Which one among the following statements regarding the properties of mixtures and compounds is not correct?

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

Welcome to our first step in mastering chemistry! To understand matter, we must look at how its smallest particles are organized. Science classifies everything around us into two main buckets: Pure Substances and Mixtures.

A Pure Substance is matter that consists of only one type of particle. Whether those particles are individual atoms or molecules, they all behave identically Science, Class VIII, Chapter 8, p.130. These are further divided into Elements (the simplest building blocks, like Gold or Iron) and Compounds (where different elements are chemically bonded in a fixed ratio, like H₂O). In a compound, the original elements lose their individual identities to create something entirely new; for instance, explosive Hydrogen and fire-sustaining Oxygen combine to form Water, which puts out fires!

On the other hand, a Mixture occurs when two or more substances are physically blended together without any chemical reaction taking place Science, Class VIII, Chapter 8, p.130. Unlike compounds, the components of a mixture retain their original properties. Think of a sprout salad or salt-water: the salt still tastes salty, and the water is still wet Science, Class VIII, Chapter 8, p.120. Because no chemical bonds are formed or broken during the mixing, there is usually no significant energy change (like heat being released) when you make a mixture.

The following table summarizes the key distinctions you need to remember:

Feature	Pure Substance (Compound)	Mixture
Composition	Fixed ratio (e.g., always 2:1 for H₂O)	Variable ratio (any amount of salt in water)
Properties	Entirely new properties	Retains properties of constituents
Separation	Only by chemical/electrochemical methods	Physical methods (filtration, evaporation)
Energy	Energy is absorbed or evolved during formation	No significant energy change during mixing

Sources: Science, Class VIII, Chapter 8: Nature of Matter: Elements, Compounds, and Mixtures, p.130; Science, Class VIII, Chapter 8: Nature of Matter: Elements, Compounds, and Mixtures, p.120

2. Chemical Bonding and the Nature of Compounds (intermediate)

When we look at the material world, we see substances combined in two distinct ways: as mixtures or as compounds. A compound is a pure substance formed when two or more elements combine chemically in a fixed proportion by mass. For instance, in a molecule of water (H₂O), the ratio of hydrogen to oxygen is always fixed, regardless of the water's source Science, Class VIII, Nature of Matter: Elements, Compounds, and Mixtures, p. 124. This is a fundamental shift from a mixture, where components are simply physically intermingled in any ratio without losing their individual identities.

The core of this transformation lies in chemical bonding. During the formation of a compound, the atoms of the constituent elements undergo a chemical reaction where bonds are broken and new ones are formed Science, Class X, Chemical Reactions and Equations, p. 6. This process almost always involves a significant energy change—energy is either absorbed (endothermic) or evolved (exothermic) as heat or light. In contrast, making a mixture (like stirring salt into sand) typically involves no such energy exchange. Furthermore, a compound is homogeneous in nature and possesses entirely new properties that are distinct from its parents. For example, sodium (a highly reactive metal) and chlorine (a poisonous gas) combine to form sodium chloride (common table salt), which is essential for life!

Feature	Mixture	Compound
Composition	Variable; no fixed ratio.	Fixed; elements combine in a specific ratio.
Properties	Retains properties of constituents.	Shows entirely new properties.
Energy Change	No significant energy change during preparation.	Energy (heat/light) is usually evolved or absorbed.
Separation	Can be separated by physical methods (e.g., filtration).	Can only be separated by chemical or electrochemical means.

Because a compound is a single substance with a fixed chemical formula, it cannot be broken down into its elements by physical means like evaporation or magnetism Science, Class VIII, Nature of Matter: Elements, Compounds, and Mixtures, p. 130. To separate the oxygen from the hydrogen in water, you would need a chemical process like electrolysis, not just a filter paper.

Sources: Science, Class VIII . NCERT(Revised ed 2025), Chapter 8: Nature of Matter: Elements, Compounds, and Mixtures, p.124, 130; Science, class X (NCERT 2025 ed.), Chapter 1: Chemical Reactions and Equations, p.6

3. Physical vs. Chemical Changes and Energy (basic)

In our study of matter, we distinguish between two fundamental types of transformations: physical changes and chemical changes. A physical change occurs when a substance alters its physical properties—such as shape, size, or state (solid, liquid, gas)—without forming a new chemical substance Science-Class VII . NCERT (Revised ed 2025), Changes Around Us: Physical and Chemical, p. 59, 68. For example, when a candle melts, the solid wax turns into liquid wax; it is still wax, just in a different state. Conversely, a chemical change results in the formation of one or more entirely new substances with different properties, often involving a chemical reaction Science-Class VII . NCERT (Revised ed 2025), Changes Around Us: Physical and Chemical, p. 68. Using the candle again: while the melting of wax is physical, the burning of the wax vapor is a chemical change because it produces new substances like CO₂ and water vapor Science-Class VII . NCERT (Revised ed 2025), Changes Around Us: Physical and Chemical, p. 65.

