A mixture of sodium chloride and naphthalene can be separated by

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

Everything we touch, breathe, or see—from the water in your bottle to the air in the atmosphere—is matter. At its most fundamental level, matter is composed of tiny particles that are constantly in motion Science, Class VIII, Particulate Nature of Matter, p.112. In the study of chemistry, we classify all matter into two broad categories based on the nature of these particles: Pure Substances and Mixtures.

In our daily lives, we use the word "pure" to mean unadulterated (like "pure milk"). However, in science, a pure substance has a much stricter definition: it is a type of matter that consists of only one type of particle throughout. Because all its particles are identical, a pure substance cannot be broken down into other kinds of matter by any physical process like filtering or heating Science, Class VIII, Nature of Matter: Elements, Compounds, and Mixtures, p.121. Common examples include distilled water (H₂O), common salt (NaCl), and gold.

On the other hand, a mixture contains two or more different substances—which could be elements or compounds—physically combined together. The components of a mixture retain their own individual properties because they aren't chemically bonded. For example, in a salt-water solution, the water is still water and the salt is still salt. Mixtures are further divided into two types based on how their particles are distributed:

• Uniform (Homogeneous) Mixtures: The components are spread so evenly that you cannot distinguish them, even with a microscope. A classic example is sugar dissolved in water Science, Class VIII, Nature of Matter: Elements, Compounds, and Mixtures, p.117.
• Non-uniform (Heterogeneous) Mixtures: The components remain physically separate and are often visible to the naked eye, like a salad or a mixture of sand and salt Science, Class VIII, Nature of Matter: Elements, Compounds, and Mixtures, p.117.

Feature	Pure Substance	Mixture
Particle Type	Only one type of particle present.	Two or more different types of particles.
Physical Separation	Cannot be separated by physical means.	Can be separated using physical methods (e.g., filtration, evaporation).
Composition	Fixed and definite.	Variable; components can be added in any ratio.

Sources: Science, Class VIII, Nature of Matter: Elements, Compounds, and Mixtures, p.117; Science, Class VIII, Nature of Matter: Elements, Compounds, and Mixtures, p.121; Science, Class VIII, Particulate Nature of Matter, p.112

2. Physical Properties: Melting, Boiling, and Solubility (basic)

To understand how matter behaves, we must first look at its physical properties—characteristics like shape, size, and state that can be observed without changing the identity of the substance. When a substance undergoes a change in these properties, such as ice melting into water, it is called a physical change, and crucially, no new substance is formed Science-Class VII, Changes Around Us: Physical and Chemical, p.68. These properties are not just descriptions; they are determined by how closely packed the constituent particles are and how strongly they attract one another Science-Class VIII, Particulate Nature of Matter, p.113.

Melting and Boiling Points are the temperature milestones where a substance changes its state. In a solid, particles are closely packed and held by strong interparticle forces. Melting occurs when we add enough heat energy to overcome these attractions, allowing particles to move past each other and become a liquid Science-Class VIII, Particulate Nature of Matter, p.113. Boiling takes this a step further, providing enough energy for particles to break free entirely into a gaseous state. Some unique substances, like naphthalene, skip the liquid phase entirely through a process called sublimation—transitioning directly from solid to gas.

Solubility is another vital physical property. It is defined as the maximum amount of a solute (the substance being dissolved) that can be dissolved in a fixed quantity of solvent (the dissolving medium, usually a liquid) at a specific temperature Science-Class VIII, The Amazing World of Solutes, Solvents, and Solutions, p.149. When a solution can hold no more solute at that temperature, we call it a saturated solution. Interestingly, physical conditions affect this property: for most solids, solubility increases as the temperature rises, while for gases, it typically decreases.

Property	Description	Key Factor
Melting Point	Solid to Liquid transition	Breaking interparticle attractions
Boiling Point	Liquid to Gas transition	Total escape from liquid forces
Solubility	Max solute in 100 mL solvent	Temperature and Nature of solute/solvent

Sources: Science-Class VII, Changes Around Us: Physical and Chemical, p.59, 68; Science-Class VIII, Particulate Nature of Matter, p.113; Science-Class VIII, The Amazing World of Solutes, Solvents, and Solutions, p.149

3. Liquid Separation: Distillation and Chromatography (intermediate)

To master the art of chemical separation, we must first look at the unique physical properties of liquids. Unlike solids, liquid particles have enough kinetic energy to move freely, giving them a definite volume but no fixed shape Science Class VIII NCERT, Particulate Nature of Matter, p.104. When two or more liquids are mixed—such as acetic acid in water (vinegar) or oil and water Science Class VIII NCERT, Nature of Matter, p.120—we use specific techniques like Distillation and Chromatography to pull them apart based on their boiling points or molecular 'stickiness.'

