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Lesson 5 of 7Chemical Reactions
Unit 2 · Lesson 5 of 7

Double Displacement Reactions

How two ionic compounds in solution can swap partners — and how solubility rules predict whether a solid precipitate will form.

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Key things to remember

  1. 1Double displacement: — cations and anions trade partners.
  2. 2A double displacement reaction "happens" (is worth writing) only if a precipitate, gas, or water forms.
  3. 3Group 1 and ammonium compounds are always soluble; nitrates and acetates are always soluble.
  4. 4Silver, lead(II), and mercury(I) halides (Cl⁻, Br⁻, I⁻) are the classic insoluble exceptions to the halide rule.
  5. 5Carbonates, phosphates, and hydroxides are insoluble except with Group 1 metals and ammonium.
  6. 6Neutralization, acid + base → salt + water, is a double displacement driven by and forming water.

Pour clear, colourless lead(II) nitrate solution into clear, colourless potassium iodide solution and a brilliant yellow cloud appears instantly, then slowly settles to the bottom. Nothing was heated and no metal was added: two dissolved compounds simply traded partners, and one of the new pairings could not stay dissolved. The same idea lets chemists test water for lead or chloride, lets sewage plants pull phosphate out of waste water, and explains how some kidney stones (calcium oxalate) form in the body.

In a double displacement (also called "metathesis") reaction, two ionic compounds in solution swap partners: the cation from one compound pairs with the anion from the other, and vice versa. Unlike single displacement, there's no activity series to check — instead, you need to know whether the newly formed compound will actually stay dissolved, or fall out of solution as a solid.

Cations (A, C) and anions (B, D) trade partners.

Why something has to leave the solution

Once an ionic compound dissolves, its ions separate and wander independently through the water. Mix NaCl(aq) with KNO₃(aq) and you simply have Na⁺, Cl⁻, K⁺ and NO₃⁻ all floating around together; calling that "KCl + NaNO₃" changes nothing real. A swap only means something when two of the ions get locked together and removed from the free-moving crowd. For a double displacement reaction to actually proceed forward and be worth writing, at least one of the products must leave the solution in some way — most commonly by forming an insoluble solid, a precipitate. It can also 'drive forward' by forming water (neutralization) or a gas that bubbles away.

Using the solubility rules

Solubility RulesIonSolubilityExceptionsGroup 1 (Li⁺, Na⁺, K⁺…), NH₄⁺always solublenoneNO₃⁻, C₂H₃O₂⁻ (acetate)always solublenoneCl⁻, Br⁻, I⁻solubleexcept with Ag⁺, Pb²⁺, Hg₂²⁺SO₄²⁻solubleexcept with Ca²⁺, Sr²⁺, Ba²⁺, Pb²⁺OH⁻insolubleexcept group 1, NH₄⁺, and slightly Ca²⁺/Sr²⁺/Ba²⁺CO₃²⁻, PO₄³⁻, S²⁻insolubleexcept group 1 and NH₄⁺
Fig 1Solubility rules: which combinations of ions form soluble compounds (stay dissolved) versus insoluble compounds (form a precipitate).
RuleGenerally solubleMajor exceptions (insoluble)
1All Group 1 (Li⁺, Na⁺, K⁺…) and ammonium (NH₄⁺) compoundsNone — always soluble
2All nitrates (NO₃⁻) and acetates (C₂H₃O₂⁻)None — always soluble
3Chlorides, bromides, iodidesAg⁺, Pb²⁺, Hg₂²⁺
4Sulfates (SO₄²⁻)Ba²⁺, Sr²⁺, Pb²⁺ (Ca²⁺ and Ag⁺ sulfates are only slightly soluble, but can still precipitate)
5Carbonates, phosphates, hydroxides, sulfidesInsoluble EXCEPT with Group 1 and NH₄⁺ (Ca/Sr/Ba hydroxides are slightly soluble)
Key idea
Method: write the skeleton double displacement equation, swap the cations/anions to determine the two new products, then check both new products against the solubility rules. If either is insoluble, mark it (s) — a precipitate has formed and a real reaction has occurred.
  • Colour clues for common precipitates: white, cream, pale yellow, bright yellow, white, white, pale blue, reddish-brown.
  • A rough reason behind the rules: ions with larger charges (such as and paired with or ) attract each other so strongly that water usually cannot pull them apart, while large, singly charged ions such as , and are held weakly and dissolve easily.
  • Charges still decide formulas: after swapping, rebuild each product so the charges total zero, e.g. with gives .
Tip
If you check both possible products and both turn out to be soluble, no precipitate forms — the ions simply remain mixed in solution, and the reaction is considered "NR" for the purposes of forming a new substance. A quick shortcut: if one reactant is a Group 1 nitrate (such as or ), then each possible product contains either a Group 1 ion or a nitrate ion, and both of those are always soluble. So a Group 1 nitrate mixed with any other soluble salt is always NR.