A critical layer to this distinction is the role of energy. Chemical changes are almost always accompanied by the absorption or evolution of energy as chemical bonds are broken and formed. We categorize these into exothermic reactions, where heat is released (like combustion or respiration), and endothermic reactions, where heat is absorbed Science, class X (NCERT 2025 ed.), Chemical Reactions and Equations, p. 7, 15. This energy aspect helps us distinguish between mixtures and compounds. When you create a mixture (like stirring salt into sand), the components are just physically intermingled, and no significant energy change occurs. However, when a compound is formed (like burning hydrogen in oxygen to form H₂O), a chemical reaction takes place, resulting in a distinct energy change and a substance with completely new properties.

Feature	Physical Change	Chemical Change
New Substance	No new substance is formed.	One or more new substances are formed.
Energy Change	Usually negligible energy change.	Significant absorption or release of energy.
Reversibility	Often reversible (e.g., freezing water).	Usually irreversible (e.g., rusting of iron).
Examples	Melting, boiling, shredding paper.	Burning, digestion, photosynthesis.

Sources: Science-Class VII . NCERT (Revised ed 2025), Changes Around Us: Physical and Chemical, p.59, 65, 68; Science, class X (NCERT 2025 ed.), Chemical Reactions and Equations, p.7, 15

4. Types of Mixtures: Solutions, Colloids, and Suspensions (intermediate)

In our journey through the nature of matter, we arrive at mixtures—physical combinations of two or more substances where no chemical reaction occurs. Unlike compounds, which require energy to form and exhibit entirely new properties, mixtures are simply intermingled. They retain the properties of their original constituents and can usually be separated by physical methods like evaporation or filtration Science, Class VIII. NCERT(Revised ed 2025), Chapter 8, p.130.

Mixtures are broadly classified based on how evenly their components are distributed. A uniform mixture (homogeneous) has components spread so evenly that they cannot be seen separately, even under a microscope, such as sugar dissolved in water. In contrast, a non-uniform mixture (heterogeneous) has visible components, like a sprout salad or sand in water Science, Class VIII. NCERT(Revised ed 2025), Chapter 8, p.117. To master this for the UPSC, we categorize these into three distinct types based on particle size: Solutions, Colloids, and Suspensions.

Feature	Solution	Colloid	Suspension
Nature	Homogeneous (Uniform)	Heterogeneous (appears uniform)	Heterogeneous (Non-uniform)
Particle Size	Extremely small (< 1 nm)	Intermediate (1–1000 nm)	Large (> 1000 nm)
Stability	Stable; particles do not settle	Quite stable; do not settle	Unstable; particles settle down
Tyndall Effect	Does not scatter light	Scatters light (visible beam)	May scatter light initially

Solutions deserve a closer look as they are the most common in chemistry. A solution consists of a solute (the substance being dissolved, usually in smaller quantity) and a solvent (the substance doing the dissolving). For example, in air, Nitrogen acts as the solvent (78%), while Oxygen and CO₂ are the solutes Science, Class VIII. NCERT(Revised ed 2025), Chapter 9, p.149. When a solvent has dissolved the maximum possible amount of solute at a given temperature, it is called a saturated solution; if it can still hold more, it is unsaturated Science, Class VIII. NCERT(Revised ed 2025), Chapter 9, p.149.

Sources: Science, Class VIII. NCERT(Revised ed 2025), Nature of Matter: Elements, Compounds, and Mixtures, p.117, 130; Science, Class VIII. NCERT(Revised ed 2025), The Amazing World of Solutes, Solvents, and Solutions, p.149

5. Methods of Separating Components (intermediate)

In our journey through chemistry, we must distinguish between how components are held together. Unlike compounds, which are chemically bonded in fixed ratios, a mixture consists of two or more substances physically intermingled. Crucially, these substances retain their individual properties because no chemical reaction occurs between them Science, Class VIII. NCERT (Revised ed 2025), Nature of Matter: Elements, Compounds, and Mixtures, p.130. Because the identity of the components is preserved, we can exploit their differing physical properties—such as size, boiling point, or solubility—to separate them back into pure substances Science, Class VIII. NCERT (Revised ed 2025), Nature of Matter: Elements, Compounds, and Mixtures, p.120.