Distillation relies on the principle that different liquids evaporate at different temperatures. In Simple Distillation, we heat a mixture; the component with the lower boiling point turns into vapor first, which is then cooled back into a liquid (distillate) in a separate container. However, when liquids have boiling points that are very close to one another, we use Fractional Distillation. This is the universal standard for oil refining GC Leong, Fuel and Power, p.269. By using a fractionating column, we can separate crude oil into various useful 'fractions' like petrol or kerosene based on their specific boiling ranges GC Leong, Fuel and Power, p.271.

Chromatography, on the other hand, does not rely on heat. Instead, it exploits the differential speed at which different substances travel through a medium. Imagine a race where some molecules are more 'attracted' to the track (the stationary phase) and others prefer the 'wind' (the mobile solvent). Because they move at different rates, they eventually separate into distinct bands. This is incredibly useful for identifying complex mixtures like ink dyes, blood toxins, or plant pigments where boiling the sample might destroy the very chemicals we are trying to study.

Method	Physical Property Used	Common Application
Simple Distillation	Large difference in boiling points	Purifying water from salt solutions
Fractional Distillation	Small difference in boiling points	Refining crude oil into petrol
Chromatography	Solubility and Adsorption rates	Separating pigments or forensic testing

Sources: Science Class VIII NCERT, Particulate Nature of Matter, p.104; Science Class VIII NCERT, Nature of Matter: Elements, Compounds, and Mixtures, p.120; Certificate Physical and Human Geography, GC Leong, Fuel and Power, p.269; Certificate Physical and Human Geography, GC Leong, Fuel and Power, p.271

4. Solid Separation: Filtration, Evaporation, and Crystallization (intermediate)

When we encounter matter in our daily lives, it is rarely in its purest form. To separate solids from liquids or from other solids, we must exploit differences in their physical properties—such as solubility and particle size. In a mixture where a solid (the solute) is dissolved in a liquid (the solvent), they form a uniform solution Science, Class VIII, The Amazing World of Solutes, Solvents, and Solutions, p.135. However, if the solid does not dissolve, we have a non-uniform mixture, and our approach to separation changes entirely.

Filtration is the most straightforward technique for heterogeneous mixtures, such as sand and water. Because the solid particles are significantly larger than the liquid molecules and do not move past each other in a fixed state Science, Class VIII, Particulate Nature of Matter, p.113, they can be trapped by a porous material like filter paper. While effective for insoluble substances, filtration cannot separate dissolved salts from water.

For homogeneous solutions, like salt in seawater, we use Evaporation. By heating the solution, the liquid solvent escapes as vapor, leaving the solid residue behind. This is how massive deposits of rock salt were formed historically when ancient seas dried up Science, Class X, Acids, Bases and Salts, p.29. However, evaporation has a downside: if the solution contains multiple soluble impurities, they all end up in the final solid. Furthermore, some substances (like sugar) may char or decompose if heated too strongly.

Crystallization is a more sophisticated "upgrade" to evaporation. Instead of boiling the liquid away entirely, we evaporate it just enough to create a saturated solution and then let it cool. As it cools, the pure solid forms beautiful, geometric crystals. This method is superior because it leaves soluble impurities behind in the liquid. A classic example is Copper Sulphate (CuSO₄); these crystals often contain "water of crystallization," which gives them a distinct blue color and structure Science, Class X, Acids, Bases and Salts, p.32.

Method	Type of Mixture	Key Advantage
Filtration	Solid-Liquid (Insoluble)	Quick and simple for suspended particles.
Evaporation	Solid-Liquid (Soluble)	Effective for recovering the solute on a large scale.
Crystallization	Solid-Liquid (Soluble)	Produces highly pure solids and prevents decomposition.

Sources: Science, Class VIII, The Amazing World of Solutes, Solvents, and Solutions, p.135; Science, Class VIII, Particulate Nature of Matter, p.113; Science, Class X, Acids, Bases and Salts, p.29; Science, Class X, Acids, Bases and Salts, p.32

5. Sublimation: The Direct Phase Transition (exam-level)

In our study of matter, we typically observe a three-step progression when heating a substance: a solid melts into a liquid, which then boils into a gas. However, certain substances possess a unique physical property called sublimation. This is a direct phase transition where a substance moves from a solid state to a gaseous state without ever passing through the intermediate liquid phase. This occurs when the vapor pressure of the solid becomes high enough to overcome atmospheric pressure before the substance reaches its melting point. Common examples of sublimable substances include camphor, ammonium chloride (NH₄Cl), naphthalene, and solid carbon dioxide (dry ice). You might have noticed this phenomenon at home; for instance, when camphor is heated, its fragrance quickly fills the room because it converts directly into vapor Science, Class VIII NCERT (Revised ed 2025), Particulate Nature of Matter, p.114.

