The chemical used as a ‘fixer’ in photography is

1. Common Chemicals in Daily Life (basic)

Everyday life is essentially a series of chemical interactions. To understand these, we must first distinguish between elements (pure substances like Iron or Sulfur) and compounds (substances formed when elements chemically combine, like Water). For instance, if water were a simple mixture of Hydrogen and Oxygen gases rather than a chemical compound (H₂O), it would behave as a highly flammable gas rather than the life-sustaining liquid we drink Science, Class VIII, NCERT (Revised ed 2025), Nature of Matter: Elements, Compounds, and Mixtures, p.132. In our homes, we frequently encounter complex carbon-based compounds such as Methane (CH₄), the simplest hydrocarbon, and various alcohols like ethanol Science, class X (NCERT 2025 ed.), Carbon and its Compounds, p.64, 67. Among the most versatile chemicals in daily use are sodium-based salts. Sodium Carbonate (Na₂CO₃), commonly known as washing soda, is a staple in the glass, soap, and paper industries. Domestically, it serves as a powerful cleaning agent and is specifically used to remove the permanent hardness of water Science, class X (NCERT 2025 ed.), Acids, Bases and Salts, p.32. When sodium carbonate or sodium hydrogencarbonate (baking soda) reacts with acids like ethanoic acid (vinegar), they release Carbon Dioxide (CO₂), a reaction often used in baking to make dough rise Science, class X (NCERT 2025 ed.), Carbon and its Compounds, p.74. Beyond cleaning and cooking, chemistry plays a critical role in specialized applications like photography. In the process of developing film, a chemical 'fixer' is required to make the image permanent. The most traditional chemical used for this is Sodium Thiosulfate (historically known as 'Hypo'). Its job is to dissolve and remove the unreacted silver halide crystals from the film after it has been exposed to light; without this step, the entire photograph would eventually turn black when exposed to sunlight. By forming soluble complexes, the fixer ensures the captured image remains stable and clear.

Sources: Science, Class VIII, NCERT (Revised ed 2025), Nature of Matter: Elements, Compounds, and Mixtures, p.132; Science, class X (NCERT 2025 ed.), Carbon and its Compounds, p.64, 67, 74; Science, class X (NCERT 2025 ed.), Acids, Bases and Salts, p.32

2. Properties and Types of Salts (basic)

In chemistry, a salt is not just the white powder on your dinner table; it is a broad class of ionic compounds formed when an acid and a base neutralize each other. This reaction, known as neutralization, replaces the hydrogen ions of an acid with metal ions or other cations Science, Class X (NCERT 2025 ed.), Acids, Bases and Salts, p.34. For instance, the common salt we consume, Sodium Chloride (NaCl), is formed from the chemical combination of sodium (a reactive metal) and chlorine (a hazardous gas), resulting in a stable compound essential for life Science, Class VIII (NCERT 2025 ed.), Nature of Matter: Elements, Compounds, and Mixtures, p.124.

One of the most important properties of salts to master for the UPSC is their pH nature. Many students mistakenly assume all salts are neutral (pH 7), but the reality depends on the "strength" of the parent acid and base that formed them. By testing salt solutions with litmus paper or pH indicators, we can categorize them into three types:

Type of Salt	Parent Components	pH Level	Example
Neutral	Strong Acid + Strong Base	Exactly 7	Sodium Chloride (NaCl), Potassium Nitrate (KNO₃)
Acidic	Strong Acid + Weak Base	Less than 7	Ammonium Chloride (NH₄Cl), Aluminium Chloride (AlCl₃)
Basic	Weak Acid + Strong Base	More than 7	Sodium Carbonate (Na₂CO₃), Sodium Acetate

Beyond pH, salts possess a fascinating structural property called water of crystallisation. This refers to the fixed number of water molecules chemically bonded within one formula unit of a salt crystal Science, Class X (NCERT 2025 ed.), Acids, Bases and Salts, p.34. For example, Copper Sulphate crystals appear blue because they contain five molecules of water (CuSO₄·5H₂O); if you heat them, the water evaporates, and the salt turns white. This "applied" side of salt chemistry explains why certain salts are used in industrial processes, from water purification to traditional film photography.

