Knowledge Organiser: Making Salts
Key Terms
- Crystallisation: forming crystals by cooling concentrated solution
- Filtration: removing insoluble solid from liquid
- Evaporation: removing water to concentrate solution
- Precipitation: forming insoluble solid by mixing two solutions
- Excess: more than needed to react completely
Must-Know Facts
- Soluble salt: react acid + insoluble base with excess base; filter, evaporate, cool, dry
- Insoluble salt: mix two solutions; filter precipitate; wash with distilled water; dry
- All nitrates are soluble; all Na/K/NH₄ salts are soluble
- Insoluble: AgCl (white), BaSO₄ (white), PbI₂ (yellow)
- Add EXCESS BASE not excess acid
- FECD: Filter, Evaporate, Cool, Dry
Key Equations
- CuO + H₂SO₄ → CuSO₄ + H₂O (insoluble base + acid → soluble salt)
- BaCl₂ + Na₂SO₄ → BaSO₄↓ + 2NaCl (precipitation)
- AgNO₃ + NaCl → AgCl↓ + NaNO₃ (silver chloride precipitation)
Common Mistakes
- Using excess acid instead of excess base: Always add excess BASE — if excess acid is present, the salt solution will be contaminated and evaporation won't remove it
- Filtering before evaporating for soluble salts: For soluble salt preparation, filter FIRST (to remove excess solid) then evaporate and crystallise
- Confusing soluble and insoluble salt methods: Insoluble salts use precipitation (mix two solutions); soluble salts use acid + excess insoluble base then FECD
- Forgetting to wash the precipitate: When making insoluble salts, wash with distilled water to remove soluble impurities before drying
Memory Aid
- Soluble salt method — "FECD": Filter, Evaporate, Cool, Dry — the four steps after the acid-base reaction is complete.
- Why add excess base: "BASE beats ACID" — always use excess of the insoluble base so all acid is consumed. "Excess base in, excess base removed by filter." Excess acid in = problem (acid stays in product).
- Insoluble salts: "Silver CHLORIDE is WHITE, Lead IODIDE is YELLOW, Barium SULFATE is WHITE." These are the three insoluble salts most commonly tested.
- Solubility rules shortcut: "ALL nitrates dissolve. ALL sodium/potassium/ammonium salts dissolve. Most chlorides and sulfates dissolve, EXCEPT the named exceptions." Learn the exceptions: silver chloride, barium sulfate, lead chloride, lead sulfate, lead iodide.