This distinct property is incredibly useful in the laboratory for separation techniques. If you have a mixture of two solids, one that sublimes and one that does not, you can easily separate them using heat. A classic example is a mixture of naphthalene and sodium chloride (NaCl). Naphthalene is a volatile organic compound with weak intermolecular forces, allowing it to sublime at relatively low temperatures. In contrast, sodium chloride is an ionic compound held together by powerful electrostatic forces, resulting in an extremely high melting point Science, class X NCERT (2025 ed.), Chapter 3: Metals and Non-metals, p.47. When the mixture is heated in a dish covered by an inverted funnel, the naphthalene turns to vapor and then deposits back as pure solid crystals on the cool inner walls of the funnel (a process sometimes called desublimation or deposition), leaving the salt behind.

Substance Type	Response to Heating	Example
Sublimable	Solid → Gas (skips liquid)	Naphthalene, NH₄Cl, Iodine
Non-Sublimable	Solid → Liquid → Gas	Sodium Chloride (NaCl), Sugar

Sources: Science, Class VIII NCERT (Revised ed 2025), Particulate Nature of Matter, p.114; Science, class X NCERT (2025 ed.), Chapter 3: Metals and Non-metals, p.47

6. Chemical Profile: Naphthalene and Sodium Chloride (NaCl) (exam-level)

When we encounter a mixture of Sodium Chloride (NaCl) and Naphthalene, we are looking at two substances with vastly different chemical personalities. Sodium Chloride is an ionic compound, held together by powerful electrostatic forces between sodium and chloride ions. Because these particles are so tightly bound, NaCl has a very high melting point and does not easily change state. In contrast, Naphthalene (the primary ingredient in mothballs) is a volatile organic compound. While the particles in all solids are held together by interparticle forces, the forces in Naphthalene are much weaker than the ionic bonds in salt Science, Class VIII, Particulate Nature of Matter, p.113.

The most effective way to separate this duo is through sublimation. Sublimation is the unique physical process where a substance transitions directly from a solid to a gaseous state without ever becoming a liquid. When the mixture is heated, the Naphthalene molecules gain enough energy to break free from their solid structure and enter the air as vapor. Meanwhile, the Sodium Chloride remains stubbornly solid because its melting point is far higher. If we place a cool surface (like an inverted funnel with a cotton plug) over the heated mixture, the Naphthalene vapor will deposite back into pure crystals on the cool glass, leaving the salt behind in the dish.

Feature	Sodium Chloride (NaCl)	Naphthalene (C₁₀H₈)
Bonding Type	Strong Ionic Bonds	Weak Covalent/Intermolecular Forces
Solubility in Water	Highly Soluble	Insoluble
Effect of Heat	Remains solid (High melting point)	Sublimes (Solid to Gas)

Aside from sublimation, we could theoretically use solubility to separate them. As noted in Science, Class VIII, The Amazing World of Solutes, Solvents, and Solutions, p.137, solubility is the maximum amount of solute that dissolves in a solvent. Sodium chloride dissolves readily in water, whereas Naphthalene does not. While this is a valid chemical distinction, sublimation remains the standard laboratory preference because it is cleaner and yields high-purity crystals of Naphthalene directly.

Sources: Science, Class VIII (Revised ed 2025), Particulate Nature of Matter, p.113; Science, Class X (NCERT 2025 ed), Carbon and its Compounds, p.67; Science, Class VIII (Revised ed 2025), The Amazing World of Solutes, Solvents, and Solutions, p.137

7. Solving the Original PYQ (exam-level)

Now that you have mastered the fundamental physical properties of matter and the diverse array of separation techniques, you can see how this question bridges theory and application. The key to solving such UPSC problems lies in identifying the distinctive physical signatures of each component in a mixture. In your previous lessons, we discussed how certain organic solids possess weak intermolecular forces that allow them to bypass the liquid phase entirely when heated. By recognizing naphthalene as a volatile substance and sodium chloride (NaCl) as a stable, high-melting-point ionic compound, you are effectively identifying the 'weak link' that allows for easy separation.

To arrive at the correct answer, you must apply the logic of state transitions. When the mixture is heated, the naphthalene molecules gain sufficient kinetic energy to transition directly from a solid to a vapor. This vapor can then be condensed back into pure crystals on a cooler surface, a process known as sublimation. Conversely, the sodium chloride remains unchanged in the container because its strong electrostatic bonds require significantly more energy to break. Therefore, (C) sublimation is the most direct and efficient laboratory method to achieve a clean separation, as highlighted in Science, Class X (NCERT 2025 ed.).

UPSC often includes distractors that are theoretically possible but practically inferior to test your precision. Options (A) and (B) involving water extraction are classic traps; while it is true that NaCl dissolves in water and naphthalene does not, this method would require the extra steps of filtration and subsequent evaporation, making it a multi-stage process rather than a single-step physical transition. Steam distillation (D) is generally used for separating liquids or volatile solids that are immiscible with water and have high boiling points, but it is unnecessarily complex for this specific solid-solid mixture. Always choose the method that exploits the most extreme difference in physical properties with the least amount of intervention.