Sources: Science, Class X (NCERT 2025 ed.), Acids, Bases and Salts, p.29; Science, Class X (NCERT 2025 ed.), Acids, Bases and Salts, p.34; Science, Class VIII (NCERT 2025 ed.), Nature of Matter: Elements, Compounds, and Mixtures, p.124

3. Industrial Applications of Sulphur Compounds (intermediate)

Sulphur is often called the 'backbone of industry' because its compounds are involved in almost every manufacturing process. At the heart of this is Sulphuric Acid (H₂SO₄), frequently termed the 'King of Chemicals.' Its reactivity is utilized in soda-acid fire extinguishers, where it reacts with sodium hydrogencarbonate (NaHCO₃) to rapidly release CO₂ gas, which smothers flames Science, Class X (NCERT 2025 ed.), Acids, Bases and Salts, p.36. Beyond the acid itself, various sulphate salts—such as potassium sulphate and magnesium sulphate (Epsom salts)—serve critical roles in agriculture as fertilizers and in medicine Science, Class X (NCERT 2025 ed.), Acids, Bases and Salts, p.28.

In our daily hygiene, sulphur compounds have revolutionized cleaning. While traditional soaps struggle in 'hard water' (water containing calcium and magnesium ions), detergents remain highly effective. Detergents are typically sodium salts of sulphonic acids. Unlike soaps, the charged ends of these sulphur-based molecules do not form insoluble precipitates (scum) with the minerals in hard water, allowing them to remain soluble and maintain their cleansing power in shampoos and laundry products Science, Class X (NCERT 2025 ed.), Carbon and its Compounds, p.76.

One of the most specialized industrial applications of sulphur is found in photographic processing. To make a photographic image permanent, it must be 'fixed.' Sodium thiosulfate (traditionally known as 'Hypo') or ammonium thiosulfate is used as a 'fixer.' Its job is to dissolve and remove unreacted silver halide crystals from the film or paper. By forming soluble silver–thiosulfate complexes, the fixer prevents the remaining silver from darkening when exposed to light, effectively 'locking' the image in place. Additionally, on a biological scale, sulphur is essential for life; it is taken up by plants as sulphates and converted into sulphur-bearing amino acids, which are the building blocks of proteins Environment, Shankar IAS Academy (10th ed.), Functions of an Ecosystem, p.21.

Sources: Science, Class X (NCERT 2025 ed.), Acids, Bases and Salts, p.28, 36; Science, Class X (NCERT 2025 ed.), Carbon and its Compounds, p.76; Environment, Shankar IAS Academy (10th ed.), Functions of an Ecosystem, p.21

4. Silver Salts and Light Sensitivity (intermediate)

At the heart of traditional photography lies a fascinating chemical property: the extreme sensitivity of silver halides to light. Silver halides are compounds formed between silver and elements from the halogen group, most notably chlorine and bromine. These salts, such as silver chloride (AgCl) and silver bromide (AgBr), possess a unique crystal lattice that is easily disrupted by photons (light energy). When exposed to sunlight or even artificial light, these compounds undergo a photochemical decomposition reaction, essentially breaking apart into their constituent elements.

To visualize this, imagine placing a small amount of white silver chloride in a china dish under direct sunlight. Over a short period, you will observe the white powder turning into a dull grey color Science, class X (NCERT 2025 ed.), Chemical Reactions and Equations, p.9. This color change is the visual evidence of a chemical transformation: the light energy provides enough power to strip electrons and break the bonds, causing the silver ions to reduce into metallic silver while the chlorine is released as a gas. The chemical equation for this process is:

2AgCl(s) + Light → 2Ag(s) + Cl₂(g)

This exact principle was the foundation of black and white photography Science, class X (NCERT 2025 ed.), Chemical Reactions and Equations, p.10. Photographic film was coated with an emulsion containing these light-sensitive crystals. When the camera shutter opened, light would strike the film, causing a microscopic amount of decomposition in proportion to the brightness of the scene. This created a "latent image" of metallic silver particles. Because these reactions absorb energy to proceed, they are classified as endothermic reactions, specifically powered by light rather than heat or electricity.

