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.
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Chemical ReactionsPrecipitation ReactionsContent level: Core 20 min
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.
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
Lesson
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.
The rules worth memorising
Start with the compounds that are essentially always soluble: all nitrates, all acetates, all group 1 salts and all ammonium salts. Then the mostly soluble families: chlorides, bromides and iodides, except with silver, lead(II) and mercury(I); sulfates, except with barium, lead(II), calcium and strontium. Finally the mostly insoluble families: carbonates, phosphates, sulfides and hydroxides, except when paired with group 1 or ammonium.
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.
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.
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
Always soluble
Nitrates, acetates, group 1 cations and ammonium have no common insoluble partners.
Rule
Usually insoluble
Carbonates, phosphates, sulfides and hydroxides are insoluble unless paired with group 1 or ammonium.
Key concept
No reaction
If both possible products are soluble, the ions stay dispersed and no chemical change occurs.
Worked examples
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-.
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Worked example 3
Is there a reaction when KNO₃(aq) and NaCl(aq) are mixed?
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Common mistakes
Carrying subscripts across from the reactants into the product formula.
Why it's wrong: Product formulas are determined by the charges of the ions being paired, not by the compounds they came from.
Check instead: Write each ion with its charge, then balance the charges to build the formula.
Assuming every mixture of two solutions must react.
Why it's wrong: If both possible products are soluble, the ions simply coexist and nothing changes.
Check instead: Check both new pairings, and if neither is insoluble, write no reaction.
Treating insoluble as meaning nothing dissolves at all.
Why it's wrong: A tiny amount always dissolves, insoluble means the amount is negligible at this scale.
Check instead: Read insoluble as forms a visible solid under normal conditions.
Practice this skill
No practice questions are available for this topic yet. You can still practice the whole unit.
What you should now know
Soluble ionic compounds exist in water as separated ions. When two solutions are mixed, the ions swap partners on paper, and any new combination that is insoluble drops out as a precipitate. A short set of memorised rules, led by the always-soluble nitrates, group 1 cations and ammonium, is enough to predict almost every precipitation reaction you will meet.
- Soluble ionic compounds exist in water as separated ions
- Nitrates, acetates, group 1 and ammonium salts are always soluble
- Carbonates, phosphates, sulfides and hydroxides are usually insoluble
- Build product formulas from ion charges, never from reactant subscripts
- If both possible products are soluble, there is no reaction
Sources and further reading
This lesson is original Chem Help content. No external sources were adapted.