Neutralization: acid + base

At the Grade 11 level, an Arrhenius acid is a substance that releases ions in water, and a base releases ions. Mixing them is a double displacement in which the from the acid pairs with the from the base to form water, while the leftover cation and anion form a salt. The driving force is the water: and bond very strongly and are pulled out of the ion crowd as neutral molecules, even when the salt stays dissolved. A thermometer shows the evidence: the mixture warms up.

Acid + base gives salt + water. Two KOH are needed because sulfuric acid supplies two H⁺ ions.
Driving forceWhat leaves the solutionExample
Precipitatean insoluble solid\text{Pb{(NO}_3)}_2 + 2\text{KI} \rightarrow \text{PbI}_2\text{{(s)}} + 2\text{KNO}_3
Water (neutralization)neutral molecules
Gasbubbles of (from carbonates)
If none of these three things happens, the ions just stay mixed and the answer is NR.
Precipitation at work
Ontario sewage treatment plants add iron(III) or aluminum salts to waste water so that dissolved phosphate ions precipitate as solid iron(III) or aluminum phosphate, which settles out with the sludge. Removing phosphate protects lakes from the algae blooms it would otherwise feed. The same logic is used in the lab to identify ions: add a reagent and see whether a precipitate of a known colour appears.

Worked examples

Reveal one step at a time and try to beat it to the next line.

Example 1
Predict the products when aqueous silver nitrate is mixed with aqueous sodium chloride.
Example 2
Predict the products when aqueous barium chloride is mixed with aqueous sodium sulfate.
Example 3
Determine whether a reaction occurs when aqueous potassium nitrate is mixed with aqueous sodium chloride.
Example 4
Exam-level: aqueous calcium chloride is mixed with aqueous sodium phosphate. Predict the products with states, balance the equation, and name the precipitate.

Where marks get lost

  • Forgetting to check BOTH new possible products against the solubility rules — only one needs to be insoluble for a reaction to occur.
  • Swapping only the elements instead of correctly identifying and swapping the cation/anion pairs (especially error-prone with polyatomic ions).
  • Forgetting to rebalance charges within each newly formed formula unit (e.g. writing BaCl instead of BaCl₂).
  • Assuming any two ionic solutions mixed together must react — many combinations produce no visible change (NR) because both products are soluble.
  • Mixing up the sulfate exception list (Ba, Sr, Pb, and slightly soluble Ca) with the halide exception list (Ag, Pb, Hg) — they are different exception sets.

Flashcards 9

Double displacement reaction
Precipitate
Which ions make a compound ALWAYS soluble?
Halide (Cl⁻, Br⁻, I⁻) solubility exceptions
+ 5 more

Practice

1
In a double displacement reaction , what happens?
2
Which compound is insoluble (a precipitate) according to the solubility rules?
3
What is the term for an insoluble solid that forms and separates out of solution during a double displacement reaction?
4
Which of these compound types is ALWAYS soluble, regardless of what cation it's paired with?
5
When aqueous Pb(NO₃)₂ is mixed with aqueous KI, a bright yellow precipitate forms. Name it.
6
Mixing aqueous NaCl and aqueous KNO₃ produces no visible reaction. Why?
7
Balance with the lowest whole numbers. What is the coefficient of water?
8
Nitric acid is neutralized by calcium hydroxide. Give the formula of the salt formed.
9
Put the steps for predicting a double displacement reaction in order.
  1. 1Assign states and balance, or write NR
  2. 2Check each new compound against the solubility rules
  3. 3Rebuild each formula so charges total zero
  4. 4Split both reactants into their ions
  5. 5Swap partners to form two new pairings
10
Match each mixture of aqueous solutions to its result.
11
Which pair of solutions produces a precipitate when mixed?

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