At high altitudes, pressure cooker is preferable for cooking, because the boiling point of water

1. Kinetic Theory and Phase Transitions (basic)

To understand thermal physics, we must first look at the very foundation of matter: the Kinetic Theory. This theory posits that all matter is composed of tiny particles (atoms or molecules) that are in constant, random motion. The state of matter—whether it is a solid, liquid, or gas—is determined by the balance between the kinetic energy of these particles and the attractive forces pulling them together. As we observe the Science, Class VIII (NCERT 2025 ed.), Chapter 7: Particulate Nature of Matter, p.107, the physical properties of a substance are heavily influenced by interparticle spacing and the freedom of movement of these particles.

When we add heat to a substance, we are essentially increasing the average kinetic energy of its particles. In a solid, particles only vibrate about fixed positions. As energy increases, they break free to slide past one another (liquid state), and eventually, they move entirely independently at high speeds (gaseous state). This transition is not just about temperature; it involves a "tug-of-war" with the environment. For a liquid to transition into a gas (boiling), its particles must gain enough energy to create a vapor pressure that matches the external atmospheric pressure pushing down on them. This is why boiling points are not fixed; they change depending on the surrounding pressure conditions.

Interestingly, the mass of the molecules also plays a role in these transitions. In a homologous series of compounds, as the molecular mass increases, the melting and boiling points also tend to rise because heavier molecules require more energy to overcome their mutual attractions and move apart Science, Class X (NCERT 2025 ed.), Carbon and its Compounds, p.67. Understanding this relationship between energy, pressure, and particle motion allows us to predict how substances will behave in different environments—from the deep ocean to the highest mountain peaks.

Sources: Science, Class VIII (NCERT 2025 ed.), Particulate Nature of Matter, p.107; Science, Class VIII (NCERT 2025 ed.), Particulate Nature of Matter, p.109; Science, Class X (NCERT 2025 ed.), Carbon and its Compounds, p.67

2. Atmospheric Pressure and Altitudinal Variation (basic)

3. Modes of Heat Transfer (intermediate)

Heat transfer is the flow of thermal energy from a region of higher temperature to a region of lower temperature. In nature, this energy doesn't just sit still; it moves through three distinct mechanisms: Conduction, Convection, and Radiation. Each of these plays a vital role in everything from how your coffee stays warm to how the Earth’s atmosphere circulates Science-Class VII . NCERT, Heat Transfer in Nature, p.101.

Conduction is the process of heat transfer through a material via molecular activity. Imagine a line of people passing buckets of water; the people stay in place, but the water moves. Similarly, in conduction, the particles of the substance do not move from their positions; they simply vibrate and pass the energy to their neighbors Science-Class VII . NCERT, Heat Transfer in Nature, p.91. This is the primary mode of heat transfer in solids. Materials like metals are good conductors because they facilitate this transfer easily, while materials like wood or air are poor conductors (insulators) Physical Geography by PMF IAS, Horizontal Distribution of Temperature, p.282.

Convection involves the actual bulk movement of molecules within a fluid (liquids or gases). When a fluid is heated, it becomes less dense and rises, while cooler, denser fluid sinks to take its place, creating a cycle. This is why water in a kettle heats up uniformly even if the flame is only at the bottom Science-Class VII . NCERT, Heat Transfer in Nature, p.97. On a larger scale, convection is responsible for land and sea breezes and atmospheric circulation Science-Class VII . NCERT, Heat Transfer in Nature, p.102.

Finally, Radiation is the most unique mode because it requires no medium at all. Heat travels through the vacuum of space as electromagnetic waves. This is how the Sun’s energy reaches the Earth across millions of miles of empty space Science-Class VII . NCERT, Heat Transfer in Nature, p.102. Every object around you, including your own body, is constantly emitting and absorbing heat through radiation.

Sources: Science-Class VII . NCERT, Heat Transfer in Nature, p.91, 97, 101, 102; Physical Geography by PMF IAS, Horizontal Distribution of Temperature, p.282

4. Latent Heat and Thermal Properties (intermediate)

In thermal physics, Latent Heat (the word 'latent' comes from the Latin latere, meaning 'to lie hidden') is the energy absorbed or released by a substance during a change in its physical state without any change in its temperature. Unlike Sensible Heat, which you can 'sense' through a thermometer as the temperature rises or falls, latent heat is used exclusively to break or form the molecular bonds during a phase change. For instance, when you boil water, the temperature stays at 100°C even as you continue to add heat; this energy is being 'hidden' in the steam as latent heat of vaporization Physical Geography by PMF IAS, Vertical Distribution of Temperature, p.294.

This concept is a cornerstone of meteorology. When water evaporates from the ocean, it stores latent heat. When that vapor eventually rises and cools in the atmosphere, it undergoes condensation, releasing that stored energy back into the air. This 'latent heat of condensation' acts as the fuel for tropical cyclones and thunderstorms. Because this process adds heat back into a rising air parcel, it cools down much more slowly than dry air—a phenomenon known as the Wet Adiabatic Lapse Rate Physical Geography by PMF IAS, Vertical Distribution of Temperature, p.299.

