Precipitation ReactionsHigh School / HonorsAP Chemistry

Solubility Rules and Precipitation Reactions

What you'll be able to do: Use solubility rules to predict whether mixing two solutions produces a precipitate, and name it.

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Introduction

When two clear solutions are mixed and a cloudy solid appears, the ions did not simply mix, one pair of them found each other and left the solution. Solubility rules are the shortcut that tells you in advance which pair that will be.

These are recommended, not required. You can start this lesson at any time.

Learning objectives

State which common ionic compounds are soluble and which are not
Predict the products of a double replacement reaction
Decide whether a precipitate forms when two solutions are mixed
Write the correct formula and name for the precipitate

What dissolving really means

An ionic compound labelled (aq) is not floating around as intact formula units. It has dissociated, so each cation and anion is surrounded by water molecules and moves independently.

A bottle labelled NaCl(aq) genuinely contains Na⁺ ions and Cl⁻ ions, in equal numbers but with no attachment to each other.

NaCl(s) → Na⁺(aq) + Cl⁻(aq)

Writing (aq) after a soluble ionic compound is shorthand for separated, hydrated ions.

Solubility rules and their exceptions

Use the course solubility table as a first prediction for ordinary aqueous mixtures. “Soluble” and “insoluble” describe relative amounts that dissolve; they are not absolute yes/no properties.

Family	Usual prediction	Important exceptions or qualifications
Group 1 and NH₄⁺ salts; nitrates	Soluble	Reliable introductory rules for common salts
Acetates	Usually soluble	Apply to the common salts used here
Chlorides, bromides, iodides	Soluble	Ag⁺, Pb²⁺ and Hg₂²⁺ salts are common exceptions
Sulfates	Soluble	Ba²⁺, Pb²⁺, Sr²⁺; CaSO₄ is sparingly soluble
Carbonates and phosphates	Usually insoluble	Group 1 and NH₄⁺ salts are soluble
Hydroxides	Usually insoluble	Group 1 soluble; Ca(OH)₂ is sparingly soluble, Sr(OH)₂ and Ba(OH)₂ appreciably more soluble
Sulfides	Often insoluble	Group 1, group 2 and NH₄⁺ exceptions are commonly listed; hydrolysis can complicate actual solutions

Do not transfer a chloride rule to a chemically different anion such as perchlorate, ClO₄⁻. Recognize the whole polyatomic ion before choosing its family.

If either ion is nitrate, sodium, potassium or ammonium, that compound stays dissolved. This single check settles most questions.

Swapping partners

A precipitation reaction is a double replacement: the cation of one compound pairs with the anion of the other. Write both new combinations, balance their charges to get correct formulas, then apply the rules to each. If both are soluble, nothing happens and there is no reaction.

For Pb(NO₃)₂(aq) + KI(aq), list Pb²⁺, NO₃⁻, K⁺ and I⁻. The new candidates are PbI₂ and KNO₃: lead requires two iodides, while potassium nitrate stays dissolved. The balanced molecular equation is Pb(NO₃)₂(aq) + 2 KI(aq) → PbI₂(s) + 2 KNO₃(aq). Balancing changes how much of each substance reacts, not the already established product formulas.

A partner swap alone does not demonstrate a reaction. NaCl(aq) mixed with KNO₃(aq) leaves the same four hydrated ions; there is no net precipitation reaction. Other mixtures can instead react by proton transfer, gas formation or redox, which need their own tests.

AB(aq) + CD(aq) → AD + CB

Getting the formula right

Charges, not coefficients, set the subscripts in the new compound. Pairing Ag⁺ with CrO₄²⁻ gives Ag₂CrO₄, not AgCrO₄. Decide the formula from charge balance first, then look it up in the rules, because the wrong formula usually leads to the wrong prediction.

Never carry a subscript across from a reactant. Rebuild each product formula from the ion charges.

