The law enforcement agencies use a chemical test to approximate a person’s blood alcohol level. The person under going the test blows into the mouthpiece of a bag containing sodium dichromate solution in acidic medium. A chemical reaction with ethanol changes the colour of the solution from

1. Basics of Redox Reactions: Oxidation and Reduction (basic)

Welcome to your first step in mastering everyday chemistry! To understand how the world around us works—from the rusting of an iron gate to the functioning of a breathalyzer—we must first grasp the fundamental concept of Redox reactions. The term 'Redox' is a portmanteau of Reduction and Oxidation. In simple terms, these are chemical processes involving the transfer of specific atoms (like oxygen) or electrons between substances.

At the most basic level, we define these processes by the movement of oxygen. Oxidation occurs when a substance gains oxygen during a reaction. Conversely, Reduction occurs when a substance loses oxygen. These two processes are like two sides of the same coin; they almost always happen simultaneously because if one substance loses oxygen, another must be there to take it. As noted in Science, class X (NCERT 2025 ed.), Chemical Reactions and Equations, p.12, when one reactant is oxidized while the other is reduced, we call the entire process an oxidation-reduction reaction.

Consider the reaction where copper oxide (CuO) reacts with hydrogen (H₂). The copper oxide loses its oxygen to become pure copper (it is reduced), while the hydrogen gains that oxygen to become water (it is oxidized). This principle is vital in industries; for instance, obtaining pure metals from their ores is essentially a reduction process where we strip away the oxygen from metal oxides using 'reducing agents' like carbon or highly reactive metals like sodium Science, class X (NCERT 2025 ed.), Metals and Non-metals, p.51.

In everyday life, these reactions often provide a visual signal. For example, in a breathalyzer test used by law enforcement, ethanol from a person's breath undergoes oxidation. During this process, orange dichromate ions (Cr₂O₇²⁻) are reduced to green chromium(III) ions (Cr³⁺). This dramatic color change from orange to green acts as a chemical 'fingerprint' that indicates the presence and concentration of alcohol.

Sources: Science, class X (NCERT 2025 ed.), Chemical Reactions and Equations, p.12; Science, class X (NCERT 2025 ed.), Metals and Non-metals, p.51; Science, class X (NCERT 2025 ed.), Chemical Reactions and Equations, p.14

2. Chemistry of Transition Metals and Color Changes (intermediate)

In the world of chemistry, transition metals (like Iron, Copper, and Chromium) are the master artists. Unlike many other elements, these metals have partially filled d-orbitals, which allows them to exist in multiple oxidation states. When white light—which we know is a spectrum of colors ranging from Violet to Red (VIBGYOR) Science class X (NCERT 2025 ed.), The Human Eye and the Colourful World, p.167—interacts with these metals, they absorb specific wavelengths. The remaining light that is reflected or transmitted gives the substance its characteristic vibrant color.

One of the most fascinating aspects of these metals is that their color is not fixed; it changes based on their chemical environment and oxidation state. For instance, while pure copper is a reddish-brown metal, heating it in air creates Copper(II) oxide, which is a distinct black oxide Science class X (NCERT 2025 ed.), Metals and Non-metals, p.41. This principle is vital in Redox reactions (Reduction-Oxidation), where the transfer of electrons shifts the metal from one oxidation state to another, resulting in a visible "chemical signal"—a color change.

A practical, everyday application of this is found in law enforcement technology: the breathalyzer. When a person exhales into the device, any ethanol (alcohol) in their breath acts as a reducing agent. It reacts with an acidified solution of Potassium Dichromate. In this reaction, the Chromium atom undergoes a transformation that is immediately visible to the naked eye. This transition is summarized in the table below:

As the ethanol is oxidized into ethanoic acid, the orange dichromate is reduced to green chromium(III) ions. The intensity of the green color is directly proportional to the amount of alcohol present in the breath. This allows the device to provide a quick, non-invasive estimate of blood alcohol levels based purely on the shifting electronic structure of a transition metal.

Sources: Science class X (NCERT 2025 ed.), The Human Eye and the Colourful World, p.167; Science class X (NCERT 2025 ed.), Metals and Non-metals, p.41

3. Properties and Oxidation of Alcohols (Ethanol) (basic)

When we talk about alcohols in chemistry, we are usually referring to a family of carbon compounds characterized by the -OH (hydroxyl) functional group. The most famous member of this family is ethanol (C₂H₅OH), a substance that is not only a common solvent but also a key player in many industrial and everyday chemical reactions Science, class X (NCERT 2025 ed.), Carbon and its Compounds, p.71.