One of the most fundamental physical methods is evaporation. This process involves transforming a liquid into a gaseous state, typically through the application of heat. It is used to separate a solid solute (like salt) from a liquid solvent (like water). The rate of evaporation is not constant; it increases with higher temperatures and greater air movement, which replaces saturated air layers with unsaturated ones FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.), Water in the Atmosphere, p.86. Interestingly, air pressure also plays a role: according to Bernoulli's principle, higher wind speeds decrease air pressure, which in turn increases the rate of evaporation Physical Geography by PMF IAS, Tropical Cyclones, p.358.

When dealing with a mixture of two or more liquids, we often use fractional distillation. This method relies on the different boiling points of the components. As the mixture is heated, the component with the lower boiling point vaporizes first, is collected, and then condensed back into a liquid. This is a universal technique in oil refining, where crude oil is separated into various useful fractions like petrol, diesel, and kerosene Certificate Physical and Human Geography, GC Leong, Fuel and Power, p.269.

Method	Property Exploited	Common Example
Evaporation	Volatility / Boiling Point	Obtaining salt from seawater
Fractional Distillation	Difference in Boiling Points	Refining crude oil into fuel
Filtration	Particle Size	Separating tea leaves from tea
Magnetic Separation	Magnetic Susceptibility	Removing iron filings from sand

Sources: Science, Class VIII. NCERT (Revised ed 2025), Nature of Matter: Elements, Compounds, and Mixtures, p.120, 130; FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.), Water in the Atmosphere, p.86; Physical Geography by PMF IAS, Tropical Cyclones, p.358; Certificate Physical and Human Geography, GC Leong, Fuel and Power, p.269

6. Comparative Analysis: Mixtures vs. Compounds (exam-level)

In our journey through the nature of matter, understanding the distinction between mixtures and compounds is a fundamental pillar. At the surface, both involve putting two or more substances together, but the scientific reality behind them is vastly different. A mixture is like a crowd where individuals stand together but remain themselves; a compound is like a team where individuals lose their solo identity to achieve a common goal.

A mixture is formed when two or more substances are physically intermingled without any chemical reaction taking place. Because no chemical bonds are formed or broken, there is usually no significant evolution or absorption of energy (heat or light) during its preparation Science, Class VIII . NCERT(Revised ed 2025), Chapter 8: Nature of Matter: Elements, Compounds, and Mixtures, p. 130. For example, if you mix sand and salt, you can still see the individual grains, and the salt still tastes salty. On the other hand, a compound is a substance formed when two or more elements chemically combine in a fixed ratio by mass. This process involves a chemical reaction, which almost always results in the absorption or release of energy Science, Class VIII . NCERT(Revised ed 2025), Chapter 8: Nature of Matter: Elements, Compounds, and Mixtures, p. 124.

The most striking difference lies in their properties. In a mixture, the constituents retain their original characteristics. In a compound, the new substance has entirely different properties from its constituent elements. A classic example involves iron (Fe) and sulfur (S). If you simply mix them, you can pull the iron out with a magnet. However, if you heat them to form the compound Iron Sulfide (FeS), the resulting black solid is no longer attracted to a magnet, proving that a new substance with new properties has been born Science, Class VIII . NCERT(Revised ed 2025), Chapter 8: Nature of Matter: Elements, Compounds, and Mixtures, p. 132.

Feature	Mixture	Compound
Composition	Variable (any ratio).	Fixed (definite ratio).
Properties	Shows properties of its constituents.	Entirely new properties.
Energy Change	No significant energy change during formation.	Energy (heat/light) is evolved or absorbed.
Separation	Physical methods (filtration, evaporation).	Chemical or electrochemical methods only.
Nature	Can be homogeneous or heterogeneous.	Always a homogeneous pure substance.

Sources: 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.130; Science, Class VIII . NCERT(Revised ed 2025), Chapter 8: Nature of Matter: Elements, Compounds, and Mixtures, p.132

7. Solving the Original PYQ (exam-level)

Now that you have mastered the fundamental differences between physical combinations and chemical reactions, this question serves as the ultimate test of your conceptual clarity. To arrive at the correct answer, you must apply the principle that the formation of a compound is a chemical process involving the breaking and forming of bonds, which inherently results in the absorption or evolution of energy. Conversely, a mixture is a simple physical intermingling where components stay side-by-side without reacting, meaning no significant energy change occurs. Therefore, statement (D) is the incorrect one because it completely reverses these scientific properties.

When navigating UPSC questions, pay close attention to reversal traps. Options (A), (B), and (C) are classic distractors that correctly describe the homogeneity of compounds, the physical separation techniques for mixtures (like filtration), and how constituents retain their original properties in a mixture but lose them in a compound. As detailed in Science, Class VIII, NCERT (Revised ed 2025), compounds are pure substances that undergo a total transformation of identity, whereas mixtures remain an assembly of their parts. By identifying that energy exchange is a hallmark of chemical change, you can confidently isolate the error in the final statement.