The Detective's Chemistry Toolkit
🔍 The Detective's Chemistry Toolkit
Imagine you are given a white powder and asked to identify it. It could be sodium chloride, calcium carbonate, iron sulfate, or dozens of other compounds. Ion tests are chemistry's detective toolkit — a series of carefully chosen chemical reactions that each target one specific ion. Add the right reagent, observe the result, and you can narrow down the identity systematically. Master this topic and you can identify any common ionic compound from scratch.
Chemistry glossary
- How do you test for sulfate ions (SO₄²⁻)?
- Add barium chloride solution + dilute HCl. White precipitate of BaSO₄ forms. Equation: Ba²⁺ + SO₄²⁻ → BaSO₄(s)
There are two main approaches to testing for metal cations: flame tests (covered in Topic 48) and adding sodium hydroxide solution to form characteristic precipitates.
Earn the mark scheme marks
🧠 Memory Aid
"NaOH for METALS, BaCl₂ for SULFATES, AgNO₃ for HALIDES"
Precipitate colours with NaOH: "Cu = Blue, Fe2 = Green, Fe3 = Brown, Al/Ca = White"
Silver halide colours: "White, Cream, Yellow — going down the halogen group"
- Cl⁻ = White
- Br⁻ = Cream
- I⁻ = Yellow
- Remember: "Why (Y) does Cream come between?" — White, Cream, Yellow
Quick Check: A solution gives a brown precipitate when sodium hydroxide is added. What metal ion is present?
Iron III ion (Fe³⁺). It forms brown/orange Fe(OH)₃ precipitate with sodium hydroxide.
Now try it yourself
Quiz · Question 1 of 20
Which reagents are used to test for carbonate ions in a solution?
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This topic in real past papers
Every real exam question we've found on tests for ions, with a full worked answer.