Which one of the following is a compound ?

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

In our daily lives, we use the word 'pure' to describe things like 'pure honey' or 'pure ghee' to mean they aren't adulterated. However, in the world of science, matter is classified much more strictly based on its chemical composition. To a scientist, matter is either a Pure Substance or a Mixture.

A Pure Substance consists of only one type of particle. These particles behave identically and have a fixed chemical composition throughout Science, Class VIII. NCERT, Chapter 8: Nature of Matter: Elements, Compounds, and Mixtures, p.121. Pure substances are further divided into Elements (like Gold or Oxygen) and Compounds (like Water or Sugar). A compound is unique because it is made of different elements chemically bonded in a fixed ratio. For example, every molecule of sugar (C₁₂H₂₂O₁₁) is exactly the same, making it a pure substance Science, Class VIII. NCERT, Chapter 8: Nature of Matter: Elements, Compounds, and Mixtures, p.130.

In contrast, most things we touch and see—like the air we breathe, the soil in a garden, or the milk we drink—are Mixtures. A mixture contains two or more substances that are physically combined but not chemically bonded. Because they aren't bonded, the individual components in a mixture retain their own original properties Science, Class VIII. NCERT, Chapter 8: Nature of Matter: Elements, Compounds, and Mixtures, p.130. For instance, in a salt-water mixture, the salt still tastes salty and the water is still liquid. This lack of chemical bonding allows us to separate mixtures using physical methods like filtration or evaporation.

Feature	Pure Substance	Mixture
Composition	Fixed and uniform throughout.	Variable; components can be in any ratio.
Properties	Different from its constituent elements.	Retains properties of its components.
Separation	Cannot be separated by physical means.	Can be separated by physical processes.

Sources: Science, Class VIII. NCERT, Chapter 8: Nature of Matter: Elements, Compounds, and Mixtures, p.117; Science, Class VIII. NCERT, Chapter 8: Nature of Matter: Elements, Compounds, and Mixtures, p.121; Science, Class VIII. NCERT, Chapter 8: Nature of Matter: Elements, Compounds, and Mixtures, p.130

2. Elements: The Simplest Form of Matter (basic)

Imagine you have a piece of pure gold. If you keep dividing it into smaller and smaller pieces, you eventually reach a point where you cannot break it down further into anything that isn't gold. This fundamental substance is what we call an element. In chemistry, an element is the simplest form of matter that cannot be broken down into simpler substances by chemical reactions. They are the basic building blocks of everything in the universe, from the air you breathe to the device you are holding Science-Class VII, The World of Metals and Non-metals, p.53.

Currently, there are 118 known elements. While some occur naturally in the earth's crust or atmosphere (like Oxygen or Iron), others are artificially created in laboratories. Every element is unique because it consists of only one type of atom. When we look at a chemical equation, such as one involving Zinc (Zn) or Hydrogen (H), we treat these elements as the primary units that rearrange themselves but do not change their fundamental identity Science, Class X, Chemical Reactions and Equations, p.3.

To make sense of these 118 building blocks, scientists classify them based on their properties. The most common classification is into metals and non-metals Science, Class X, Metals and Non-metals, p.40.

• Metals: Usually shiny, hard, and good conductors (e.g., Iron, Gold, Zinc).
• Non-metals: Often gases or brittle solids (e.g., Oxygen, Nitrogen, Carbon).

It is important to distinguish these pure elements from everyday materials like plastic, glass, or wood. Those materials are complex combinations, whereas elements like Oxygen (O₂) or Carbon (C) are the pure, uncombined "alphabet" of the chemical world Science-Class VII, The World of Metals and Non-metals, p.53.

