Which one of the following is not a mixture ?

1. Classification of Matter: Pure Substances vs. Mixtures (basic)

To understand the material world, scientists use the process of classification. Much like we categorize economic activities into sectors to analyze growth, we classify matter to understand its chemical behavior Understanding Economic Development. Class X, SECTORS OF THE INDIAN ECONOMY, p.32. At the most fundamental level, matter is divided into two categories: Pure Substances and Mixtures.

A Pure Substance is a form of matter that consists of the same type of particles throughout. For a chemist, "pure" means more than just being clean; it means the substance cannot be separated into other kinds of matter by any physical process like filtration or evaporation Science, Class VIII, Nature of Matter: Elements, Compounds, and Mixtures, p.121. Pure substances are further divided into Elements (simplest forms like Oxygen or Gold) and Compounds (substances like Baking Soda—NaHCO₃—where elements are chemically combined in a fixed ratio) Science, Class VIII, Nature of Matter: Elements, Compounds, and Mixtures, p.130.

In contrast, a Mixture consists of two or more substances physically blended together without reacting chemically. In a mixture, each component retains its individual properties. For example, in a solution like vinegar (acetic acid in water) or a complex formulation like soap, the ingredients are mixed but not chemically transformed into a single new type of particle Science, Class VIII, Nature of Matter: Elements, Compounds, and Mixtures, p.130. Because they are only physically combined, mixtures can typically be separated back into their original parts using physical methods.

Feature	Pure Substance	Mixture
Composition	Fixed and uniform (e.g., H₂O is always 2H:1O).	Variable; components can be present in any ratio.
Properties	Distinct and different from its constituent elements.	Reflects the properties of its individual components.
Separation	Only by chemical or electrochemical reactions.	Physical methods (filtration, boiling, magnetism).

Sources: Understanding Economic Development. Class X, SECTORS OF THE INDIAN ECONOMY, p.32; Science, Class VIII, Nature of Matter: Elements, Compounds, and Mixtures, p.121; Science, Class VIII, Nature of Matter: Elements, Compounds, and Mixtures, p.130

2. Elements and Compounds: The Building Blocks (basic)

To understand the universe at its most fundamental level, we must distinguish between the ingredients and the recipes of matter. In chemistry, we start with pure substances—materials that consist of only one type of particle. These are broadly classified into two categories: elements and compounds. An element is the most basic form of matter that cannot be broken down into simpler substances by ordinary chemical reactions. Think of them as the individual letters of the chemical alphabet. Each element, such as Gold, Sulfur, or Oxygen, is made of identical particles called atoms, which are unique to that specific element Science, Class VIII, NCERT (Revised ed 2025), Chapter 8, p.123.

When these "letters" combine to form "words," we get compounds. A compound is formed when two or more different elements chemically combine in a fixed ratio. This is a critical distinction: in a compound like water (H₂O), the hydrogen and oxygen atoms are so tightly bonded that they cannot be separated by physical methods like filtering or boiling Science, Class VIII, NCERT (Revised ed 2025), Chapter 8, p.124. Most importantly, a compound often has entirely different properties from the elements that make it up. For instance, Hydrogen is a highly flammable gas and Oxygen supports combustion, yet when they bond to form the compound Water, the result is a liquid used to extinguish fires!

Feature	Element	Compound
Composition	Made of only one type of atom.	Made of two or more types of atoms chemically bonded.
Breakdown	Cannot be simplified further.	Can be broken down into elements via chemical reactions.
Properties	Unique to that specific atom.	Entirely different from its constituent elements.
Ratio	N/A (Single substance).	Elements are always present in a fixed, definite ratio Science, Class VIII, NCERT (Revised ed 2025), Chapter 8, p.131.

In our daily lives, we encounter these in various forms. For example, Baking Soda (sodium bicarbonate, NaHCO₃) is a pure chemical compound because it has a fixed composition and specific chemical properties. In contrast, items like toothpaste or vinegar are usually mixtures, where different substances are put together without chemically bonding into a new, single structure Science, Class VIII, NCERT (Revised ed 2025), Chapter 8, p.132.

