🎓 Prepared by students from Boğaziçi University

What are Double Displacement Reactions?

Double displacement reactions occur when ions from two compounds exchange places to form two new compounds. They follow the pattern AB + CD → AD + CB. These reactions often produce a gas, precipitate, or water.

Short answer

A double displacement reaction exchanges ions between two compounds: AB + CD → AD + CB. Example: HCl + NaOH → NaCl + H₂O (hydrogen and sodium ions swap partners, forming salt and water).

Double Displacement: Ions Exchange Partners

1
Compound AB
Ions A and B bonded
↓
2
Compound CD
Ions C and D bonded
↓
3
Ions exchange
A bonds with D, B bonds with C
↓
4
Products AD and CB
Two new compounds form

Step-by-step worked examples

Hydrochloric acid (HCl) reacts with sodium hydroxide (NaOH). Ions exchange to form salt and water. Write and balance.

Unbalanced: HCl + NaOH → NaCl + H₂O
H: 1 on left, 2 on right (unbalanced)
Cl: 1 on each side ✓
Na: 1 on each side ✓
O: 1 on each side ✓
Balance: equation already balanced as written (1:1:1:1)
Balanced: HCl + NaOH → NaCl + H₂O

Sodium carbonate (Na₂CO₃) reacts with calcium chloride (CaCl₂). They form calcium carbonate precipitate and NaCl. Balance.

Unbalanced: Na₂CO₃ + CaCl₂ → CaCO₃ + NaCl
Na: 2 on left, 1 on right (unbalanced)
Ca: 1 on each side ✓
C: 1 on each side ✓
Cl: 2 on left, 1 on right (unbalanced)
Balance: need 2 NaCl on right
Balanced: Na₂CO₃ + CaCl₂ → CaCO₃↓ + 2NaCl

Silver nitrate (AgNO₃) reacts with sodium chloride (NaCl). They form AgCl precipitate. Write and balance.

Unbalanced: AgNO₃ + NaCl → AgCl + NaNO₃
Ag: 1 on each side ✓
N: 1 on each side ✓
Cl: 1 on each side ✓
Na: 1 on each side ✓
O: 3 on each side ✓
Balanced: AgNO₃ + NaCl → AgCl↓ + NaNO₃

Flashcards

Quick quiz

Q1.Which equation shows double displacement?

Correct answer: B. HCl + NaOH → NaCl + H₂O: H and Na ions exchange partners.

Q2.In AB + CD → AD + CB, what happens?

Correct answer: B. Ions from the two compounds trade partners: A with D, B with C.

Q3.What often forms in double displacement?

Correct answer: B. Double displacement often produces an insoluble precipitate, gas, or weakly molecular compound like water.

Q4.In AgNO₃ + NaCl → AgCl↓ + NaNO₃, the precipitate is…

Correct answer: C. AgCl (silver chloride) is insoluble in water and forms a white precipitate (↓ indicates this).

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Common mistakes

✗

All double displacement produces a precipitate. — Correct: Double displacement can produce a precipitate, gas, water, or any insoluble/weakly molecular product.

✗

Only one type of ion exchanges. — Correct: Both cations and anions exchange partners: A and C swap, B and D swap.

✗

Double displacement is the same as synthesis. — Correct: Synthesis combines two substances into one (A + B → AB). Double displacement exchanges ions (AB + CD → AD + CB).

✗

The products are always soluble. — Correct: One or both products in double displacement often form an insoluble solid (precipitate), gas, or weakly molecular compound.

FAQ

What is double displacement?

Double displacement is when ions from two compounds exchange places: AB + CD → AD + CB.

What is an example?

HCl + NaOH → NaCl + H₂O. Hydrogen and sodium ions swap partners, forming salt and water.

Is acid-base neutralization double displacement?

Yes. Acid + Base → Salt + Water follows the double displacement pattern.

What does the ↓ symbol mean?

The ↓ symbol indicates a precipitate — an insoluble solid that forms and settles. Example: AgCl↓.