Solubility Rules

These rules tell you whether an ionic compound dissolves in water. You need them to predict double displacement reactions in Predicting Products and to write the equations in Precipitation and Net Ionic Equations.

They are measured results, not something you could reason out from the periodic table. Look them up without apology; so does everyone else.

What “soluble” means here

A compound is called soluble if more than about 0.1 mol/L of it dissolves at room temperature, and insoluble below that. Some tables add a middle band, slightly soluble, for compounds close to the line.

Two consequences follow from the word “about”:

• Nothing is truly insoluble. Silver chloride is the standard example of an insoluble salt and a genuinely tiny amount of it still dissolves. That is one honest reason a precipitation reaction never recovers the full theoretical yield — see Limiting Reagent and Yield.
• A “slightly soluble” compound may or may not visibly precipitate, depending on how concentrated your solutions were. A faint cloudiness is a real result.

The rules

Read the table as: this ion makes soluble compounds, except with these partners.

Ion	Compounds are	Exceptions
Group 1 cations — $\mathrm{Li}{}^{+}$, $\mathrm{Na}{}^{+}$, $\mathrm{K}{}^{+}$, $\mathrm{Rb}{}^{+}$, $\mathrm{Cs}{}^{+}$	soluble	none worth listing
Ammonium, $\mathrm{NH}{}_4{}^{+}$	soluble	none
Nitrate, $\mathrm{NO}{}_3{}^{-}$	soluble	none
Chlorate, $\mathrm{ClO}{}_3{}^{-}$ and perchlorate, $\mathrm{ClO}{}_4{}^{-}$	soluble	none worth listing
Acetate, $\mathrm{CH}{}_3\mathrm{COO}{}^{-}$	soluble	silver acetate is only slightly soluble
Chloride, bromide, iodide	soluble	$\mathrm{Ag}{}^{+}$, $\mathrm{Pb}{}^{2+}$, and $\mathrm{Hg}{}_2{}^{2+}$ are insoluble
Sulfate, $\mathrm{SO}{}_4{}^{2-}$	soluble	$\mathrm{Ba}{}^{2+}$, $\mathrm{Sr}{}^{2+}$, $\mathrm{Pb}{}^{2+}$ insoluble; $\mathrm{Ca}{}^{2+}$ and $\mathrm{Ag}{}^{+}$ slightly soluble
Sulfide, $\mathrm{S}{}^{2-}$	insoluble	group 1, group 2, and $\mathrm{NH}{}_4{}^{+}$ are soluble
Hydroxide, $\mathrm{OH}{}^{-}$	insoluble	group 1, $\mathrm{NH}{}_4{}^{+}$, and $\mathrm{Ba}{}^{2+}$ soluble; $\mathrm{Ca}{}^{2+}$ and $\mathrm{Sr}{}^{2+}$ slightly soluble
Carbonate, $\mathrm{CO}{}_3{}^{2-}$	insoluble	group 1 and $\mathrm{NH}{}_4{}^{+}$ are soluble
Phosphate, $\mathrm{PO}{}_4{}^{3-}$	insoluble	group 1 and $\mathrm{NH}{}_4{}^{+}$ are soluble
Sulfite, $\mathrm{SO}{}_3{}^{2-}$	insoluble	group 1 and $\mathrm{NH}{}_4{}^{+}$ are soluble
Chromate, $\mathrm{CrO}{}_4{}^{2-}$	insoluble	group 1, $\mathrm{NH}{}_4{}^{+}$, and $\mathrm{Mg}{}^{2+}$ are soluble
Oxide, $\mathrm{O}{}^{2-}$	insoluble	group 1 and the heavier group 2 oxides react with water rather than simply dissolving, giving hydroxide solutions

The short version, if you only remember four lines

1. Everything with group 1 or ammonium dissolves. No exceptions worth the ink.
2. Everything with nitrate dissolves. This is why nitrates are the reagent of choice when you want a particular cation in solution and nothing else to precipitate.
3. Chlorides, bromides, iodides, and sulfates dissolve, except for a short list dominated by silver, lead, and barium.
4. Carbonates, phosphates, sulfides, and hydroxides do not dissolve, except when rule 1 overrides them — and rule 1 always overrides them.

Rules 1 and 2 win against every other row in the table. Sodium carbonate is soluble even though carbonates are not, because sodium is group 1.

Using it on a real prediction

Will a precipitate form?

Barium chloride solution is mixed with sodium sulfate solution.

Swap the partners: the possible products are barium sulfate and sodium chloride.

Barium sulfate — the sulfate row lists $\mathrm{Ba}{}^{2+}$ as an exception, so it is insoluble. It precipitates.

Sodium chloride — sodium is group 1, so it is soluble and stays in solution as ions.

$\mathrm{BaCl}{}_2(\mathrm{aq})+\mathrm{Na}{}_2\mathrm{SO}{}_4(\mathrm{aq})\to \mathrm{BaSO}{}_4(\mathrm{s})+2\mathrm{NaCl}(\mathrm{aq})$

Net ionic: $\mathrm{Ba}{}^{2+}(\mathrm{aq})+\mathrm{SO}{}_4{}^{2-}(\mathrm{aq})\to \mathrm{BaSO}{}_4(\mathrm{s})$

Potassium nitrate solution is mixed with sodium chloride solution.

Possible products: potassium chloride and sodium nitrate. Both are group 1 salts, and one of them is a nitrate as well. Both soluble.

The answer is no reaction, written out in those words. Nothing left the solution, so nothing happened.

Two things this table does not tell you

How fast. Solubility rules say what the final state is, not how long it takes to get there. A precipitate can take a while to become visible, and warming or stirring a mixture is sometimes what makes it appear.

What happens at other temperatures. Every value here is for room temperature. Solubility changes with temperature, usually upwards for solids, and a compound that will not dissolve in cold water may dissolve in hot — which is exactly how you recrystallise something to purify it. The reasoning is in Water and Solutions, and reading a solubility curve is covered in Reading a Data Table.