Which one of the following is the correct order in which the gases H2, Ne, O2, and N2 are evolved on fractional distillation of liquid air?

1. States of Matter and Phase Transitions (basic)

At the very heart of chemistry lies the understanding of matter—anything that has mass and occupies space. Matter exists primarily in three states: solid, liquid, and gas. The difference between these states isn't just what we see, but how the tiny particles (atoms or molecules) inside them behave. In a solid, particles are tightly packed and vibrate in place; in a liquid, they have more room to move; and in a gas, they move vigorously and spread out to fill all available space Science, Class VIII NCERT (Revised ed 2025), Particulate Nature of Matter, p.115.

Phase transitions occur when we add or remove energy (usually heat). For instance, as we heat a liquid, its particles gain kinetic energy and move more violently. Eventually, they move so fast that they overcome the interparticle forces of attraction holding them together. At this specific temperature—known as the boiling point—the particles break free and the liquid turns into a gas or vapour Science, Class VIII NCERT (Revised ed 2025), Particulate Nature of Matter, p.105. This is why boiling occurs throughout the bulk of the liquid, as particles everywhere gain enough energy to escape the liquid state.

However, the state of matter isn't determined by temperature alone; ambient pressure plays a critical role. Think of atmospheric pressure as a "lid" pressing down on the surface of a liquid. If the pressure is high, it is harder for molecules to escape into the air, meaning you need more heat (a higher temperature) to reach the boiling point. Conversely, if you reduce the pressure, the molecules meet less resistance and can escape into the gas phase much more easily Physical Geography by PMF IAS, Geological Time Scale, p.43. This is why water boils at lower temperatures at high altitudes where the air is thinner.

State of Matter	Particle Arrangement	Interparticle Forces
Solid	Closely packed, fixed positions	Very Strong
Liquid	Less closely packed, can slide past each other	Moderate
Gas	Far apart, random rapid motion	Negligible / Very Weak

Sources: Science, Class VIII NCERT (Revised ed 2025), Particulate Nature of Matter, p.115; Science, Class VIII NCERT (Revised ed 2025), Particulate Nature of Matter, p.105; Physical Geography by PMF IAS, Geological Time Scale, p.43

2. Composition of the Earth's Atmosphere (basic)

The Earth's atmosphere is not a single substance but a uniform mixture of various gases, water vapor, and microscopic solid particles. To understand its chemistry, we first look at its permanent gases—those whose proportions remain nearly constant in the lower atmosphere. Nitrogen (N₂) is the most abundant, making up about 78.08% of the volume, followed by Oxygen (O₂) at roughly 20.95% Physical Geography by PMF IAS, Earth's Atmosphere, p.271. While Nitrogen is relatively inert and does not support combustion, Oxygen is the life-sustaining component essential for respiration and burning Science, Class VIII NCERT, Nature of Matter, p.118.

Beyond these two giants, the atmosphere contains Argon (Ar) at 0.93% and several "trace gases." Among these, Carbon Dioxide (CO₂) is chemically significant despite its small concentration (0.036%) because it acts as a greenhouse gas—transparent to incoming solar radiation but opaque to outgoing terrestrial heat Fundamentals of Physical Geography, Class XI NCERT, Composition and Structure of Atmosphere, p.66. Other trace components include noble gases like Neon (Ne), Helium (He), and Hydrogen (H₂).

The distribution of these gases is influenced by gravity and altitude. Heavier gases like Nitrogen and Oxygen tend to be concentrated in the lower layers. As we move upward, the composition changes drastically: Oxygen becomes almost negligible at a height of about 120 km, while Carbon Dioxide and water vapor are confined mostly to the first 90 km from the Earth's surface Fundamentals of Physical Geography, Class XI NCERT, Composition and Structure of Atmosphere, p.64.

Gas	Percentage (%)	Key Property
Nitrogen (N₂)	78.08	Inert, dilutes Oxygen
Oxygen (O₂)	20.95	Vital for life and combustion
Argon (Ar)	0.93	Noble gas, chemically unreactive
Carbon Dioxide (CO₂)	0.036	Greenhouse effect

Sources: Physical Geography by PMF IAS, Earth's Atmosphere, p.271; Science, Class VIII NCERT, Nature of Matter: Elements, Compounds, and Mixtures, p.118; Fundamentals of Physical Geography, Class XI NCERT, Composition and Structure of Atmosphere, p.64, 66

3. Principles of Separation: Distillation vs. Fractional Distillation (intermediate)

To understand how we separate complex mixtures, we must first understand the principle of boiling points. Every pure liquid has a specific temperature at which it turns into vapor. Simple Distillation is a technique used to separate a solvent from a solution or two liquids with significantly different boiling points (usually a difference of more than 25°C). In this process, the mixture is heated, the component with the lower boiling point vaporizes first, and these vapors are then passed through a condenser where they cool down and turn back into liquid form, collected separately. While this works well for simple mixtures, it is often insufficient for complex industrial needs.

