JAMB - Chemistry (1993 - No. 39)
A gas that will turn orange potassium heptaoxodichromate (lV) solution to clear green is?
sulphur (ii) oxide
hydrogen sulphide
sulphide (lV) oxide
hydrogen chloride
Spiegazione
Sulphur dioxide (SO₂) gas will turn an orange solution of acidified potassium dichromate (K₂Cr₂O₇) clear green. This colour change occurs because sulphur dioxide is a reducing agent, and it reduces the dichromate ion (Cr₂O₇²⁻) from its orange form to the green chromium(III) form (Cr³⁺).