Which one of the following statements about a compound is NOT correct ?

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

To master chemistry, we must first look at how scientists categorize the world around us. In everyday language, we call milk or honey 'pure' if they haven't been tampered with. However, in science, a pure substance has a much stricter definition: it is a form of matter that consists of only one type of particle throughout Science, Class VIII, Chapter 8, p.121. These particles can be individual atoms (as in elements like Gold or Iron) or molecules made of different atoms bonded together in a fixed ratio (as in compounds like Water, H₂O). Because every particle is identical, a pure substance cannot be separated into other kinds of matter by simple physical means like filtration or evaporation.

In contrast, mixtures are considered 'impure' substances because they are made of two or more different substances physically combined. Crucially, in a mixture, the individual components retain their own properties and can usually be separated by physical methods Science, Class VIII, Chapter 8, p.130. For example, in a mixture of salt and sand, the salt still tastes salty and the sand remains gritty. However, when elements form a compound, they undergo a chemical change to create something entirely new. Water (a liquid used to extinguish fires) is a compound formed from Hydrogen and Oxygen—two gases that are highly flammable or support combustion. This transformation of properties is a hallmark of chemical combination.

To help you distinguish between the two for your exams, remember that the key difference lies in how the components are joined and how they can be taken apart:

Feature	Pure Substance (Compound)	Mixture
Composition	Fixed ratio (e.g., H₂O is always 2:1)	Variable ratio (e.g., more or less sugar in tea)
Properties	Entirely different from its elements	Components retain their original properties
Separation	Only via chemical or electrochemical reactions	Physical methods (filtration, distillation, etc.)

Sources: Science, Class VIII, Nature of Matter: Elements, Compounds, and Mixtures, p.121; Science, Class VIII, Nature of Matter: Elements, Compounds, and Mixtures, p.124; Science, Class VIII, Nature of Matter: Elements, Compounds, and Mixtures, p.130

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

In our journey to understand the building blocks of the universe, we begin with Elements. Imagine trying to break down a piece of pure gold; no matter how much you divide it through chemical means, it remains gold. This is because an element is the simplest form of matter that cannot be broken down into simpler substances by ordinary chemical reactions. Each element is a pure substance because it consists of only one type of atom throughout Science, Class VIII, Chapter 8: Nature of Matter, p.121.

Currently, there are 118 known elements, and they serve as the primary alphabet of chemistry. Scientists classify these elements into three main categories based on their properties: Metals, Non-metals, and Metalloids Science, Class X, Chapter: Metals and Non-metals, p.37. Metals are typically shiny (lustrous), can be hammered into sheets (malleable), or drawn into wires (ductile), and are excellent conductors of heat and electricity. Non-metals, on the other hand, are often poor conductors and exist in various states—solid, liquid, or gas—at room temperature Science, Class X, Chapter: Metals and Non-metals, p.55.

It is fascinating to observe how elements behave under standard conditions. While the vast majority of elements are solids, there are notable exceptions that are frequent favorites in competitive exams:

Property	Details
Gaseous State	11 elements are gases at room temperature (e.g., Oxygen O₂, Nitrogen N₂, Helium He).
Liquid State	Only two elements are liquid at room temperature: Mercury (Hg), which is a metal, and Bromine (Br), which is a non-metal Science, Class VIII, Chapter 8, p.123.
Low Melting Solids	Gallium and Caesium are solids that can melt in your palm, turning liquid at around 30°C.

Understanding these elements isn't just academic; it’s the foundation of modern engineering. For instance, while pure Iron (Fe) is an element, we mix it with other elements to create alloys like stainless steel, which are far more durable for building the bridges and skyscrapers we see today Science, Class VIII, Chapter 8, p.129.

Sources: Science, Class VIII, Chapter 8: Nature of Matter, p.121, 123, 129; Science, Class X, Metals and Non-metals, p.37, 55

3. Physical vs. Chemical Changes (basic)

In our study of matter, understanding change is fundamental. Everything around us is in a state of flux, but scientifically, we categorize these shifts into two primary types based on whether a new substance is created. A physical change is one where only the physical properties—such as shape, size, or state (solid, liquid, gas)—are altered. Crucially, the chemical identity of the substance remains exactly the same. For instance, when ice melts into water (H₂O), it is still the same molecule; only its physical state has changed Science - Class VII, Changes Around Us: Physical and Chemical, p. 59. Processes like the erosion of rocks by wind or water are also physical changes, as they merely break down the size and shape of the material Science - Class VII, Changes Around Us: Physical and Chemical, p. 68.

