A7LAM CSCA ACADEMYCSCA Academy / Chemistry / Substances and their reactions
CSCA Chemistry · Lesson 11 of 21Ionic reactions and ion tests
Réactions ioniques et tests d'ions · التفاعلات الأيونية والكشف عن الأيونات
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In solution, salts, acids and bases exist as ions. A reaction happens when ions are removed as a precipitate, a gas or water.
Key ideas
- Ionic equations keep only the ions that react; spectator ions are left out.
- A reaction goes when it forms a precipitate, a gas or a weak electrolyte such as water.
- Ions that react together cannot coexist in solution (Ag⁺ with Cl⁻, H⁺ with OH⁻).
- Tests: Cl⁻ + Ag⁺ → white AgCl; SO₄²⁻ + Ba²⁺ → white BaSO₄; Fe³⁺ + SCN⁻ → blood red.
Formulas to know
- Ag⁺ + Cl⁻ → AgCl↓ (white)
- Cu²⁺ + 2OH⁻ → Cu(OH)₂↓ (blue)
- Fe³⁺ + 3OH⁻ → Fe(OH)₃↓ (red-brown)
Worked example
Write the ionic equation for HCl + NaOH → NaCl + H₂O.
- Ions: H⁺ + Cl⁻ + Na⁺ + OH⁻ → Na⁺ + Cl⁻ + H₂O
- Spectators: Na⁺ and Cl⁻ are on both sides
- Keep: H⁺ + OH⁻ → H₂O
Answer: H⁺ + OH⁻ → H₂O
Practice questions
1. Which ions cannot coexist in solution?
Na⁺ and NO₃⁻ · Ba²⁺ and SO₄²⁻ ✓ · K⁺ and Cl⁻ · H⁺ and Cl⁻
They form a BaSO₄ precipitate.
2. Adding NaOH gives a blue precipitate. The solution contains:
Fe³⁺ · Cu²⁺ ✓ · Al³⁺ · Na⁺
Cu(OH)₂ is blue.
3. To test for Cl⁻ you add:
BaCl₂ · AgNO₃ and dilute HNO₃ ✓ · NaOH · litmus
White AgCl that does not dissolve in HNO₃.