Scuba divers are: at risk due to high concentration of dissolved gases while breathing air at high pressure under water. The tanks used by Scuba divers are filled with

1. Solubility of Gases and Henry's Law (basic)

When we think of a solution, we often imagine salt or sugar dissolved in water. However, some of the most vital solutions for life involve gases dissolved in liquids. In these mixtures, the gas acts as the solute and the liquid acts as the solvent Science, Class VIII, Chapter 9, p.139. For example, the dissolved oxygen in our oceans and rivers is what allows fish and aquatic plants to breathe, even though it is present in relatively small amounts Science, Class VIII, Chapter 9, p.149.

The solubility of a gas—meaning how much of it can actually stay dissolved in the liquid—is not fixed; it is highly sensitive to the environment. Two main factors govern this behavior:

• Temperature: Unlike solids (which usually dissolve better when heated), gases become less soluble as temperature increases. When water warms up, gas molecules gain kinetic energy and "escape" back into the air Science, Class VIII, Chapter 9, p.139.
• Pressure (Henry's Law): This is the most critical factor for understanding how gases behave under depth. Henry's Law states that at a constant temperature, the solubility of a gas in a liquid is directly proportional to the pressure of that gas above the liquid. Simply put: the harder you "push" the gas down onto the liquid, the more of it will be forced to dissolve into the solution.

Think of a sealed bottle of carbonated soda. Inside the factory, CO₂ is pumped into the bottle at high pressure, forcing a large amount of gas to dissolve. As soon as you twist the cap, you hear a hiss—this is the pressure dropping to match the atmosphere. Because the pressure has decreased, the solubility of the CO₂ drops instantly, and the gas rushes out of the liquid in the form of bubbles. This same principle of pressure and solubility is what governs how the human body absorbs gases when we move between different environments, such as from the surface to the deep sea.

Sources: Science, Class VIII, The Amazing World of Solutes, Solvents, and Solutions, p.139; Science, Class VIII, The Amazing World of Solutes, Solvents, and Solutions, p.149

2. Gas Laws: Pressure-Volume Relationship (basic)

To understand how gases behave in the world around us, we must first look at their microscopic nature. Unlike solids or liquids, where particles are tightly packed, gas particles have significant interparticle spacing. This allows them to move energetically and freely in all directions Science, Class VIII, Particulate Nature of Matter, p.107. Because of this vast empty space between molecules, gases are uniquely compressible. This means that when we apply force to a gas, we can significantly change the space it occupies—a property that is much less prominent in other states of matter.

The core rule governing this behavior is the Pressure-Volume Relationship (often known as Boyle’s Law). It states that at a constant temperature, the volume of a fixed amount of gas is inversely proportional to the pressure applied to it. In simpler terms, if you increase the pressure on a gas, its volume will decrease; if you release that pressure, the gas expands. This happens because increasing the pressure forces the widely spaced particles closer together, reducing the total volume they occupy Science, Class VIII, Particulate Nature of Matter, p.112.

It is also important to remember that gases, like liquids, are fluids that exert pressure in all directions—not just downwards, but also against the walls of their container Science, Class VIII, Pressure, Winds, Storms, and Cyclones, p.85. This pressure is the result of billions of gas particles constantly colliding with the surfaces around them. When we compress a gas into a smaller volume, these collisions happen more frequently, which is why the internal pressure rises.

Sources: Science, Class VIII (NCERT 2025 ed.), Particulate Nature of Matter, p.107; Science, Class VIII (NCERT 2025 ed.), Particulate Nature of Matter, p.112; Science, Class VIII (NCERT 2025 ed.), Pressure, Winds, Storms, and Cyclones, p.85

3. Human Respiratory System and Gas Exchange (intermediate)

To understand how chemistry applies to our breath, we must first distinguish between breathing and respiration. Breathing is the physical act of moving air in and out of the lungs, whereas respiration is the biochemical process occurring within cells where oxygen is used to break down glucose into energy, releasing CO₂ and water as byproducts Science-Class VII, Life Processes in Animals, p.132. The formula for this vital chemical reaction is: Glucose + Oxygen → Carbon dioxide + Water + Energy.

