Misconception 1: "All white precipitates indicate the same ion"
White precipitates can form from several different ions: Al³⁺ + NaOH gives white Al(OH)₃; Ca²⁺ + NaOH gives white Ca(OH)₂; BaSO₄ is white; AgCl is white. You must use the specific reagent and look for the specific distinguishing feature. The key difference between Al³⁺ and Ca²⁺ is that only aluminium hydroxide redissolves in excess NaOH.
Misconception 2: "You don't need to add acid to the barium chloride test"
You must add dilute HCl before or alongside the BaCl₂. Without HCl, carbonate ions (CO₃²⁻) can also react with Ba²⁺ to form barium carbonate — another white precipitate — giving a false positive for sulfate. The acid removes carbonate ions first: CO₃²⁻ + 2H⁺ → CO₂ + H₂O.
Misconception 3: "Iron II and iron III give the same NaOH result"
No — this is a key distinction. Iron II (Fe²⁺) gives a green precipitate of Fe(OH)₂. Iron III (Fe³⁺) gives a brown/rust orange precipitate of Fe(OH)₃. The colour difference is very clear in practice and commonly tested in exams. Remember: green = two, brown = three (like "3 = rust = iron rust colour").
This common misconceptions covers Common Misconceptions within Tests for Ions for GCSE Chemistry. Revise Tests for Ions in Chemical Analysis for GCSE Chemistry with 20 exam-style questions and 14 flashcards. This is a high-frequency topic, so it is worth revising until the explanation feels precise and repeatable. It is section 9 of 15 in this topic. Use this common misconceptions to connect the idea to the wider topic before moving on to questions and flashcards.
Practice questions for Tests for Ions
Which reagents are used to test for carbonate ions in a solution?
Describe how sodium hydroxide solution can be used to distinguish between iron(II) ions and iron(III) ions in solution, including the expected observations.