Sources: Science-Class VII, The World of Metals and Non-metals, p.53; Science, Class X, Chemical Reactions and Equations, p.3; Science, Class X, Metals and Non-metals, p.40

3. Compounds: Chemical Bonds and Fixed Proportions (intermediate)

In our journey through the building blocks of the universe, we arrive at one of nature's most transformative processes: the formation of a chemical compound. Unlike a simple mixture where substances are just physically tossed together, a compound represents a chemical "marriage." When two or more elements combine chemically, they form a new substance with a fixed composition and properties that are often radically different from the original elements. For instance, while hydrogen is an explosive gas and oxygen supports combustion, their chemical union results in water (H₂O), a liquid used to extinguish fires. This change in identity is the hallmark of a compound Science, Class VIII (Revised ed 2025), Nature of Matter: Elements, Compounds, and Mixtures, p.130.

A defining rule of compounds is the Law of Constant Proportions. This means that in a pure chemical compound, the constituent elements are always present in fixed proportions by mass. Take Sucrose (C₁₂H₂₂O₁₁), common table sugar, as an example. Every single molecule of sugar, whether it comes from a cane in India or a beet in Europe, contains exactly 12 carbon atoms, 22 hydrogen atoms, and 11 oxygen atoms chemically bonded together Science, Class X (NCERT 2025 ed.), Carbon and its Compounds, p.67. If you were to change these ratios even slightly, you would no longer have sugar; you would have an entirely different substance.

To master this for the UPSC, you must distinguish between a compound and a mixture. In a mixture like tea or coffee, the ingredients (water, sugar, caffeine, tannins) retain their individual chemical identities and can be mixed in varying amounts. In contrast, a compound is a pure substance where the elements are locked together by chemical bonds and cannot be separated by simple physical methods like filtration or evaporation.

Feature	Compound (e.g., Sugar)	Mixture (e.g., Milk/Tea)
Composition	Fixed and definite ratio.	Variable ratio.
Properties	Entirely different from constituents.	Shows properties of constituents.
Separation	Only by chemical or electrochemical reactions.	By physical methods (filtration, boiling).

Sources: Science, Class VIII (Revised ed 2025), Nature of Matter: Elements, Compounds, and Mixtures, p.130-131; Science, Class X (NCERT 2025 ed.), Carbon and its Compounds, p.67-68

4. Homogeneous and Heterogeneous Mixtures (intermediate)

In our journey through the nature of matter, we find that most substances around us are not pure but are combinations of different substances. These are called mixtures. The defining characteristic of a mixture is that its components are not chemically bonded and retain their individual properties. However, how these components distribute themselves determines whether a mixture is homogeneous or heterogeneous.

A heterogeneous mixture is non-uniform in its composition. If you look at a sprout salad or a mixture of sand and salt, you can easily distinguish the different components with your naked eye or a magnifying glass Science, Class VIII, Nature of Matter: Elements, Compounds, and Mixtures, p.117. Because the particles are not spread evenly, one spoonful of the mixture might contain more of one component than the next. Common examples include muddy water, a bowl of cereal, or a mixture of oil and water.

On the other hand, a homogeneous mixture possesses a uniform composition throughout. The components are so thoroughly mixed that they cannot be seen separately, even under a powerful microscope Science, Class VIII, Nature of Matter: Elements, Compounds, and Mixtures, p.117. A classic example is sugar dissolved in water; once mixed, every drop of the liquid tastes equally sweet because the sugar molecules are evenly distributed. Interestingly, homogeneous mixtures aren't just liquids. Alloys, such as stainless steel (a mix of iron, nickel, and chromium) or brass, are solid homogeneous mixtures Science, Class X, Metals and Non-metals, p.54.

Feature	Homogeneous Mixture	Heterogeneous Mixture
Uniformity	Uniform throughout; looks like a single substance.	Non-uniform; different parts are visible.
Particle Distribution	Evenly distributed at the molecular level.	Unevenly distributed; often exists in different phases.
Visibility	Components are not visible separately.	Components are usually visible to the eye or microscope.
Examples	Salt solution, Air, Alloys (Brass).	Mixture of pulses, Chalk in water, Smoke.

