble

from 10 to 30

Sparingly soluble

from 30 to 100

Slightly soluble

from 100 to 1,000

Very slightly soluble

from 1,000 to 10,000

Practically insoluble

more than 10,000