Sources: Science, class X (NCERT 2025 ed.), Chemical Reactions and Equations, p.9; Science, class X (NCERT 2025 ed.), Chemical Reactions and Equations, p.10

5. Borax and Cleaning Agents (intermediate)

Borax, chemically known as sodium tetraborate decahydrate (Na₂B₄O₇·10H₂O), is a naturally occurring mineral and a powerhouse in the world of domestic and industrial chemistry. Often found in the evaporite deposits of salt lakes, such as those in the Mojave Desert, it is a versatile salt that has been used for centuries as a cleaning agent, disinfectant, and even a flux in metallurgy Certificate Physical and Human Geography, Lakes, p.86. Interestingly, while it is mined naturally, it is also chemically linked to other common household chemicals; for instance, sodium carbonate (washing soda) is a primary raw material used in the industrial manufacture of borax Science, class X (NCERT 2025 ed.), Acids, Bases and Salts, p.32.

At the molecular level, borax acts as an effective cleaning agent due to its alkaline nature. When dissolved in water, it creates a basic solution that helps to neutralize acidic stains (like those from oils or certain foods) and facilitates the emulsification of fats. Furthermore, borax serves as a water softener. Hard water contains high concentrations of calcium and magnesium ions which react with soap to form an insoluble "scum," making cleaning difficult. Borax helps by chelating these metal ions, preventing them from interfering with the cleaning process and allowing detergents to work more efficiently Science, class X (NCERT 2025 ed.), Carbon and its Compounds, p.76.

Beyond simple pH adjustment, borax has a unique trick up its sleeve: when added to hot water, it undergoes a chemical reaction that converts some water molecules into hydrogen peroxide (H₂O₂). This makes it an excellent natural bleaching agent and disinfectant. It is frequently used as a "laundry booster" because it enhances the stain-removing power of standard soaps. However, chemistry is only half the battle; as noted in practical cleaning methods, physical agitation—whether by hand or machine—is still essential to physically dislodge the dirt particles that the borax has helped to loosen from the fabric fibers Science, class X (NCERT 2025 ed.), Carbon and its Compounds, p.76.

Sources: Certificate Physical and Human Geography, Lakes, p.86; Science, class X (NCERT 2025 ed.), Acids, Bases and Salts, p.32; Science, class X (NCERT 2025 ed.), Carbon and its Compounds, p.76

6. The Chemistry of Traditional Photography (exam-level)

At its heart, traditional photography is a brilliant application of photochemistry—the study of chemical reactions triggered by light. The process begins with a film or paper coated in a light-sensitive emulsion containing silver halides, primarily silver bromide (AgBr) or silver chloride (AgCl). These compounds are unique because they undergo a photochemical decomposition reaction. When light strikes the film, the energy from the photons breaks the chemical bonds in the silver halide, producing tiny particles of metallic silver. As noted in Science, Class X (NCERT 2025 ed.), Chemical Reactions and Equations, p.10, this is an endothermic reaction where light energy is absorbed to facilitate the breakdown: 2AgBr(s) → 2Ag(s) + Br₂(g).

While light creates the initial 'latent image' by producing these silver specks, a significant amount of unreacted silver halide remains on the film. If you were to take the film into the sunlight at this stage, the remaining AgBr would also decompose, causing the entire image to turn black or 'fog.' To prevent this and make the image permanent, we must 'fix' the photograph. This is done using a chemical known as a fixer, most commonly sodium thiosulfate (Na₂S₂O₃), which was historically called 'hypo.'