One of the most important chemical properties of ethanol is its ability to undergo oxidation. While ethanol can burn completely in air to produce carbon dioxide and water (combustion), it can also be converted into ethanoic acid (CH₃COOH) through a controlled chemical reaction. This is achieved by using substances called oxidizing agents, such as alkaline potassium permanganate (KMnO₄) or acidified potassium dichromate (K₂Cr₂O₇), which are capable of adding oxygen to the ethanol molecule Science, class X (NCERT 2025 ed.), Carbon and its Compounds, p.70.

This oxidation principle has a very practical application in road safety. Law enforcement agencies use devices called breathalyzers to estimate a person's blood alcohol level. When someone who has consumed alcohol blows into the device, the ethanol in their breath undergoes a redox reaction (reduction-oxidation). The ethanol is oxidized to ethanoic acid, while the orange-colored dichromate ions (Cr₂O₇²⁻) in the device are reduced to chromium(III) ions (Cr³⁺), which are green. The shift in color is a direct indicator of the presence of alcohol.

Sources: Science, class X (NCERT 2025 ed.), Carbon and its Compounds, p.70; Science, class X (NCERT 2025 ed.), Carbon and its Compounds, p.71

4. Inorganic Reagents: Sodium and Potassium Dichromate (intermediate)

In the realm of inorganic chemistry, Sodium Dichromate (Na₂Cr₂O₇) and Potassium Dichromate (K₂Cr₂O₇) stand out as some of the most powerful and frequently used oxidizing agents. As you may recall from basic chemistry, an oxidizing agent is a substance that has the capability to add oxygen to another substance or remove electrons from it. Specifically, in organic chemistry, acidified potassium dichromate is a classic reagent used to transform alcohols into carboxylic acids by adding oxygen to the starting material Science, Class X (NCERT 2025 ed.), Carbon and its Compounds, p.71.

The most fascinating aspect of these dichromates is their chromogenic property—their ability to change color during a reaction. The dichromate ion (Cr₂O₇²⁻) is naturally a vivid orange. When it reacts with a reducing agent, such as the ethanol found in a person's breath, a redox (reduction-oxidation) reaction occurs. During this process, the ethanol is oxidized to ethanoic acid, while the chromium in the dichromate is reduced from an oxidation state of +6 to +3. This results in the formation of Chromium(III) ions (Cr³⁺), which are distinctively green Science, Class X (NCERT 2025 ed.), Carbon and its Compounds, p.70.

This dramatic color shift from orange to green is the scientific backbone of the breathalyzer test used by law enforcement. When a driver blows into the device, any ethanol present reacts with the acidified dichromate solution inside. The more alcohol present, the more the orange solution turns green. By measuring the intensity of this green color, the device can accurately estimate the Blood Alcohol Concentration (BAC). This is a perfect example of how complex transition metal chemistry is applied to keep our roads safe through non-invasive, real-time testing.

Sources: Science, Class X (NCERT 2025 ed.), Carbon and its Compounds, p.70; Science, Class X (NCERT 2025 ed.), Carbon and its Compounds, p.71

5. Chemical Sensors in Public Safety (intermediate)

In the realm of public safety, chemical sensors act as the front line for law enforcement and industrial monitoring. One of the most ubiquitous applications of these sensors is the breathalyzer, a device designed to detect ethanol (alcohol) levels in a person's breath to ensure road safety. This process relies on a fundamental principle of chemistry: the redox reaction (reduction-oxidation). When a person exhales into the device, any ethanol present acts as a reducing agent, reacting with a specific chemical reagent stored inside the sensor.

Traditionally, these sensors use an oxidizing agent—a substance capable of adding oxygen to another molecule—to facilitate the change. As explained in Science, class X (NCERT 2025 ed.), Carbon and its Compounds, p.71, substances like acidified potassium dichromate (K₂Cr₂O₇) are powerful oxidizers. In a breathalyzer, the ethanol (C₂H₅OH) is oxidized into ethanoic acid (CH₃COOH). Simultaneously, the orange-colored dichromate ions (Cr₂O₇²⁻) are reduced to chromium(III) ions (Cr³⁺), which are 1distinctly green. This visible shift from orange to green serves as a quantitative indicator: the more alcohol present, the more intense the green color becomes, allowing the device to estimate the person's blood alcohol concentration.