On the other hand, a chemical change (or a chemical reaction) occurs when substances interact to form entirely new substances with unique properties. Unlike a physical change, the internal molecular structure is rearranged. Common indicators of a chemical change include the evolution of gas, a change in color, or the release of heat and light. Examples include combustion (burning), cooking food, and the rusting of iron Science - Class VII, Changes Around Us: Physical and Chemical, p. 68. In these cases, you cannot simply "undo" the change to get your original ingredients back because a new chemical compound has been forged.

Interestingly, some processes involve both types of changes simultaneously. Consider a burning candle: as the wax melts and flows down, it is undergoing a physical change (change of state). However, the wax vapor that actually burns in the flame is undergoing a chemical change, producing new substances like carbon dioxide and water vapor Science - Class VII, Changes Around Us: Physical and Chemical, p. 65.

Feature	Physical Change	Chemical Change
New Substance	No new substance formed.	One or more new substances formed.
Properties	Original properties stay the same.	New substance has entirely different properties.
Reversibility	Usually reversible (e.g., freezing water).	Usually irreversible (e.g., burning wood).

Sources: Science - Class VII, Changes Around Us: Physical and Chemical, p.59; Science - Class VII, Changes Around Us: Physical and Chemical, p.65; Science - Class VII, Changes Around Us: Physical and Chemical, p.68

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

To master chemistry, we must first distinguish between pure substances and mixtures. A compound, such as water (H₂O), is a pure substance because it consists of the same type of particles throughout, chemically bonded in a fixed proportion. In contrast, a mixture is considered an impure substance because it is a physical blend of different particles that retain their individual properties and can usually be separated by physical means Science, Class VIII, Chapter 8, p. 121, 130. Mixtures are broadly classified into three types based on the size of the particles involved: solutions, suspensions, and colloids.

A Solution is a homogeneous mixture where the solute particles are so tiny (less than 1 nanometer) that they dissolve completely and do not scatter light. At the other extreme is a Suspension, a heterogeneous mixture containing large particles that are visible to the naked eye. These particles are unstable and will eventually settle down due to gravity if left undisturbed. The earth's atmosphere itself is a complex heterogeneous mixture, containing air molecules alongside suspended dust and smoke particles Science, Class X, The Human Eye and the Colourful World, p. 169.

Colloids represent the fascinating middle ground. While they may appear homogeneous to the naked eye (like milk or fog), they are technically heterogeneous. Their particles are larger than those in a solution but small enough to remain dispersed without settling. The hallmark of a colloid is the Tyndall Effect—the phenomenon where fine particles scatter a beam of light, making the path of the beam visible Science, Class X, The Human Eye and the Colourful World, p. 169.

Feature	Solution	Colloid	Suspension
Particle Size	Extremely Small (< 1 nm)	Intermediate (1-1000 nm)	Large (> 1000 nm)
Stability	Very Stable	Stable	Unstable (Settles)
Tyndall Effect	No scattering	Scatters light	Scatters light (until settled)

Sources: Science, Class VIII . NCERT(Revised ed 2025), Chapter 8: Nature of Matter: Elements, Compounds, and Mixtures, p.121, 124, 130; Science, class X (NCERT 2025 ed.), The Human Eye and the Colourful World, p.169

5. Separation Techniques for Impure Substances (intermediate)

In our journey through chemistry, we often encounter materials that aren't "pure" in the scientific sense. In everyday life, we separate tea leaves from tea to improve the taste, but in science, separation techniques are employed specifically to obtain pure substances from a mixture Science, Class VIII NCERT, Chapter 8, p.120. Because the components of a mixture are only physically combined—meaning they aren't locked together by chemical bonds—we can use their different physical properties (like size, density, or boiling point) to pull them apart.

One of the most vital techniques is Distillation, which relies on differences in boiling points. When we have a mixture of two liquids, we heat them; the one with the lower boiling point vaporizes first and is then condensed back into a liquid in a separate container. When dealing with complex mixtures like crude oil, where many components have very similar boiling points, we use Fractional Distillation GC Leong, Fuel and Power, p.269. This process is the backbone of modern industry, allowing us to separate "fractions" such as petrol, kerosene, and diesel from raw petroleum Geography of India, Majid Husain, Energy Resources, p.15.

