An agricultural scientist uses a nichrome wire loop to carry out a flame test on a solid sample of crop ash.
Before the loop touches the ash, it gives a yellow-orange colour to a blue Bunsen flame. The ash is then placed on the loop and the flame stays yellow-orange.
The scientist concludes that the ash contains sodium ions.
Why is this conclusion not valid?
The loop was not clean, so the sodium ions may have come from the loop and not the ash.
The ash is a solid, so it must be dissolved before its ions can colour a flame.
The blue Bunsen flame masks the yellow-orange colour produced by sodium ions.
A yellow-orange flame shows potassium ions, so the ash does not contain sodium ions.
107 exam-style questions on OCR GCSE Chemistry 4.2 Identifying the products of chemical reactions, covering 4.2.1 Tests to identify gases, 4.2.2 Tests for aqueous ions and flame tests, and 4.2.3 Instrumental methods of analysis. Each one has a worked solution and a mark scheme showing where the marks go.