Double Trouble: Double Decomposition
Exchange ions to form precipitates or water.
What this lesson covers
Why it matters
Two clear liquids meet, and suddenly a white cloud traps the ions in a solid cage. Watch how partners swap in a double decomposition reaction.
The idea in plain words
Balance BaCl₂ + H₂SO₄ → BaSO₄ + HCl — how many HCl must form for every atom to be accounted for?
Predict first
What happens when barium chloride meets dilute sulphuric acid?
Barium ions grab sulphate ions to form insoluble BaSO₄, leaving HCl in solution.
- A white precipitate of barium sulphate forms — correct
- The solution turns blue
- Bubbles of hydrogen gas escape
- Nothing happens; both remain clear
What you do
Mix the solutions. Observe the white precipitate forming instantly.
Check yourself
Which equation correctly represents this double decomposition?
Ions exchange partners: Ba²⁺ pairs with SO₄²⁻, and H⁺ pairs with Cl⁻.
Why does a precipitate form in this reaction?
Solubility rules dictate that barium sulphate falls out of solution as a solid.
In the reaction CuSO₄ + 2NaOH → Cu(OH)₂ + Na₂SO₄, what is the observation?
Copper(II) hydroxide is insoluble and appears as a pale-blue solid.
- BaCl₂ + H₂SO₄ → BaSO₄ + 2HCl — correct
- BaCl₂ + H₂SO₄ → BaS + 2HCl + O₂
- BaCl₂ + H₂SO₄ → Ba(HSO₄)₂ + Cl₂
- BaSO₄ is insoluble in water — correct
- The solution became too cold
- HCl is a solid at room temperature
- Pale-blue precipitate of Cu(OH)₂ — correct
- White precipitate of Na₂SO₄
- Reddish-brown solid forms