A 'breath test' used by traffic police to check drunken driving uses?

Analyze the question: The question asks which reagent is used in the breathalyser, the device traffic police use to detect alcohol (ethanol) in a driver's breath.

Concept: Ethanol is a primary alcohol, so it can be oxidized. The classic breathalyser uses acidified potassium dichromate(VI), K₂Cr₂O₇/H₂SO₄.

• The dichromate ion, Cr₂O₇²⁻, is orange.
• When ethanol is present, it is oxidized to ethanal (and then ethanoic acid).
• The dichromate is reduced to the chromium(III) ion, Cr³⁺, which is green.

Step-by-step solution:

1. The driver blows into a tube containing crystals or silica gel soaked in potassium dichromate(VI) and sulphuric acid.
2. If alcohol vapour is present, the following reaction occurs:

3CH₃CH₂OH + Cr₂O₇²⁻ + 8H⁺ → 3CH₃CHO + 2Cr³⁺ + 7H₂O (orange → green)

3. The colour change from orange to green shows that the driver has alcohol in the breath. The extent of the change indicates how much alcohol is present.

Checking the options:

• A. Turmeric on filter paper: turmeric is used to test for boric acid or alkalis, not alcohol.
• B. Silica gel coated with "saliva nitrate": this is not a real reagent for alcohol. It is probably a distortion of silver nitrate, which is not used here.
• C. Potassium permanganate-sulphuric acid: this can oxidize alcohol (purple → colourless), but it is not the standard breathalyser reagent.
• D. Potassium dichromate-sulphuric acid: this is correct. It is the standard reagent, and the orange-to-green change is easy to see.

Answer: D

Exam tip: Remember "Dichromate Detects Drunk drivers", and "Orange to Green means alcohol has been seen." Any question about oxidizing alcohols with a colour change from orange to green points to acidified K₂Cr₂O₇.