Sources: Science, Class VIII, NCERT (Revised ed 2025), Chapter 8: Nature of Matter: Elements, Compounds, and Mixtures, p.123; Science, Class VIII, NCERT (Revised ed 2025), Chapter 8: Nature of Matter: Elements, Compounds, and Mixtures, p.124; Science, Class VIII, NCERT (Revised ed 2025), Chapter 8: Nature of Matter: Elements, Compounds, and Mixtures, p.131; Science, Class VIII, NCERT (Revised ed 2025), Chapter 8: Nature of Matter: Elements, Compounds, and Mixtures, p.132

3. Understanding Solutions and Solubility (basic)

Hello there! Today we are diving into the heart of chemistry by exploring how substances interact to form mixtures. At its simplest, a solution is a uniform or homogeneous mixture of two or more substances. Unlike a simple pile of sand and pebbles, the components of a solution are so thoroughly mixed that they appear as a single phase. Think of salt dissolved in water or the very air you breathe—which is actually a solution of various gases Science, Class VIII, Chapter: The Amazing World of Solutes, Solvents, and Solutions, p.135.

To master this concept, you must distinguish between the two parts of a solution. The solvent is the substance present in the larger amount (the 'host'), while the solute is the substance present in the smaller amount (the 'guest'). While we often think of water as the universal solvent, solutions can exist in any state of matter. For instance, in our atmosphere, Nitrogen acts as the solvent because it makes up about 78% of the air, while Oxygen and Carbon Dioxide are the solutes Science, Class VIII, Chapter: The Amazing World of Solutes, Solvents, and Solutions, p.149.

Component	Role	Example (Salt Water)
Solute	The substance that is dissolved (usually smaller quantity).	Salt
Solvent	The medium that does the dissolving (usually larger quantity).	Water

The journey of a solute into a solvent has a limit, which we call solubility. If you keep adding sugar to a glass of tea, you will eventually reach a point where no more sugar can dissolve at that temperature; this is a saturated solution. If more can still be dissolved, it is unsaturated. Interestingly, temperature plays a vital role here: generally, you can dissolve more solid (like sugar) if you heat the liquid, but the opposite is true for gases—the solubility of gases in liquids actually decreases as the temperature rises Science, Class VIII, Chapter: The Amazing World of Solutes, Solvents, and Solutions, p.149.

Finally, we describe solutions using the term concentration. A dilute solution has very little solute compared to the solvent, while a concentrated solution has a high amount of solute. In specialized contexts like geography or advanced chemistry, we even look at the concentration of specific ions, such as hydrogen ions (H⁺), to determine if a solution is acidic or basic Physical Geography by PMF IAS, Geomorphic Movements, p.90.

Sources: Science, Class VIII (Revised ed 2025), The Amazing World of Solutes, Solvents, and Solutions, p.135; Science, Class VIII (Revised ed 2025), The Amazing World of Solutes, Solvents, and Solutions, p.149; Physical Geography by PMF IAS, Geomorphic Movements, p.90

4. Colloids and Suspensions in Daily Life (intermediate)

To understand the matter around us, we must first distinguish between pure substances and mixtures. While a pure compound like baking soda (NaHCO₃) consists of identical particles throughout, most daily items like toothpaste, soap, or vinegar are complex mixtures where different substances are physically combined Science Class VIII, Nature of Matter, p.132. Mixtures are categorized primarily by their particle size. In a suspension, particles are large enough to be seen with the naked eye and will eventually settle at the bottom if left undisturbed, such as sand in water. In contrast, colloids are the 'middle ground' of chemistry; their particles are larger than those in a true solution but small enough to remain suspended indefinitely without settling.

