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 topic appears regularly enough that it should still be part of a steady revision cycle. It is section 9 of 14 in this topic. Use this common misconceptions to connect the idea to the wider topic before moving on to questions and flashcards.
Topic position
Section 9 of 14
Practice
20 questions
Recall
14 flashcards
⚠️ Common Misconceptions
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").