Swap the partners
Double displacement: two compounds trade ions.
Two compounds, two new compounds
Sodium sulphate and barium chloride are both compounds. When their solutions are mixed, a white solid (barium sulphate) forms and sodium chloride stays dissolved.
Two compounds went in and two different compounds came out. How is that possible?
What this lesson covers
The idea
Reactions in which there is an exchange of ions between the reactants are double displacement reactions.
Two compounds, two new compounds
Sodium sulphate and barium chloride are both compounds. When their solutions are mixed, a white solid (barium sulphate) forms and sodium chloride stays dissolved.
Two compounds went in and two different compounds came out. How is that possible?
- They trade partners: each positive ion joins the other compound’s negative ion
- One of the compounds simply disappears
- The two compounds just add up into one
Trade the ions
In water, each compound is positive ions (+) and negative ions (−). Tap the two groups of positive ions and watch them swap.
An exchange of ions
Reactions in which there is an exchange of ions between the reactants are double displacement reactions.
In Na₂SO₄ + BaCl₂ the white solid forms from SO₄²⁻ and Ba²⁺. The other product, sodium chloride, stays in the solution.
- Kind | Example
- Displacement: one element pushes another out | Fe + CuSO₄ → FeSO₄ + Cu
- Double displacement: two ions swap partners | Na₂SO₄ + BaCl₂ → BaSO₄ + 2NaCl
Notes
Reactions in which there is an exchange of ions between the reactants are double displacement reactions.
Check yourself
In Na₂SO₄ + BaCl₂ → BaSO₄ + 2NaCl, who joined with the sulphate ion (SO₄²⁻)?
Which one is a double displacement reaction?
What is exchanged in a double displacement reaction?
- Na⁺, the same partner as before. Na⁺ left the sulphate and joined the chloride.
- Cl⁻. Both are negative ions. They do not join each other.
- Ba²⁺, which came from barium chloride — correct. Yes! The ions traded partners.
- AgNO₃ + NaCl → AgCl + NaNO₃ — correct. Yes! Ag⁺ and Na⁺ exchanged partners.
- Fe + CuSO₄ → FeSO₄ + Cu. That is a displacement reaction: an element pushes another element out.
- CaCO₃ → CaO + CO₂. One reactant breaks into two products: a decomposition.
- Only heat. Heat is not what is exchanged.
- Ions — correct. Yes! Two different groups of ions swap places.
- Whole flasks. Think of what is inside the solution: ions.