### Correct Option: D. Potassium dichromate-sulphuric acid #### Explanation of the Correct Answer: The 'breath test' used by traffic police to check for drunken driving primarily relies on the chemical reaction between ethanol (the alcohol found in beverages) and potassium dichromate in the presence of sulfuric acid. Here’s a step-by-step breakdown of how this works: 1. **Chemical Reaction**: When a person exhales into the breathalyzer, the breath contains ethanol if they have been drinking. The breathalyzer contains a solution of potassium dichromate (K2Cr2O7) and sulfuric acid (H2SO4). The reaction can be summarized as follows: \[ \text{C}_2\text{H}_5\text{OH} + \text{K}_2\text{Cr}_2\text{O}_7 + \text{H}_2\text{SO}_4 \rightarrow \text{Acetic Acid} + \text{Cr}_2\text{O}_3 + \text{K}_2\text{SO}_4 + \text{H}_2\text{O} \] In this reaction, ethanol is oxidized to acetic acid, and potassium dichromate is reduced, changing its color from orange to green. 2. **Color Change**: The degree of color change in the solution is proportional to the amount of ethanol present in the breath sample. A more significant color change indicates a higher concentration of alcohol. 3. **Quantitative Measurement**: The breathalyzer can be calibrated to provide a quantitative measure of blood alcohol concentration (BAC) based on the intensity of the color change. This allows law enforcement to determine whether a driver is over the legal limit for alcohol consumption. #### Why Other Options Are Incorrect: - **Option A: Turmeric on filter paper**: - Turmeric is a natural pH indicator that changes color in response to acidity or alkalinity. It does not react specifically with ethanol and is not used in breath tests. Therefore, it cannot provide a reliable measurement of alcohol concentration. - **Option B: Silica gel coated with saliva nitrate**: - This option does not relate to the detection of alcohol. Silica gel is typically used as a desiccant or in chromatography, and saliva nitrate does not have a known reaction with ethanol. This option is irrelevant to breath alcohol testing. - **Option C: Potassium permanganate-sulfuric acid**: - While potassium permanganate (KMnO4) is a strong oxidizing agent and can react with alcohols, it is not used in breathalyzers. The reaction with KMnO4 is more complex and less specific for ethanol compared to potassium dichromate. Additionally, the color change is less distinct and harder to quantify accurately for breath alcohol testing. #### Common Pitfalls: - **Misunderstanding the Chemistry**: Students may confuse the reactions of different oxidizing agents. It’s crucial to remember that potassium dichromate is specifically used for its clear and quantifiable reaction with ethanol. - **Ignoring the Role of Sulfuric Acid**: Some may overlook the importance of sulfuric acid in the reaction, which acts as a catalyst and helps to facilitate the oxidation of ethanol. ### Revision Summary: - The breath test for alcohol detection uses potassium dichromate and sulfuric acid due to their specific reaction with ethanol. - The color change from orange to green indicates the presence of alcohol and is used to quantify blood alcohol concentration. - Other options like turmeric and silica gel do not provide reliable measurements for alcohol detection. - Understanding the specific chemical reactions involved is key to grasping how breathalyzers work.