Solubility Rules Chart and Memorization Tips

The solubility rules in chemistry are a set of guidelines for identifying inorganic compounds that are soluble in water near room temperature.

Key Takeaways: Solubility Rules

• Solubility rules predict whether ionic compounds dissolve in water
• Most rules depend on the anion, with key metal exceptions
• “Always soluble” ions include nitrates, acetates, alkali metals, and ammonium
• Many “insoluble” compounds are only slightly soluble, not zero
• Solubility rules help predict precipitates in reactions
• These are guidelines, not absolute laws

What Is Solubility

Solubility is how easily a substance dissolves in a solvent to form a solution. The dissolving substance is the solute. The chemical it dissolves into is called the solvent.

A soluble chemical freely dissolves in a solvent in any ratio. For example, ethanol is soluble in water. In insoluble chemical does not dissolve in the solvent. But, solubility is not an all-or-nothing process. Solubility exists on a spectrum, from highly soluble to slightly soluble to insoluble. Many chemicals are slightly soluble, meaning they don’t completely dissolve, but they partially dissociate into their ions. Many “insoluble” chemicals are still very slightly soluble in a solvent, so a tiny fraction of the substance dissolves.

What Are the Solubility Rules?

The solubility rules are a set of guidelines predicting solubility of inorganic compounds in water at or near room temperature. Soluble compounds form aqueous solutions.

How to Read the Solubility Rules Quickly

• Identify the anion first (CO₃²⁻, SO₄²⁻, Cl⁻, etc.)
• Check if it is generally soluble or insoluble
• Look for exception cations (Ag⁺, Pb²⁺, Ba²⁺, etc.)
• If an exception applies, override the general rule
• If no exception applies, follow the general rule

Solubility Chart

Here is a solubility chart you can download or print. Either right-click and save the image or else download the PDF file.

How to Memorize Solubility Rules

The easiest way of memorizing solubility rules is using mnemonic devices. Four mnemonics that cover most compounds are NAG, SAG, PMS, and Castro Bear. NAG and SAG are always soluble, with PMS and Castro Bear being the exceptions.

NAG

• Nitrates (NO₃^–)
• Acetates (C₂H₃O₂^–)
• Group 1 (the alkali metals: Li⁺, Na⁺, K⁺, etc.)

SAG

• Sulfates (SO₄^2-)
• Ammonium (NH₄⁺)
• Group 17 (the halogens: F^–, Cl^–, Br^–, etc.)

PMS

The exceptions are particular metal compounds.

• P: Pb²⁺, lead
• M: Mercury, Hg²⁺
• S: Silver, Ag⁺

Castro Bear

Saying “castro bear” makes it easier to distinguish between these metals and other with similar names and symbols.

• Calcium (Ca²⁺)
• Strontium (Sr²⁺)
• Barium (Ba²⁺)

Factors That Affect Solubility

Several factors affect solubility:

• Temperature: If the dissolution reaction is endothermic, solubility tends to increase with temperature. If the dissolution is exothermic, solubility tends to decrease as temperature increases. Dissolving most solids and liquids is endothermic, so usually solubility increases with temperature. Solubility of organic compounds almost always increases with temperature (exception is cyclodextrin). Gas behavior is more complex and harder to predict.
• Phase: Solubility depends on phase. For example, the solubility of aragonite differs from that of calcite, even though both are forms of calcium carbonate (CaCO₃).
• Presence of other species: Other species in a solution affect solubility. Factors include ligands, common ions, and ionic strength of solution.
• Pressure: Pressure plays a small role in the solubility of solids and liquids. While typically ignored in most applications, it is important in petroleum chemistry, where calcium sulfate fouling of oil wells occurs. Calcium sulfate solubility decreases with decreasing pressure.
• Particle shape and size: Increasing surface area tends to increase solubility, particularly approaching saturation. So, a fine powder is more soluble than a single chunk. Whether a substance is crystalline or amorphous matters. Typically, increasing order reduces solubility.
• Polarity: “Like dissolves like” means polar solvents dissolve polar compounds, while nonpolar solvents dissolve nonpolar compounds.

Solubility vs Ksp

Solubility rules provide a qualitative way to predict whether a compound dissolves in water. The solubility product constant (Ksp) provides a quantitative measure of how much dissolves.

A compound labeled “insoluble” by solubility rules often still dissolves slightly and establishes an equilibrium.

For example:
AgCl(s) ⇌ Ag⁺(aq) + Cl⁻(aq)

Even though silver chloride is considered insoluble, it has a small but measurable solubility described by its Ksp value.

Key Differences

• Solubility rules: quick predictions (soluble vs insoluble)
• Ksp: exact equilibrium concentration of ions
• Solubility rules: used in introductory chemistry
• Ksp: used for calculations and equilibrium problems

In practice, chemists use solubility rules for fast decisions and Ksp for precise calculations.

How to Use Solubility Rules

The solubility rules have multiple uses, including predicting whether a chemical will dissolve, predicting precipitate formation, and purifying samples. To use the solubility rules, check the anion (the negative part of the ion) and see whether it is soluble or insoluble. Pay attention to exceptions to the rules.