One of the most distinctive features of a colloid is the Tyndall Effect. In a true solution (like salt water), a beam of light passes through invisibly. However, in a colloidal solution, the particles are large enough to scatter light, making the path of the beam visible Science Class X, The Human Eye and the Colourful World, p.169. This phenomenon explains why we can see sunlight streaming through a canopy of trees in a misty forest—the tiny water droplets in the mist act as colloidal particles. The color of this scattered light is a function of particle size: very fine particles scatter shorter wavelengths like blue light, while larger particles scatter longer wavelengths Science Class X, The Human Eye and the Colourful World, p.169.

In our environment, colloids often appear as aerosols—solid or liquid particles suspended in a gas, typically ranging from 1/100 micrometer to 1 micrometer in diameter Environment and Ecology, Major Crops and Cropping Patterns in India, p.98. These include naturally occurring mist and clouds, as well as atmospheric pollutants like brown carbon. Brown carbon is a light-absorbing organic aerosol that has become a significant focus in climate research because of how its colloidal particles interact with solar radiation Environment, Climate Change, p.258. Understanding these mixtures is essential for everything from industrial formulation to environmental science.

Feature	Solution	Colloid	Suspension
Particle Size	Smallest (< 1 nm)	Intermediate (1-1000 nm)	Largest (> 1000 nm)
Tyndall Effect	No	Yes	Yes (until settled)
Stability	Highly Stable	Stable	Unstable (settles)

Sources: Science Class VIII, Nature of Matter: Elements, Compounds, and Mixtures, p.132; Science Class X, The Human Eye and the Colourful World, p.169; Environment and Ecology (Majid Hussain), Major Crops and Cropping Patterns in India, p.98; Environment (Shankar IAS), Climate Change, p.258

5. Common Organic Acids and Bases (intermediate)

In our daily lives, we encounter a variety of organic acids, which are naturally occurring acidic compounds found in plants and animals. Unlike strong mineral acids (such as Hydrochloric acid) that ionize completely in water, organic acids—specifically the group known as carboxylic acids—are weak acids. This means they only partially dissociate to release hydrogen ions (H⁺) when dissolved Science, Class X, Carbon and its Compounds, p.73. For example, Ethanoic acid (commonly called acetic acid) is a vital organic acid; its pure form is known as Glacial Acetic Acid because its melting point is 290 K, often causing it to freeze into ice-like crystals in cold climates Science, Class X, Carbon and its Compounds, p.73.

Many common household substances are actually dilute solutions of these acids. Vinegar, for instance, is a 5–8% mixture of acetic acid in water and is widely used as a preservative Science, Class X, Carbon and its Compounds, p.73. Below is a reference for other common natural sources and the acids they contain:

Natural Source	Organic Acid Present
Lemon / Orange	Citric acid
Tamarind	Tartaric acid
Tomato	Oxalic acid
Sour milk (Curd)	Lactic acid
Ant sting / Nettle sting	Methanoic acid

Science, Class X, Acids, Bases and Salts, p.28

To identify the presence of these acids, we can observe their reaction with bases or carbonates. A classic test involves adding Baking Soda (sodium hydrogen carbonate) to an acid like vinegar. This results in a chemical reaction that produces Carbon Dioxide (CO₂), visible as fizzing bubbles. If this gas is passed through lime water, the lime water turns milky, confirming the identity of the gas Science, Class VII, Changes Around Us: Physical and Chemical, p.61. Understanding these reactions is crucial because while organic acids are mild, they are reactive enough to cause the stinging sensation in ant bites or the souring of milk.

Sources: Science, Class X (NCERT 2025), Carbon and its Compounds, p.73; Science, Class X (NCERT 2025), Acids, Bases and Salts, p.26-28; Science, Class VII (NCERT 2025), Changes Around Us: Physical and Chemical, p.61

6. Important Chemical Compounds in Households (exam-level)

In our daily lives, we are surrounded by chemical substances that we often identify by their common names rather than their scientific identities. To master this for the exam, we must distinguish between pure chemical compounds (which have a fixed chemical formula) and mixtures (which are combinations of substances that retain their individual properties). For instance, while Baking Soda is a pure compound known as Sodium Bicarbonate (NaHCO₃), items like toothpaste or vinegar are actually mixtures or solutions of multiple components Science, Class VIII NCERT (2025 ed.), Nature of Matter: Elements, Compounds, and Mixtures, p.132.

