Writing the swapped formulas
Two clear solutions can make a solid. Now you write the new formulas yourself.
- Two clear solutions: copper sulphate and sodium hydroxide.
- In water, each compound splits into free ions.
- Copper ions meet hydroxide ions. Sodium ions meet sulphate ions.
- Copper hydroxide does not dissolve. It sinks as a solid.
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Two solutions meet. Swap the partners on paper. Tap the new pair.
The rule
In a double displacement reaction, two compounds swap partners. Each new formula comes from crossing the charges, then cutting down. A solid that does not dissolve is a precipitate.
An ion is an atom or group of atoms with a charge. These are the ions in the lab.
| Ion | Charge |
|---|---|
| Sodium Na⁺, potassium K⁺ | 1+ |
| Copper Cu²⁺, calcium Ca²⁺, magnesium Mg²⁺ | 2+ |
| Hydroxide OH⁻, nitrate NO₃⁻, chloride Cl⁻ | 1− |
| Sulphate SO₄²⁻, carbonate CO₃²⁻ | 2− |
Cross the charges. Cu²⁺ and OH⁻ give Cu(OH)₂. One copper needs two hydroxides.
The 2 of Cu²⁺ goes to OH. The 1 of OH⁻ goes to Cu. That gives Cu₁(OH)₂, written Cu(OH)₂.
Ca²⁺ and CO₃²⁻ cross to Ca₂(CO₃)₂. Cut down to CaCO₃.
Cutting down means dividing both small numbers by the same number. Here both 2s become 1.
CuSO₄(aq) + 2NaOH(aq) → Cu(OH)₂(s) + Na₂SO₄(aq)
(aq) means dissolved in water. (s) means a solid. The solid is the precipitate.
Compounds of sodium and potassium dissolve in water. Most carbonates and hydroxides of other metals do not.
| Mix | Swapped on paper | A solid? |
|---|---|---|
| Copper sulphate and sodium hydroxide | Copper hydroxide, sodium sulphate | Yes: copper hydroxide |
| Calcium nitrate and potassium carbonate | Calcium carbonate, potassium nitrate | Yes: calcium carbonate |
| Magnesium chloride and potassium hydroxide | Magnesium hydroxide, potassium chloride | Yes: magnesium hydroxide |
| Sodium nitrate and potassium chloride | Sodium chloride, potassium nitrate | No. All four ions stay dissolved. |
In displacement, one free element pushes another out. Here, two compounds swap partners.