The efficiency of this system relies on the Alveoli—millions of tiny, balloon-like sacs at the end of the respiratory tree. These structures are the primary site for gas exchange Science-Class VII, Life Processes in Animals, p.135. Their walls are incredibly thin and surrounded by a vast network of capillaries. When we inhale, the diaphragm flattens and the rib cage expands, creating a vacuum that pulls air into these sacs Science, class X, Life Processes, p.90. Here, oxygen diffuses into the blood, and carbon dioxide diffuses out. Interestingly, our lungs always maintain a residual volume of air; this ensures that gas exchange continues even between breaths, preventing the lungs from collapsing and providing a steady oxygen supply Science, class X, Life Processes, p.90.

In everyday chemistry, the behavior of these gases changes based on ambient pressure. Under normal conditions, nitrogen (which makes up 78% of our air) does not dissolve much in our blood. However, for deep-sea divers, the high pressure underwater forces more nitrogen to dissolve into the tissues. This can lead to nitrogen narcosis (a feeling of drunkenness) or decompression sickness (the "bends") if the diver surfaces too quickly and the gas forms bubbles in the blood. To prevent this, divers use specialized gas mixtures like Trimix or Heliox, where nitrogen is partially or fully replaced by Helium. Helium is used because it is an inert gas with much lower solubility in blood and is less dense, making it easier to breathe under high pressure.

Sources: Science-Class VII, Life Processes in Animals, p.132; Science-Class VII, Life Processes in Animals, p.135; Science, class X, Life Processes, p.90

4. Noble Gases: Properties of Helium (intermediate)

To understand why Helium is a staple in deep-sea exploration, we must first look at its fundamental nature. Helium is a noble gas, a group of elements defined by having a completely filled valence shell. As a result, these gases show very little chemical activity because they have no "desire" to gain or lose electrons to reach stability Science, Class X, Metals and Non-metals, p.46. Among the 118 known elements, Helium is one of the few that remains in a gaseous state at room temperature Science, Class VIII, Nature of Matter, p.123 and exists only in trace amounts in our atmosphere Physical Geography by PMF IAS, Earth's Atmosphere, p.270.

The primary challenge for scuba divers is pressure. As a diver descends, the increasing water pressure forces gases from their breathing tank to dissolve more readily into their blood and tissues. When using ordinary compressed air (which is ~78% Nitrogen), two dangerous conditions arise: Nitrogen Narcosis (a feeling of intoxication that impairs judgment) and Decompression Sickness (the "bends"), where nitrogen bubbles form in the blood during a rapid ascent. Because Nitrogen is relatively soluble in lipids (fats) and blood under pressure, it stays in the body longer and causes these toxic effects.

Helium is the ideal substitute for Nitrogen in deep-sea diving for three specific reasons:

• Low Solubility: Helium is significantly less soluble in blood and body tissues than Nitrogen. This means less gas is absorbed under pressure, and it can leave the body more safely during ascent.
• Chemical Inertness: It does not react with any biological molecules in the body, ensuring no toxic side effects at high concentrations.
• Low Density: Helium is much lighter than Nitrogen. At great depths, high-pressure air becomes "thick" and hard to move through the lungs; adding Helium reduces the work of breathing for the diver.

In practice, divers use specialized mixtures like Heliox (Helium and Oxygen) or Trimix (Helium, Nitrogen, and Oxygen). By diluting the Oxygen and Nitrogen with Helium, divers can descend to depths that would otherwise be fatal or incapacitating.