For example, predict whether FeCO₃ is soluble.

From the solubility rules, carbonates (compounds containing CO₃^2-) tend to be insoluble. So, FeCO₃ likely is not soluble. As the product of a reaction, it forms a precipitate.

For example, predict whether a precipitate forms from this reaction:

2AgNO₃ + Na₂S → Ag₂S + 2NaNO₃

A precipitate form if either Ag₂S or NaNO₃ is insoluble. From the solubility rules, sulfides tend to be insoluble, so Ag₂S likely forms a precipitate. NaNO₃ is soluble and does not form a precipitate because most nitrates are soluble. Since Ag₂S forms a precipitate, one does form in this reaction.

Predicting Precipitates Step-by-Step

1. Write the balanced equation
2. Identify the products (double replacement)
3. Check each product using solubility rules
4. Any insoluble product forms a precipitate (s)
5. Write the net ionic equation (optional advanced step)

The solubility rules do not predict behavior under all circumstances. For example, they don’t necessarily work with organic compounds or at extremely high or low temperatures. The rules apply best to pure solutions of a single compound in water, so real behavior may deviate from predicted behavior in mixtures. While they are called “rules”, they are really “guidelines.”

Net Ionic Equations

Chemical reactions in aqueous solution often involve ions that do not participate in the actual chemical change. Writing the net ionic equation helps identify the species that undergo reaction.

A complete ionic equation shows all strong electrolytes as dissociated ions. A net ionic equation removes spectator ions and shows only the species that change.

How to Write a Net Ionic Equation

1. Write the balanced molecular equation
2. Break soluble ionic compounds into ions (aq only)
3. Keep solids (s), liquids (l), and gases (g) intact
4. Cancel spectator ions that appear on both sides
5. Write the remaining species as the net ionic equation

Example

Predict the reaction:

AgNO₃ + NaCl → AgCl + NaNO₃

Step 1: Write the complete ionic equation

Ag⁺(aq) + NO₃⁻(aq) + Na⁺(aq) + Cl⁻(aq) → AgCl(s) + Na⁺(aq) + NO₃⁻(aq)

Step 2: Remove spectator ions (Na⁺ and NO₃⁻)

Net ionic equation:
Ag⁺(aq) + Cl⁻(aq) → AgCl(s)

This equation shows the formation of a solid precipitate from aqueous ions.

Common Student Mistakes

Students often struggle with solubility rules because they rely on memorization without understanding how to apply them.

• Assuming “insoluble” means zero solubility
• Forgetting to check exceptions like Ag⁺, Pb²⁺, and Ba²⁺
• Looking at the cation first instead of the anion
• Assuming all sulfates are soluble without checking exceptions
• Confusing solubility with reaction rate
• Forgetting that only aqueous ions dissociate in ionic equations
• Writing net ionic equations without removing spectator ions
• Mixing up Hg₂²⁺ (mercury(I)) with Hg²⁺ (mercury(II))

FAQs

What are the solubility rules in chemistry?

Solubility rules are guidelines that predict whether an ionic compound dissolves in water based on its ions.

Which compounds are always soluble?

Compounds containing nitrates (NO₃⁻), acetates (C₂H₃O₂⁻), alkali metal ions, and ammonium (NH₄⁺) are always soluble.

What does insoluble mean in chemistry?

Insoluble means a compound dissolves only slightly in water, not that it has zero solubility.

How do you predict if a precipitate forms?

Write the products of the reaction and use solubility rules. If a product is insoluble, it forms a precipitate.

Are solubility rules always correct?

No. They are guidelines that work best for simple ionic compounds in water at room temperature.

Why are there exceptions to solubility rules?

Solubility depends on ionic interactions, lattice energy, and hydration energy, which vary between compounds.

References

• Hefter, G. T.; Tomkins, R. P. T (eds.) (2003). The Experimental Determination of Solubilities. Wiley-Blackwell. ISBN 978-0-471-49708-0.
• IUPAC (1997). “Solubility”. Compendium of Chemical Terminology (the “Gold Book”) (2nd ed.). Blackwell Scientific Publications. doi:10.1351/goldbook.S05740
• Jain, N.; Yalkowsky, S. H. (2001). “Estimation of the aqueous solubility I: application to organic nonelectrolytes”. Journal of Pharmaceutical Sciences. 90 (2): 234–252. doi:10.1002/1520-6017(200102)90:2<234::aid-jps14>3.0.co;2-v
• Petrucci, Ralph H.; et al. (2011). General Chemistry: Principles and Modern Applications (10th ed.). Upper Saddle River, New Jersey: Pearson Education. ISBN: 978-0132064521.
• Ran, Y.; N. Jain; S. H. Yalkowsky (2001). “Prediction of Aqueous Solubility of Organic Compounds by the General Solubility Equation (GSE)”. Journal of Chemical Information and Modeling. 41 (5): 1208–1217. doi:10.1021/ci010287z