One of the most versatile household compounds is Sodium Hydrogencarbonate (Baking Soda). It is inherently alkaline, which makes it an effective antacid for neutralizing stomach acidity. In the kitchen, it is a key component of Baking Powder. It is vital to note that baking powder is a mixture of baking soda and a mild edible acid like tartaric acid. When heated or mixed with water, it releases Carbon Dioxide (CO₂) gas, which causes bread and cakes to rise, giving them a soft, spongy texture Science, Class X NCERT (2025 ed.), Acids, Bases and Salts, p.31. Beyond the kitchen, its ability to release CO₂ also makes it useful in soda-acid fire extinguishers.

When it comes to cleaning, we see a fascinating distinction between Soaps and Detergents. Soaps are sodium or potassium salts of long-chain carboxylic acids (fatty acids). They work by forming micelles—spherical structures where the hydrophobic "tail" attaches to oil/dirt and the hydrophilic "head" interacts with water Science, Class X NCERT (2025 ed.), Carbon and its Compounds, p.75. However, soaps perform poorly in hard water because they react with calcium and magnesium ions to form an insoluble precipitate called "scum." This is why we use Detergents, which are typically sodium salts of sulphonic acids. Detergents do not form insoluble precipitates with hard water ions, allowing them to remain effective for cleaning clothes even in challenging water conditions Science, Class X NCERT (2025 ed.), Carbon and its Compounds, p.76.

Compound/Mixture	Chemical Name / Nature	Primary Household Use
Baking Soda	Sodium Bicarbonate (NaHCO₃)	Antacid, cooking, fire extinguishers
Vinegar	Dilute Acetic Acid (CH₃COOH) in water	Preservative, flavoring
Soap	Sodium salts of fatty acids	Cleaning in soft water
Detergent	Sodium salts of sulphonic acids	Cleaning in hard water

Sources: Science, Class VIII NCERT (2025 ed.), Nature of Matter: Elements, Compounds, and Mixtures, p.132; Science, Class X NCERT (2025 ed.), Acids, Bases and Salts, p.31; Science, Class X NCERT (2025 ed.), Carbon and its Compounds, p.75-76

7. Solving the Original PYQ (exam-level)

To solve this question, you must apply the fundamental distinction between pure substances and mixtures. In your recent modules, you learned that while a mixture consists of two or more substances that are physically combined and retain their individual properties, a compound is a substance formed when two or more elements are chemically bonded in a fixed ratio. The key to cracking UPSC science questions is identifying which option represents a single chemical entity with a specific molecular formula rather than a commercial formulation or a solution.

Walking through the options, we analyze their chemical nature. Baking soda stands out because it is the common name for sodium bicarbonate (NaHCO₃). Because it has a fixed chemical composition and its constituent elements (Sodium, Hydrogen, Carbon, and Oxygen) are chemically bonded, it is classified as a compound. Therefore, (C) Baking soda is the correct answer. Unlike mixtures, its properties are entirely different from the elements that form it, and it cannot be separated by simple physical means, which is a hallmark of a pure substance.

The other options are classic UPSC "real-world" traps designed to test your application of theory. Toothpaste and Toilet soap are complex formulations—mixtures of various surfactants, abrasives, binders, and fragrances. Crucially, Vinegar is often mistaken for a pure liquid, but it is actually a homogeneous mixture (a solution) of acetic acid diluted in water. As highlighted in Science, Class VIII NCERT (Revised ed 2025), mixtures like these do not have a fixed chemical formula, which distinguishes them from the pure, single-substance nature of a compound like baking soda.