The chemistry of 'fixing' involves a double displacement and complexation reaction. The sodium thiosulfate reacts with the remaining, unexposed silver halide to form a soluble complex. In this reaction, silver ions from the AgBr are sequestered into a stable, water-soluble complex: Na₃[Ag(S₂O₃)₂]. Because this complex is soluble, it can be easily washed away with water, leaving behind only the stable metallic silver that forms the actual black-and-white image. This step is crucial for the longevity of the photograph, ensuring that the 'whites' of the image stay white even when exposed to light later. This transformation of ions is a sophisticated example of the exchange reactions discussed in Science, Class X (NCERT 2025 ed.), Chemical Reactions and Equations, p.12.

Sources: Science, Class X (NCERT 2025 ed.), Chemical Reactions and Equations, p.10; Science, Class X (NCERT 2025 ed.), Chemical Reactions and Equations, p.12

7. Sodium Thiosulphate: The 'Hypo' Fixer (exam-level)

Sodium Thiosulphate (Na₂S₂O₃), commonly known in the world of photography as 'Hypo', is a fascinating salt that plays a critical role in preserving images. While we often study salts like sodium chloride or sodium sulphate in the context of their chemical formulas and pH values Science, Class X (NCERT 2025 ed.), Acids, Bases and Salts, p.28, sodium thiosulphate stands out for its unique ability to dissolve specific metal compounds that are otherwise insoluble in water.

The primary application of 'Hypo' is as a photographic fixer. In traditional film photography, the film is coated with light-sensitive silver halides (usually silver bromide, AgBr). When you take a photo, only some of these silver halide crystals react to light. After the image is developed, the unreacted silver halides remain on the film. If they aren't removed, they will eventually react with ambient light, causing the photo to fog or turn completely black. Sodium thiosulphate solves this by reacting with the insoluble silver bromide to form a soluble silver-thiosulphate complex. This complex can then be easily washed away with water, leaving behind only the stable, metallic silver image.

Beyond the darkroom, sodium thiosulphate is a versatile chemical tool. It is used in water treatment as a dechlorinating agent to neutralize excess chlorine, similar to how other sodium compounds are utilized for specific industrial tasks like softening hard water Science, Class X (NCERT 2025 ed.), Acids, Bases and Salts, p.33. It is also an essential reagent in analytical chemistry for iodometric titrations and serves a life-saving role in medicine as an antidote for cyanide poisoning. Its ability to act as a reducing agent makes it a staple in 'Applied Chemistry'—the science of using chemical properties to solve real-world problems.

Sources: Science, Class X (NCERT 2025 ed.), Acids, Bases and Salts, p.28; Science, Class X (NCERT 2025 ed.), Acids, Bases and Salts, p.33

8. Solving the Original PYQ (exam-level)

To solve this question, you must bridge your knowledge of coordination compounds and the light-sensitivity of silver halides. In photography, the film is coated with silver bromide, which reacts to light; however, once the image is captured, any unreacted silver halide remaining on the film would continue to darken if exposed to light, eventually ruining the picture. The process of 'fixing' involves making the image permanent by removing these excess salts. As you learned in the study of complex salts, this requires a reagent that can transform insoluble silver halides into a soluble coordination complex that can be easily washed away with water.

Walking through the reasoning, the substance historically and scientifically recognized for this role is sodium thiosulphate (Na₂S₂O₃), commonly known in darkroom chemistry as 'hypo'. When the film is treated with this solution, the thiosulphate ions act as ligands, bonding with the silver ions to form a stable, water-soluble complex. Therefore, Option (B) is the correct answer. This application is a classic example of how chemical properties like complexation are applied in industrial and artistic processes, a favorite theme for UPSC in the General Science section.

It is important to avoid the common traps found in the other options. UPSC often uses similar-sounding chemical names to test your precision. Sodium sulphate is a simple salt used in detergents but lacks the complexing ability required here. Ammonium persulphate is a powerful oxidizing agent used in etching printed circuit boards, not fixing film. Finally, borax is frequently used as a cleaning agent or a pH buffer in developers, but it cannot dissolve silver halides. Understanding these distinctions helps you move beyond rote memorization to a conceptual clarity often emphasized in NCERT Class 12 Chemistry.