It is crucial to distinguish between separating a mixture and breaking down a compound. A compound, like water (H₂O), is a pure substance because it consists of the same type of molecules throughout. Unlike mixtures, the elements in a compound are held together by strong chemical bonds and cannot be separated by physical means like filtration or distillation. To separate a compound into its constituent elements, one must use chemical or electrochemical reactions Science, Class VIII NCERT, Chapter 8, p.124.

Feature	Mixtures (Impure)	Compounds (Pure)
Separation Method	Physical methods (Filtration, Distillation)	Chemical or Electrochemical methods
Nature of Bond	Physically mixed; no new bonds	Chemically bonded in fixed proportions
Properties	Shows properties of its constituents	Properties differ from its constituents

Sources: Science, Class VIII NCERT, Nature of Matter: Elements, Compounds, and Mixtures, p.120, 124; Certificate Physical and Human Geography, GC Leong, Fuel and Power, p.269; Geography of India, Majid Husain, Energy Resources, p.15

6. Understanding Compounds and the Law of Constant Proportions (exam-level)

When we talk about matter in chemistry, we often distinguish between things that are simply "thrown together" and things that are "chemically bonded." A compound falls into the latter category. It is defined as a pure substance composed of two or more elements that are chemically combined in a fixed proportion. Unlike a mixture, where you can see or feel the individual parts (like sand in water), a compound is homogeneous at the molecular level because all its constituent particles behave identically Science, Class VIII, NCERT (Revised ed 2025), Chapter 8, p.130.

One of the most fundamental rules governing compounds is the Law of Constant Proportions. This principle states that in a chemical substance, the elements are always present in definite proportions by mass, regardless of the source or method of preparation. For example, in pure water (H₂O), the ratio of the mass of hydrogen to the mass of oxygen is always 1:8. Whether you take water from a river in India or create it in a laboratory in London, this ratio never changes Science, Class VIII, NCERT (Revised ed 2025), Chapter 8, p.124. Because these atoms are held together by strong chemical bonds, you cannot separate them using physical methods like filtration or distillation; you would need a chemical or electrochemical reaction (like electrolysis) to break them apart.

Crucially, when elements form a compound, they undergo a "personality change." The properties of a compound are entirely different from its constituent elements. Consider iron (a magnetic metal) and sulfur (a yellow powder). 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 result is a black solid that is no longer attracted to a magnet and has completely new chemical behaviors Science, Class VIII, NCERT (Revised ed 2025), Chapter 8, p.128.

Feature	Mixture	Compound
Purity	Impure substance	Pure substance
Composition	Variable (any ratio)	Fixed (constant proportion)
Properties	Shows properties of constituents	Entirely new properties
Separation	Physical methods (filtration, etc.)	Chemical/Electrochemical methods

Sources: 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.124; Science, Class VIII . NCERT(Revised ed 2025), Chapter 8: Nature of Matter: Elements, Compounds, and Mixtures, p.128

7. Solving the Original PYQ (exam-level)

Now that you have mastered the classification of matter, this question tests your ability to distinguish between pure substances and mixtures. You’ve learned that a compound is not just a random assembly of elements; it is a result of a chemical reaction where atoms bond in a fixed proportion. According to Science, Class VIII NCERT, Chapter 8, this fundamental shift in identity is why compounds are classified as pure substances—they consist of only one type of molecule throughout their structure, regardless of how many different elements are inside that molecule. Therefore, stating that a compound is impure is a direct contradiction of its chemical nature.

To arrive at the correct answer (C), we must use the process of elimination. Statement (A) is the standard definition of a compound, and statement (B) highlights the transformation of properties—for example, liquid water behaves nothing like the hydrogen and oxygen gases that form it. Statement (D) is a common UPSC check on your understanding of chemical bonds; because elements in a compound are chemically locked, they cannot be separated by physical means like filtration. The trap here lies in the word "impure." Beginners often mistake the presence of "two or more elements" as a sign of impurity, but in the UPSC syllabus, "pure" refers to a substance having uniform chemical properties throughout. Since a compound is a pure substance, statement (C) is NOT correct.