What are solubility rules?

Solubility rules are a set of generalisations that predict whether an ionic compound dissolves in water at room temperature. They group anions and cations by behaviour — for example, all nitrates are soluble, while most carbonates are insoluble — with a short list of well-known exceptions.

What does insoluble mean here?

An insoluble salt does not dissolve appreciably in water; in a mixing reaction it appears as a solid precipitate. In practice nothing is perfectly insoluble, so the term means very low solubility, typically under about 0.01 mol per litre.

How do I predict a precipitate?

In a double-displacement reaction, swap the ion partners and check each new pairing against the rules. If either product is insoluble, that compound precipitates. For example, mixing silver nitrate with sodium chloride produces insoluble silver chloride, which drops out as a white solid.

Why are all nitrates and Group 1 salts soluble?

Nitrate and Group 1 cations like sodium and potassium have low charge density and weak attraction to oppositely charged ions, so water molecules easily surround and separate them. This makes their salts reliably soluble, which is why these are the first rules to learn.

What is the Solubility Rules Reference?

Reference chart of the standard solubility rules for ionic compound classes in aqueous solution, plus an ion-pair predictor that tells you whether a salt is soluble, slightly soluble, or forms a precipitate. Runs in your browser. It runs free in your browser on Gera Tools, with nothing uploaded.

Solubility Rules Reference

Name: Solubility Rules Reference
Creator: Gera Tools
License: https://creativecommons.org/licenses/by/4.0/

Solubility rules let you predict, without experiment, whether an ionic compound dissolves in water or drops out as a precipitate. This reference lists the standard rules by ion class and includes a predictor that combines a cation and anion and tells you the expected outcome.

How it works

The rules sort common ions into “usually soluble” and “usually insoluble” groups, each with a short exception list. Apply them in order of reliability: the strongest rules — all Group 1, ammonium, and nitrate salts are soluble — override weaker ones. To judge any salt, find the rule for its anion, then check whether its cation appears in that rule’s exceptions. To predict a reaction, swap the ion partners and test each product; an insoluble product is the precipitate.

The hierarchy of rules

Always soluble — no exceptions to worry about:

All nitrates (NO₃⁻)
All acetates (CH₃COO⁻)
All chlorates (ClO₃⁻) and perchlorates (ClO₄⁻)
All Group 1 salts (Li⁺, Na⁺, K⁺, Rb⁺, Cs⁺)
All ammonium (NH₄⁺) salts

Generally soluble, with a short exception list:

Chlorides, bromides, iodides — insoluble with Ag⁺, Pb²⁺, Hg₂²⁺ (and slightly soluble with Tl⁺)
Sulfates — insoluble with Ba²⁺, Pb²⁺, Ca²⁺, Sr²⁺, and Ag⁺ (slightly)

Generally insoluble, with a short soluble exception:

Carbonates (CO₃²⁻) — soluble with Group 1 and ammonium only
Phosphates (PO₄³⁻) — soluble with Group 1 and ammonium only
Hydroxides (OH⁻) — soluble with Group 1, plus Ba²⁺, Sr²⁺, and Ca²⁺ (slightly)
Sulfides (S²⁻) — soluble with Group 1, 2, and ammonium

Predicting double-displacement reactions

When two soluble salts are mixed in aqueous solution, the ions swap partners (double displacement). To predict whether a precipitate forms:

Write out both reactant salts and their ion components.
Swap cation partners to form the two possible products.
Apply the solubility rules to each product.
Any insoluble product precipitates; soluble products remain in solution.

The net ionic equation shows only the ions actually involved in the precipitate formation; spectator ions (those that remain dissolved) are omitted.

Three worked examples

Mixing lead(II) nitrate with potassium iodide: New pairings: lead(II) iodide and potassium nitrate. Potassium nitrate is always soluble. Iodides are generally soluble, but lead is an exception — lead(II) iodide is insoluble and precipitates as a bright yellow solid. Net ionic equation: Pb²⁺(aq) + 2I⁻(aq) → PbI₂(s).

Mixing barium chloride with sodium sulfate: New pairings: barium sulfate and sodium chloride. Sodium chloride is always soluble. Sulfates are generally soluble, but barium is an exception — barium sulfate precipitates as a fine white solid. This reaction is so reliable it is used in analytical chemistry to confirm sulfate presence.

Mixing sodium carbonate with calcium chloride: New pairings: calcium carbonate and sodium chloride. Sodium chloride is always soluble. Carbonates are generally insoluble, and calcium is not a Group 1 exception — calcium carbonate precipitates as a white solid. This is essentially the chemistry of limescale formation.

Notes

These rules describe aqueous behaviour near 25°C. Solubility generally increases with temperature, and the rules do not transfer to non-aqueous solvents. For precise quantities or marginal cases, consult a quantitative solubility table giving grams per 100 mL at a stated temperature.