Sources: Science, Class X, Metals and Non-metals, p.46; Science, Class VIII, Nature of Matter: Elements, Compounds, and Mixtures, p.123; Physical Geography by PMF IAS, Earth's Atmosphere, p.270

5. Physiology of High Altitudes: Hypoxia and Anoxia (intermediate)

To understand why we feel breathless on a mountain peak, we must first look at the physics of the atmosphere. Imagine the atmosphere as a vast ocean of air. At sea level, we are at the bottom of this ocean, supporting the weight of a massive column of air above us. This weight creates atmospheric pressure, which averages about 1,013.2 millibars at sea level Fundamentals of Physical Geography, Geography Class XI (NCERT 2025 ed.), Atmospheric Circulation and Weather Systems, p.76. However, as you climb higher, there is less air above you. Consequently, the air becomes rarefied (thinner), and the pressure decreases rapidly—roughly 34 millibars for every 300 metres of ascent Physical Geography by PMF IAS, Pressure Systems and Wind System, p.305.

The chemistry of breathing depends on partial pressure. While the percentage of oxygen in the air remains constant (about 21%) up to very high altitudes, the total pressure drops. This means there are fewer actual oxygen molecules in every lungful of air you take. In our bodies, haemoglobin in red blood cells acts as a carrier, binding to oxygen to transport it to tissues Science, class X (NCERT 2025 ed.), Life Processes, p.90. For haemoglobin to "grab" oxygen effectively, the pressure of oxygen in the lungs must be high enough to push it into the blood. When that pressure is low, the blood becomes less saturated with oxygen.

This leads to two critical physiological states:

• Hypoxia: A condition where the body or a region of the body is deprived of adequate oxygen supply at the tissue level. This causes the breathlessness, dizziness, and fatigue often felt by trekkers Exploring Society: India and Beyond, Social Science-Class VII, Understanding the Weather, p.35.
• Anoxia: A more extreme state representing a total lack of oxygen supply. While hypoxia is common at high altitudes, anoxia is typically a medical emergency resulting from complete airway obstruction or severe altitude sickness.

Sources: Fundamentals of Physical Geography, Geography Class XI (NCERT 2025 ed.), Atmospheric Circulation and Weather Systems, p.76; Physical Geography by PMF IAS, Pressure Systems and Wind System, p.305; Science, class X (NCERT 2025 ed.), Life Processes, p.90; Exploring Society: India and Beyond, Social Science-Class VII, Understanding the Weather, p.35

6. Decompression Sickness and The Bends (exam-level)

To understand Decompression Sickness (DCS), commonly known as 'the bends,' we must first look at a fundamental principle of chemistry: Henry’s Law. This law states that the solubility of a gas in a liquid is directly proportional to the pressure applied to it. In our daily lives, we see this when opening a soda bottle—the sudden drop in pressure causes dissolved CO₂ to form bubbles. For a scuba diver, the human body acts like that soda bottle. Under the immense pressure of the deep sea, the Nitrogen (N₂) that makes up about 78% of the air we breathe Environment and Ecology, Majid Hussain, Basic Concepts of Environment and Ecology, p.20 dissolves much more readily into the blood plasma and tissues.

The danger arises during the ascent. If a diver rises to the surface too quickly, the external pressure drops rapidly, and the dissolved nitrogen can no longer stay in solution. It escapes the blood and tissues to form micro-bubbles. These bubbles can block blood flow or exert pressure on nerves, leading to symptoms like intense joint pain (which causes the diver to 'bend' over, hence the name), dizziness, or even paralysis Science, Class VIII, Health: The Ultimate Treasure, p.31. While our blood is designed to transport dissolved gases like CO₂ and nitrogenous wastes Science, Class X, Life Processes, p.91, it is not equipped to handle a sudden 'fizz' of gas bubbles within the circulatory system.

To mitigate these risks, modern diving chemistry utilizes specialized gas mixtures. Instead of using ordinary compressed air, deep-sea divers often use Heliox (Helium and Oxygen) or Trimix (Oxygen, Nitrogen, and Helium). Helium is the preferred diluent for several reasons:

• Low Solubility: Helium is much less soluble in human blood and fatty tissues than Nitrogen, meaning fewer bubbles form during ascent.
• Low Density: Helium is less dense than Nitrogen, which reduces the work of breathing in the high-pressure environment of the deep.
• Non-Narcotic: Unlike Nitrogen, which can cause 'Nitrogen Narcosis' (a feeling of drunkenness or disorientation) at great depths, Helium is chemically inert and does not affect the diver's mental state.

