International General Certificate of Secondary Education (IGCSE) Chemistry Practice Exam

What color precipitate is formed when testing for iodide ions?

1. A white precipitate

2. A yellow precipitate

3. A cream precipitate

4. A red precipitate

The correct answer is: A yellow precipitate

When testing for iodide ions, a yellow precipitate signifies the presence of these ions. This occurs during the reaction between iodide ions and a suitable reagent, typically lead(II) nitrate or another similar lead(II) compound. Upon addition of the reagent, lead(II) ions react with iodide ions to form lead(II) iodide, which is insoluble in water and appears as a bright yellow solid. This distinct color helps in confirming the presence of iodide ions in a sample, making it an important consideration in qualitative analysis in chemistry. The other colors listed do not correspond to iodide ion reactions. For instance, a white precipitate could indicate the presence of chloride ions or certain sulfate ions, while a cream precipitate generally suggests the presence of bromide ions. A red precipitate is typically not associated with halide ions in this context and is not a characteristic result of testing for iodides.