When the boiling points of the liquids in a mixture are very close to each other (less than 25°C), simple distillation fails because both liquids would vaporize and condense almost simultaneously. This is where Fractional Distillation becomes essential. This method employs a fractionating column—a tall tube packed with glass beads or plates—placed between the distillation flask and the condenser. These beads provide a large surface area for the vapors to collide with, leading to repeated cycles of evaporation and condensation. As the vapors rise, they become increasingly enriched with the more volatile component (the one with the lower boiling point), allowing for a much sharper and more precise separation. This is one of the most universal forms of refining Certificate Physical and Human Geography, Fuel and Power, p.269.

The most famous application of this principle is in oil refineries. Crude oil is a complex mixture of various hydrocarbons. Through fractional distillation, it is separated into different 'fractions' like petrol, kerosene, and diesel based on their specific boiling ranges. For instance, the lighter motor fuel fraction (petrol) may only account for about 15% of the total oil distilled Certificate Physical and Human Geography, Fuel and Power, p.271. This same principle is applied to liquid air to separate oxygen, nitrogen, and argon, which are vital for medical and industrial use.

Feature	Simple Distillation	Fractional Distillation
Boiling Point Difference	Large (typically > 25°C)	Small (typically < 25°C)
Equipment	Standard distillation apparatus	Includes a Fractionating Column
Efficiency	Lower; one evaporation-condensation cycle	Higher; multiple cycles in one go
Common Use	Purifying water, separating salt from water	Crude oil refining, air separation

Sources: Certificate Physical and Human Geography, Fuel and Power, p.269; Certificate Physical and Human Geography, Fuel and Power, p.271

4. Cryogenics and Industrial Applications of Gases (intermediate)

Cryogenics is the branch of physics and engineering that studies the production of extremely low temperatures (typically below -150°C) and the behavior of materials at those levels. In an industrial context, the most common application of cryogenics is the fractional distillation of liquid air. Because air is a mixture of several gases like Nitrogen (78.08%), Oxygen (20.95%), and Argon (0.93%), we can separate them by first cooling air until it turns into a liquid and then gradually warming it up. Physical Geography by PMF IAS, Earths Atmosphere, p.270

The separation depends entirely on the boiling points of the individual gases. During distillation, as the liquid air is warmed, the gas with the lowest boiling point (the one that stays liquid only at the coldest temperatures) will evaporate or "evolve" first. For instance, Hydrogen (H₂) has a boiling point of approximately -252.9°C, while Oxygen (O₂) boils at -183.0°C. Since -252.9°C is a much colder "limit" than -183.0°C, Hydrogen will turn back into a gas long before Oxygen does. This sequence of evaporation allows industries to capture pure gases for specific uses.

Gas Component	Boiling Point (Approx)	Industrial/Atmospheric Role
Hydrogen (H₂)	-252.9°C	Used as liquid fuel in cryogenic rocket stages. Geography of India, Transport, Communications and Trade, p.58
Nitrogen (N₂)	-195.8°C	Relatively inert; prevents spontaneous combustion of Oxygen and prevents food rancidity. Physical Geography by PMF IAS, Earths Atmosphere, p.272
Oxygen (O₂)	-183.0°C	Vital for respiration and combustion; used as an oxidizer in rockets.

These gases serve critical roles beyond just breathing. For example, Nitrogen is used to fill chip packets because it is relatively inert and prevents the oxidation of fats (rancidity). It is also used in electric bulbs to protect the tungsten filament from burning out in the presence of oxygen. Physical Geography by PMF IAS, Earths Atmosphere, p.272. In the high-stakes world of space exploration, ISRO utilizes cryogenic stages (using liquid oxygen and hydrogen) to provide the massive thrust needed for heavy satellite launches like the GSLV series. Geography of India, Transport, Communications and Trade, p.58

Sources: Physical Geography by PMF IAS, Earths Atmosphere, p.270, 272; Geography of India, Transport, Communications and Trade, p.58

5. Characteristics of Noble Gases and Hydrogen (intermediate)

To understand how we separate and use atmospheric gases, we must first look at their chemical stability and physical properties. Noble gases, such as Helium (He), Neon (Ne), and Argon (Ar), are defined by their chemically inert nature. This is because they possess a full outer shell of electrons—an "octet"—which makes them highly stable and unlikely to react with other elements Science Class X NCERT, Metals and Non-metals, p.47. Hydrogen (H₂), by contrast, is highly reactive and is a primary constituent of acids, which typically release hydrogen gas when they react with metals Science Class X NCERT, Acids, Bases and Salts, p.22.

In the context of the Earth's atmosphere, these gases exist in varying proportions. While Nitrogen (N₂) and Oxygen (O₂) make up about 99% of the air, Noble gases and Hydrogen exist in trace amounts. For instance, Neon accounts for only 0.002% of the atmosphere, and Hydrogen even less at 0.00005% Physical Geography by PMF IAS, Earths Atmosphere, p.271. Interestingly, when we excite these gases with electricity, they can transition into plasma—the fourth state of matter. A common example of this is seen in "neon lights," which are technically plasma lights where gas atoms have been ionized Physical Geography by PMF IAS, The Solar System, p.24.