By diluting the air with Helium, we reduce the concentration of Nitrogen, thereby lowering the risk of both narcosis and the painful bubbles of decompression sickness.

Sources: Environment and Ecology, Majid Hussain (Access publishing 3rd ed.), BASIC CONCEPTS OF ENVIRONMENT AND ECOLOGY, p.20; Science, class X (NCERT 2025 ed.), Life Processes, p.91; Science, Class VIII NCERT (Revised ed 2025), Health: The Ultimate Treasure, p.31

7. Modern Diving Gas Mixtures: Heliox and Trimix (exam-level)

When we breathe at sea level, the Nitrogen (N₂) that makes up approximately 78% of our atmosphere is relatively inert and harmless PMF IAS, Physical Geography, Earths Atmosphere, p.271. However, as a scuba diver descends, the ambient pressure increases significantly. According to Henry’s Law, the solubility of gases in liquids increases with pressure. This means that at great depths, much more nitrogen dissolves into the diver's blood and tissues than at the surface.

This high concentration of dissolved nitrogen leads to two major risks: Nitrogen Narcosis (a state of disorientation similar to alcohol intoxication) and Decompression Sickness (the 'bends'), where nitrogen forms painful bubbles in the body during ascent. To mitigate these risks, professional and deep-sea divers use specialized gas mixtures like Heliox and Trimix. By replacing or diluting nitrogen with Helium (He), divers can safely reach depths where ordinary air would be toxic or narcotic.

Helium is the preferred choice for deep diving for several scientific reasons:

• Low Solubility: Helium is much less soluble in human blood and fatty tissues than nitrogen, which reduces the risk of narcosis and simplifies decompression.
• Low Density: Helium is a very light gas. At high pressures, air becomes thick and 'syrupy,' making it hard to breathe. Adding helium reduces the work of breathing because the gas mixture is less dense.
• Inertness: Like nitrogen, helium does not react chemically with the body, making it safe for respiration when mixed with oxygen PMF IAS, Physical Geography, Earths Atmosphere, p.272.

Sources: Physical Geography by PMF IAS, Earths Atmosphere, p.271; Physical Geography by PMF IAS, Earths Atmosphere, p.272; FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.), Composition and Structure of Atmosphere, p.66

8. Solving the Original PYQ (exam-level)

This question perfectly integrates your understanding of Henry’s Law and the physiological effects of gas solubility under pressure. As you learned, the solubility of a gas in a liquid is directly proportional to its partial pressure. When a diver descends, the increasing hydrostatic pressure forces more atmospheric gases into the bloodstream. If a diver breathes standard air, the high concentration of dissolved nitrogen can lead to Nitrogen Narcosis (a sedative effect) and, upon ascent, the formation of painful bubbles in the blood known as Decompression Sickness or 'the bends.' To prevent this, we must apply the principle of dilution using a gas that is less soluble and more inert than nitrogen.

The reasoning leads us directly to Helium. By using air diluted with Helium (often referred to as Trimix), we reduce the partial pressure of nitrogen, thereby minimizing the risk of narcosis. Helium is the ideal choice because it has very low solubility in human blood and tissues compared to nitrogen, and its low density makes the gas mixture easier to breathe at depth. This is a classic application of NCERT Chemistry Class 12, where the behavior of solutions under pressure is explained through real-world safety applications.

UPSC often uses common misconceptions as traps in the other options. For instance, using pure O2 (Option B) is extremely dangerous at depth because high partial pressures of oxygen become toxic to the central nervous system. Pure N2 (Option C) would be fatal due to immediate nitrogen narcosis and the lack of life-sustaining oxygen. Finally, a mixture of N2 and Helium (Option D) is incorrect because it excludes the Oxygen necessary for survival. Therefore, the balanced solution is to use air diluted with Helium to ensure a safe, breathable concentration of oxygen while mitigating the physical risks of high-pressure nitrogen.