Sources: Science, Class VIII. NCERT (Revised ed 2025), Nature of Matter: Elements, Compounds, and Mixtures, p.117; Science, Class X (NCERT 2025 ed.), Metals and Non-metals, p.54

5. Colloids and Solutions: Milk and Infusions (exam-level)

To understand the chemistry of everyday substances like milk, tea, or sugar, we must first distinguish between a chemical compound and a mixture. A compound, such as sugar (sucrose, C₁₂H₂₂O₁₁), is a pure substance formed when elements chemically bond in a fixed ratio. In a compound, the constituent elements lose their individual properties to create something entirely new. In contrast, mixtures are physical combinations of two or more substances where each component retains its chemical identity Science, Class VIII (NCERT 2025), Nature of Matter: Elements, Compounds, and Mixtures, p.130.

Substances like milk are fascinating because they are colloids—a specific type of heterogeneous mixture. While milk may look uniform to the naked eye, it is actually a complex suspension of tiny fat globules and protein molecules (casein) dispersed in water. These particles are large enough to scatter light, a phenomenon known as the Tyndall effect, which is a hallmark of colloidal systems Science, Class X (NCERT 2025), The Human Eye and the Colourful World, p.169. Unlike a compound, milk does not have a single chemical formula; its composition can vary depending on its source.

Similarly, beverages like tea and coffee are classified as infusions. These are mixtures created by extracting soluble compounds—such as caffeine, tannins, and volatile oils—from plant matter into a solvent (usually water). Because these components are physically mixed and can be separated (for instance, by evaporation or filtration), they remain mixtures rather than compounds Science, Class VIII (NCERT 2025), Nature of Matter: Elements, Compounds, and Mixtures, p.120.

Feature	Compound (e.g., Sugar)	Mixture (e.g., Milk, Tea)
Composition	Fixed ratio of elements.	Variable proportions.
Bonding	Chemical bonds formed.	Physical mixing; no new bonds.
Properties	Distinct from its elements.	Retains properties of components.

Sources: Science, Class VIII (NCERT 2025), Nature of Matter: Elements, Compounds, and Mixtures, p.120, 121, 130; Science, Class X (NCERT 2025), The Human Eye and the Colourful World, p.169

6. Common Chemical Names and Formulas (exam-level)

In our study of matter, we distinguish between substances based on how their atoms are organized. A chemical compound is a pure substance formed when two or more elements are chemically bonded in a fixed ratio. This fixed ratio is represented by a chemical formula, such as H₂O for water or C₁₂H₂₂O₁₁ for sucrose (common sugar). Crucially, the properties of a compound are entirely different from the elements that compose it. For instance, while hydrogen and oxygen are gases, they combine to form liquid water Science, Class VIII NCERT (Revised ed 2025), Nature of Matter: Elements, Compounds, and Mixtures, p.132. In contrast, a mixture is a physical combination of substances where each component retains its individual chemical identity and properties. Common items like milk, tea, or coffee are complex mixtures containing water, proteins, fats, and various dissolved compounds. Unlike sugar, these do not have a single fixed chemical formula because their composition can vary Science, Class VIII NCERT (Revised ed 2025), Nature of Matter: Elements, Compounds, and Mixtures, p.130. Understanding these formulas is vital for chemistry, as seen in metabolic processes where glucose (C₆H₁₂O₆) is broken down into carbon dioxide and water to release energy Science, Class X NCERT (2025 ed.), Life Processes, p.87. To master this for the exam, you should be familiar with the common names and chemical formulas of salts and substances we encounter daily:

Common Name	Chemical Name	Formula
Common Salt	Sodium chloride	NaCl
Baking Soda	Sodium hydrogen carbonate	NaHCO₃
Washing Soda	Sodium carbonate	Na₂CO₃
Blue Vitriol	Copper sulphate	CuSO₄

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

Sources: Science, Class VIII NCERT (Revised ed 2025), Nature of Matter: Elements, Compounds, and Mixtures, p.130, 132; Science, Class X NCERT (2025 ed.), Life Processes, p.87; Science, Class X NCERT (2025 ed.), Acids, Bases and Salts, p.28

7. Solving the Original PYQ (exam-level)

Review the concepts above and try solving the question.