The most critical concept for industrial separation is Fractional Distillation. This process separates components of a mixture based on their boiling points. When air is liquefied (by cooling it to extremely low temperatures) and then gradually warmed, the gases "evolve" or evaporate in a specific order: the gas with the lowest boiling point evaporates first. Because Hydrogen has a lower boiling point than Neon, and Neon has a lower boiling point than Nitrogen or Oxygen, the sequence of evolution as the temperature rises is H₂ → Ne → N₂ → O₂. This physical property allows us to capture pure Hydrogen for use in fuel cells, where it reacts with Oxygen to generate clean electricity and water Environment Shankar IAS, Renewable Energy, p.296.

Gas	Approx. Boiling Point (°C)	Nature in Distillation
Hydrogen (H₂)	-252.9 °C	Evolves First (Lowest BP)
Neon (Ne)	-246.1 °C	Evolves Second
Nitrogen (N₂)	-195.8 °C	Evolves Third
Oxygen (O₂)	-183.0 °C	Evolves Last (Highest BP in group)

Sources: Science Class X NCERT, Metals and Non-metals, p.47; Science Class X NCERT, Acids, Bases and Salts, p.22; Physical Geography by PMF IAS, Earths Atmosphere, p.271; Physical Geography by PMF IAS, The Solar System, p.24; Environment Shankar IAS, Renewable Energy, p.296

6. Fractional Distillation of Liquid Air (exam-level)

To separate the various components of air—such as Nitrogen, Oxygen, and Argon—we rely on a process called fractional distillation. Because air is a homogeneous mixture of gases, we cannot simply filter them out. Instead, we use the unique physical property of each gas: its boiling point. Before distillation can begin, the air must be purified (to remove dust and CO₂) and then liquefied. This is achieved by increasing the pressure and decreasing the temperature until the air turns into a pale blue liquid. Science Class X NCERT, Carbon and its Compounds, p.59

Once we have liquid air, it is fed into a fractional distillation column and allowed to warm up slowly. In chemistry, the substance with the lowest boiling point (the most volatile) will reach its gaseous state first and "evolve" or evaporate at the top of the column. Conversely, the substance with the highest boiling point remains a liquid the longest and is collected at the bottom. For instance, Nitrogen is often used in industrial applications, such as filling chip packets to prevent oxidation, because of its relatively low boiling point and inert nature. Physical Geography by PMF IAS, Earths Atmosphere, p.272

To master this concept for the exam, you must remember the specific sequence of boiling points. A gas with a boiling point of -250°C is actually "colder" and turns into gas sooner than a gas at -180°C when you are warming them up from absolute zero.

Gas	Boiling Point (Approx. °C)	Order of Evolution
Hydrogen (H₂)	-252.9°C	1st (First to boil)
Neon (Ne)	-246.1°C	2rd
Nitrogen (N₂)	-195.8°C	3rd
Oxygen (O₂)	-183.0°C	4th (Last to boil)

Sources: Science Class X NCERT, Carbon and its Compounds, p.59; Physical Geography by PMF IAS, Earths Atmosphere, p.272

7. Solving the Original PYQ (exam-level)

This question perfectly synthesizes your understanding of matter states and physical separation techniques. As you learned in NCERT Class 9 Science, fractional distillation operates on the principle of varying boiling points. When liquid air is gradually warmed, the substance with the lowest boiling point (the most negative value in Celsius) overcomes intermolecular forces first to transition from liquid to gas. To solve this, you must apply the hierarchy of thermal properties: Hydrogen (H2) is the most volatile with the lowest boiling point, followed closely by Neon (Ne), then Nitrogen (N2), and finally Oxygen (O2).

To arrive at Option (B), think like a scientist: identify the "lightest" and "coldest" gases first. Hydrogen evolves at approximately -252.9°C, followed by Neon at -246.1°C. The real test of your preparation lies in the final two: Nitrogen (-195.8°C) and Oxygen (-183.0°C). Because -195.8 is a lower temperature than -183.0, Nitrogen must boil off before Oxygen. This logical progression from the lowest to the highest boiling point confirms that the sequence H2, Ne, N2, O2 is the only scientifically accurate order of evolution.

UPSC often includes distractors like Option (C) or (D) to exploit two common student errors. The first is reversing the order by starting with the highest boiling point (Oxygen), which would be the order of liquefaction rather than evolution. The second trap is the Nitrogen-Oxygen swap; many candidates mistakenly believe Oxygen evolves first because it is more reactive or has a different molecular weight, but in fractional distillation, the boiling point is the absolute decider. Always remember: the more negative the number, the sooner it escapes the liquid phase.