Beyond phase changes, substances also differ in their Specific Heat—the amount of heat required to raise the temperature of a unit mass by one degree. Water has a specific heat roughly 2.5 times higher than land. This is why coastal areas have moderate climates; the ocean acts as a massive thermal reservoir that takes a long time to heat up and a long time to cool down compared to the rapidly fluctuating temperature of solid ground Physical Geography by PMF IAS, Horizontal Distribution of Temperature, p.286.

Finally, it is important to understand the role of Atmospheric Pressure. The boiling point of a liquid is the temperature at which its internal vapor pressure equals the surrounding atmospheric pressure. At high altitudes (like the Himalayas), the air pressure is lower, meaning water molecules need less energy to escape into a gaseous state. Consequently, water boils at a lower temperature (e.g., 90°C), which unfortunately means food takes longer to cook because the 'boiling' water isn't hot enough to cook the fibers quickly. This is why we use pressure cookers: they artificially raise the pressure to increase the boiling point, allowing for faster cooking at higher temperatures.

Sources: Physical Geography by PMF IAS, Vertical Distribution of Temperature, p.294; Physical Geography by PMF IAS, Vertical Distribution of Temperature, p.299; Physical Geography by PMF IAS, Horizontal Distribution of Temperature, p.286

5. Vapor Pressure and Boiling Point Relationship (exam-level)

To understand why water behaves differently on a mountaintop versus a kitchen in the plains, we must first look at the invisible tug-of-war happening at the surface of any liquid. Every liquid exerts an upward "push" called Vapor Pressure. This represents the tendency of molecules to escape the liquid state and become a gas. Opposing this is the Ambient Pressure (usually atmospheric pressure), which acts like a lid, pushing down and keeping the molecules trapped in their liquid form Physical Geography by PMF IAS, Chapter 23, p. 358.

Boiling occurs only when the liquid is heated enough that its internal vapor pressure becomes exactly equal to the external atmospheric pressure. At this precise moment, the movement of particles becomes so vigorous that they can overcome interparticle forces of attraction and escape not just from the surface, but from within the liquid itself, forming bubbles Science, Class VIII. NCERT (Revised ed 2025), Chapter 7, p. 105. Therefore, the Boiling Point is not a fixed number; it is a moving target that depends entirely on the surrounding pressure.

This relationship leads to two fascinating real-world scenarios that are frequently tested in competitive exams:

Essentially, by decreasing ambient pressure, we make it easier for molecules to escape because they meet less resistance from air molecules Physical Geography by PMF IAS, Chapter 3, p. 43. This is why, in a laboratory vacuum, it is even possible to make water boil at room temperature without any heating at all!

Sources: Physical Geography by PMF IAS, Chapter 23: Pressure Systems and Wind System, p.358; Science, Class VIII. NCERT (Revised ed 2025), Chapter 7: Particulate Nature of Matter, p.105; Physical Geography by PMF IAS, Chapter 3: Geological Time Scale, p.43

6. The Pressure Cooker: Working Principle (exam-level)

7. Solving the Original PYQ (exam-level)

Having mastered the concepts of atmospheric pressure and the states of matter, you can now see how these fundamental physical laws apply to everyday survival at high altitudes. As detailed in FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.), the density of air molecules decreases with height, leading to a lower atmospheric pressure. Since the boiling point is defined as the temperature at which a liquid's vapor pressure equals the surrounding atmospheric pressure, water requires less thermal energy to turn into steam when that external pressure is reduced. This results in water boiling at a lower temperature—often as low as 90°C—which makes cooking food in an open pot incredibly slow, as the water cannot get hot enough to cook the ingredients efficiently.

To solve this, we arrive at the correct answer (B) reduces due to lower atmospheric pressure. The logic follows a clear chain: Increasing Altitude → Decreasing Pressure → Decreased Boiling Point. A pressure cooker solves this problem by sealing the vessel and trapping steam, which artificially increases the internal pressure, thereby raising the boiling point back up to higher temperatures. This allows food to cook much faster. When analyzing the options, always watch for reversal traps like Option (A), which incorrectly suggests pressure increases at altitude, or irrelevant distractors like Options (C) and (D).

UPSC frequently tests your ability to distinguish between correlation and causation. While it is true that gravitational force changes with distance from the Earth's center and ozone levels vary in the atmosphere, these factors do not directly determine the boiling point of water in a cooking context. As explained in Science, Class VIII. NCERT (Revised ed 2025), the phase change of a liquid is primarily governed by the particulate nature of matter and the external pressure acting upon those particles. Always focus on the direct physical mechanism—pressure—rather than atmospheric composition or gravity.