Modern sensors have evolved beyond simple color changes to use electrochemical fuel cells. Instead of a colorimetric test, these devices convert the chemical energy of the alcohol directly into electricity. As noted in Environment, Shankar IAS Academy (ed 10th), Renewable Energy, p.296, fuel cells consist of electrodes and an electrolyte where a fuel reacts electrochemically to generate a current. In this context, the ethanol is oxidized at an anode, producing an electrical current proportional to the amount of alcohol in the breath. This digital approach provides higher precision and is less likely to be triggered by non-alcohol substances in the environment.

Sources: Science, class X (NCERT 2025 ed.), Carbon and its Compounds, p.71; Environment, Shankar IAS Academy (ed 10th), Renewable Energy, p.296

6. The Specific Reaction: Dichromate-Ethanol Interaction (exam-level)

Imagine a roadside police check where a driver is asked to blow into a tube. This isn't just a legal procedure; it's a real-time Redox reaction in action. At the heart of many traditional breathalyzer tests is the Dichromate-Ethanol interaction. When a person consumes alcohol, ethanol (C₂H₅OH) enters the bloodstream and is eventually exhaled. In the breathalyzer device, this ethanol reacts with an acidified solution of Potassium Dichromate (K₂Cr₂O₇). In this chemical process, ethanol is oxidized, while the dichromate ion serves as a powerful oxidizing agent.

The visual signal of this reaction is a distinct color shift. The dichromate ion (Cr₂O₇²⁻) contains Chromium in the +6 oxidation state (also known as Hexavalent Chromium), which is characterized by a vibrant orange color. As it reacts with the ethanol, the Chromium is reduced to the +3 oxidation state (Cr³⁺). This transition results in a dramatic color change from orange to green. While color changes in nature can be caused by many factors—such as the red or brown hues of an algal bloom Shankar IAS Academy, Aquatic Ecosystem, p.39—in this specific laboratory setting, the shift is a precise indicator of the reduction of a transition metal complex.

This reaction is not just qualitative; it is quantitative. The intensity of the green color produced is directly proportional to the amount of ethanol present in the breath sample. By measuring this change, law enforcement can approximate a person's level of intoxication. Interestingly, while the Chromium (III) produced is relatively stable, its parent form, Chromium VI, is noted for being extremely toxic and capable of causing DNA damage Shankar IAS Academy, Environmental Pollution, p.93. This chemistry mirrors other metal-based reactions we see in science, such as the formation of blue-green copper(II) chloride when copper oxide reacts with acid NCERT Class X, Acids, Bases and Salts, p.21.

Sources: Shankar IAS Academy, Aquatic Ecosystem, p.39; Shankar IAS Academy, Environmental Pollution, p.93; NCERT Class X, Acids, Bases and Salts, p.21

7. Solving the Original PYQ (exam-level)

This question is a perfect application of the Redox chemistry and Transition Metal properties you have just mastered. In the UPSC Science and Technology section, examiners often bridge the gap between theoretical chemistry and real-world applications like the breathalyzer test. You have learned that Sodium Dichromate acts as a powerful oxidizing agent in an acidic medium. When it encounters a reducing agent like ethanol (the alcohol in breath), a specific electron transfer occurs. This causes the Chromium atom to change its oxidation state, leading to a distinct electronic transition that shifts the solution's appearance across the visible spectrum.

To arrive at the correct answer, think through the chemical transformation step-by-step. The starting Dichromate ion (Cr₂O₇²⁻), where Chromium exists in a +6 oxidation state, is characterized by a vibrant orange color. As the ethanol is oxidized to acetic acid, the Chromium is simultaneously reduced to the Chromium(III) ion (Cr³⁺). As you recall from your study of NCERT Chemistry Class XII: d-and f-Block Elements, the Cr³⁺ ion in an aqueous environment is famously green. Therefore, the visual confirmation of alcohol presence is the transition from orange to green, which makes (A) the correct choice.

UPSC frequently uses "distractor" options to test the precision of your memory. Option (B), orange to colourless, is a common trap for students who confuse this reaction with a Potassium Permanganate (KMnO₄) titration, where the deep purple solution turns colorless. Option (C), yellow to orange, targets students who vaguely remember the Chromate-Dichromate equilibrium—which is a pH-dependent shift—but forget that a redox reaction involving alcohol must result in the formation of the green Cr³⁺ ion. By focusing on the specific oxidation state change of Chromium, you can avoid these traps and confidently identify the green signature of a positive test.