Check whether the solid can actually form

A sparingly soluble salt precipitates only if the mixture contains enough of its ions to exceed its equilibrium solubility. Extremely dilute Ag⁺ and Cl⁻ solutions can remain clear even though AgCl is classed as insoluble. In Unit 10, this is made quantitative by Q_sp > K_sp. Concentrations for that test are those after mixing and dilution.

For introductory prediction questions without concentration data, assume the stated solutions are concentrated enough for the usual precipitate. Name the solid as well as writing its formula: Ca₃(PO₄)₂ is calcium phosphate; PbI₂ is lead(II) iodide. The Roman numeral gives the metal's charge, not the number of metal atoms. Finish by checking formula neutrality, atom balance and state symbols.

The first mixed-solution snapshot contains two Ag+, two Cl−, two Na+ and two NO3− labels. In the second, the same two Ag+ and two Cl− are in an AgCl solid lattice fragment at the bottom, while both sodium ions and both nitrate ions remain dissolved. No particles are created or lost; the solid is not represented as isolated AgCl molecules. Precipitation requires Qsp greater than Ksp. — Only the selected ions enter the solid lattice. The starting concentrations must still be high enough for Q_sp > K_sp after mixing.

Charge balance determines the formula; coefficients balance the reaction.

Key ideas

Definition

Precipitate

An insoluble solid that forms and separates when two solutions are mixed.

Definition

Dissociation

The separation of an ionic compound into free hydrated ions when it dissolves.

Rule

Common soluble families

Nitrates, acetates, group 1 cations and ammonium have no common insoluble partners.

Rule

Usually insoluble

Carbonates and phosphates are usually insoluble except with group 1 or ammonium; hydroxide and sulfide families have additional exceptions described in the teaching table.

Key concept

No reaction

If no precipitation, gas formation, acid-base change or redox process occurs, the hydrated ions simply coexist. A soluble partner swap alone is not a net reaction.

Worked examples

Worked example 1

Does a precipitate form when solutions of AgNO₃ and NaCl are mixed? If so, identify it.

Try it first: Write the two new cation-anion pairings before checking any rule.

Worked example 2

Predict what happens when Na₂CO₃(aq) and CaCl₂(aq) are mixed.

Try it first: Watch the charges, one of these ions is 2+ and one is 2-.

Worked example 3

Is there a reaction when KNO₃(aq) and NaCl(aq) are mixed?

Worked example 4

What precipitate forms when CaCl₂(aq) and Na₃PO₄(aq) are mixed? Balance the molecular equation.

Try it first: Combine Ca²⁺ with PO₄³⁻ in a neutral ratio.

Common mistakes

Common mistakes and better checks
Mistake	Why it's wrong	Check instead
Carrying subscripts across from the reactants into the product formula.	Product formulas are determined by the charges of the ions being paired, not by the compounds they came from.	Write each ion with its charge, then balance the charges to build the formula.
Assuming every mixture of two solutions must react.	If both possible products are soluble, the ions simply coexist and nothing changes.	Check both new pairings, and if neither is insoluble, write no reaction.
Treating insoluble as meaning nothing dissolves at all.	A tiny amount always dissolves, insoluble means the amount is negligible at this scale.	Read insoluble as sparingly soluble; a solid forms only if the post-mixing ion concentrations exceed the solubility limit.

Scroll horizontally if needed to read all columns.

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What you should now know

Predict candidate precipitates by identifying ions, building neutral formulas from their charges and applying the solubility table with its exceptions. Balance the molecular equation only after choosing the formulas. “Insoluble” means sparingly soluble: actual precipitation requires post-mixing ion activities high enough to exceed K_sp. A partner swap leaving all ions dissolved is not by itself a net reaction.

Soluble ionic compounds exist in water as separated ions
Common nitrates, acetates, group 1 and ammonium salts are treated as soluble under the introductory aqueous conditions here
Use the full table for the usually insoluble families and their exceptions
Build product formulas from ion charges, never from reactant subscripts
If both candidate salts stay dissolved, there is no net precipitation reaction

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