A breath test used by traffic police to check drunken driving commonly uses which chemical system to detect the presence of alcohol in a driver's breath?

Introduction / Context:
Drunken driving is a major safety concern, and traffic police often use chemical breath tests to estimate the presence of alcohol (ethanol) in a driver's breath. One traditional chemical method employs an oxidising agent that changes colour when it reacts with ethanol. This question asks which chemical system is commonly used in such breath analysers, testing your knowledge of applied chemistry and basic redox reactions.

Given Data / Assumptions:
- The test is a breath test for detecting alcohol in exhaled air. - Options include turmeric, silver nitrate on silica gel, potassium permanganate-sulphuric acid, and potassium dichromate-sulphuric acid. - We assume older or classical chemical breath analysers, not modern electronic fuel-cell devices. - The test relies on a noticeable colour change caused by the presence of ethanol.

Concept / Approach:
Traditional breath analysers often used acidified potassium dichromate solution as the oxidising agent. Ethanol in the breath is oxidised to acetic acid, and during this process, orange dichromate ions are reduced to green chromium(III) ions, causing a visible colour change. Potassium permanganate can also oxidise alcohols but is less commonly associated with standard breathalyser kits in general knowledge questions. Turmeric and silver nitrate have entirely different uses. The correct approach is to recall that potassium dichromate in sulphuric acid is the classic system used to detect ethanol through a colour change.

Step-by-Step Solution:
Step 1: Recall that potassium dichromate (K2Cr2O7) in acidic medium is a strong oxidising agent that can oxidise ethanol to acetic acid. Step 2: During this oxidation, the chromium in dichromate is reduced from the +6 oxidation state to the +3 state, producing a distinct colour change from orange to green. Step 3: In a breathalyser, this colour change is correlated with the amount of alcohol in the breath, giving an indirect measure of blood alcohol content. Step 4: Turmeric is an acid-base indicator that changes colour primarily with pH, not specifically with ethanol, so option a is not suitable. Step 5: Silver nitrate on silica gel can be used to detect halide ions, not alcohol, so option b is unrelated to breath tests. Step 6: Potassium permanganate in acidic medium is also an oxidising agent, but general knowledge questions about breath tests traditionally refer to acidified potassium dichromate, not permanganate. Step 7: Therefore, the correct choice is potassium dichromate-sulphuric acid, option d.

Verification / Alternative check:
Descriptions of classical breathalyser designs in chemistry texts and general science references explicitly mention orange potassium dichromate in sulphuric acid solution being used to oxidise ethanol, with the extent of colour change indicating how much alcohol was present. While modern devices may use fuel cells or infrared sensors, exam questions about basic chemistry usually focus on this traditional dichromate-based test. This consistency across sources supports selecting potassium dichromate-sulphuric acid as the answer.

Why Other Options Are Wrong:
Turmeric (option a) is used as a simple indicator in some acid-base demonstrations and in testing household substances, but it does not specifically detect ethanol. Silica gel coated with silver nitrate (option b) is used for detecting halides such as chloride and bromide, not alcohol. Potassium permanganate with sulphuric acid (option c) is indeed a strong oxidising mixture, but it is not the classic breathalyser reagent given in school-level chemistry and general knowledge texts, which emphasise potassium dichromate instead. As such, these options do not match the standard description of breath tests for drunken driving.

Common Pitfalls:
Some candidates may confuse potassium permanganate and potassium dichromate because both are strong oxidising agents and undergo colour changes in reactions. Another pitfall is to think of any familiar indicator, like turmeric, as a possible answer without considering whether it is sensitive to alcohol. To avoid errors, remember that the iconic school-level example of a breath test uses acidified potassium dichromate which turns from orange to green upon reacting with ethanol.

Final Answer:
The breath test commonly used by traffic police relies on potassium dichromate mixed with sulphuric acid to detect alcohol in the driver's breath.