Mastering Double Displacement Reactions- Practice Problems and Solutions

What Are Double Displacement Reactions?

Double displacement reactions—also called metathesis reactions—happen when two compounds swap their ions. The general form looks like this:

AB + CD → AD + CB

Think of it as a chemical trade. Cation A leaves compound AB and hooks up with anion D from compound CD. Meanwhile, cation C pairs with anion B.

These reactions happen in solution. One of three things usually comes next:

• A precipitate forms
• A gas bubbles out
• Water appears (neutralization)

If nothing changes—no gas, no solid, no water—you probably don't have a reaction.

Types of Double Displacement Reactions

Not all double displacement reactions look the same. Here's how they break down:

Precipitation Reactions

Two soluble salts mix and form an insoluble solid called a precipitate. This is the most common type you'll see in chemistry class.

Example: When you mix silver nitrate with sodium chloride, silver chloride crashes out as a white solid.

Gas-Forming Reactions

One product breaks down and releases a gas. You'll usually see this when mixing a carbonate or sulfide with an acid.

Example: Vinegar (acetic acid) + baking soda (sodium bicarbonate) → sodium acetate + water + carbon dioxide gas

Neutralization Reactions

Acid meets base. You get water and a salt. This is what happens in your stomach when you take an antacid.

Example: HCl + NaOH → NaCl + H₂O

How to Predict Products

Here's the straightforward method:

1. Write the reactants as separated ions (complete ionic form)
2. Identify the cation-anion pairs that could form
3. Check solubility rules for each possible product
4. Apply the rule: If it forms a precipitate, gas, or water—reaction happens

You need to memorize solubility rules. There's no shortcut here.

Key Solubility Rules to Know

• Always soluble: Na⁺, K⁺, NH₄⁺, NO₃⁻, most alkali metal salts
• Usually soluble: Cl⁻, Br⁻, I⁻ (except with Ag⁺, Pb²⁺, Hg₂²⁺)
• Usually insoluble: CO₃²⁻, PO₄³⁻, OH⁻, S²⁻ (except with Ca²⁺, Ba²⁺ which are slightly soluble)
• Always insoluble: AgCl, PbCl₂, BaSO₄, CaCO₃

Practice Problems

Test yourself before checking the answers. Work through each one.

Problem 1

Predict the products and identify the precipitate:

Pb(NO₃)₂(aq) + 2KI(aq) → ?

Problem 2

Complete and balance the reaction:

CaCl₂(aq) + Na₂CO₃(aq) → ?

Problem 3

Write the complete ionic equation:

Ba(NO₃)₂(aq) + Na₂SO₄(aq) → ?

Problem 4

Does this reaction occur? Why or why not?

KNO₃(aq) + NaCl(aq) → ?

Problem 5

Write the net ionic equation:

FeCl₃(aq) + 3NaOH(aq) → ?

Problem 6

Balance and identify the type:

H₂SO₄(aq) + Ba(OH)₂(aq) → ?

Solutions

Solution 1

Pb(NO₃)₂ + 2KI → PbI₂ + 2KNO₃

Lead iodide (PbI₂) is the yellow precipitate. This is a precipitation reaction.

💡 PbI₂ is famously bright yellow. You'll see it instantly when the solutions mix.

Solution 2

CaCl₂ + Na₂CO₃ → CaCO₃ + 2NaCl

Calcium carbonate (CaCO₃) precipitates out as a white solid. This is also a precipitation reaction.

The reaction is already balanced as written.

Solution 3

Complete ionic equation:

Ba²⁺(aq) + 2NO₃⁻(aq) + 2Na⁺(aq) + SO₄²⁻(aq) → BaSO₄(s) + 2Na⁺(aq) + 2NO₃⁻(aq)

Notice how Na⁺ and NO₃⁻ appear on both sides unchanged. These are spectator ions.

Solution 4

No reaction.

All possible products—KCl and NaNO₃—are soluble in water. Nothing precipitates, no gas forms, no water produced. The ions just stay in solution.

When both products are soluble, double displacement doesn't produce anything observable. No reaction occurs.

Solution 5

Full reaction: FeCl₃ + 3NaOH → Fe(OH)₃ + 3NaCl

Net ionic equation:

Fe³⁺(aq) + 3OH⁻(aq) → Fe(OH)₃(s)

Fe(OH)₃ is a reddish-brown precipitate. Remove all spectator ions (Na⁺ and Cl⁻) to get the net ionic equation.

Solution 6

H₂SO₄ + Ba(OH)₂ → BaSO₄ + 2H₂O

This is a neutralization reaction that also produces a precipitate. Two things happen at once: water forms AND barium sulfate crashes out.

The equation is balanced. BaSO₄ is white and insoluble.

Writing Ionic Equations: A Quick Reference

Different equation types confuse students. Here's the difference:

Net ionic equations always exclude spectator ions—the ions that don't participate in the actual reaction.

Common Mistakes to Avoid

• Forgetting to balance. Check your charges. They must match on both sides.
• Not memorizing solubility rules. You can't predict precipitates without them.
• Writing reactions that don't happen. If both products are soluble, nothing occurs.
• Dropping state symbols. (s), (aq), (g), (l) matter. They tell you what formed.

Getting Started: Your Action Plan

If you want to get better at double displacement reactions:

1. Memorize solubility rules until they're automatic
2. Practice writing ionic equations starting with molecular equations
3. Identify the precipitate first—everything else follows
4. Balance charges when writing formulas for new compounds
5. Check your work by verifying nothing cancels incorrectly in net ionic form

That's it. There's no magic here—just pattern recognition and practice.