Why do two ice blocks join to form one block when pressed together?

1. Basics of States of Matter and Phase Change (basic)

Concept: Basics of States of Matter and Phase Change

2. Heat and Latent Heat (basic)

When we add heat to a substance, we usually expect its temperature to rise. However, there are specific moments where the thermometer stops moving even though heat is still being applied. This "hidden" energy is known as Latent Heat. It is the energy absorbed or released by a substance during a change in its physical state (phase change) that occurs without changing its temperature Physical Geography by PMF IAS, Vertical Distribution of Temperature, p.294.

Think of it this way: heat energy does two jobs. First, it can increase the kinetic energy of molecules, which we feel as a rise in temperature. Second, it can work to break the bonds holding molecules together. During a phase change, the energy stops raising the temperature and focuses entirely on breaking these bonds to turn a solid into a liquid or a liquid into a gas. We categorize this into two main types:

• Latent Heat of Fusion: The heat required to change a substance from solid to liquid (or released when liquid turns to solid) at its melting point. For example, ice at 0°C stays at 0°C until every crystal has melted into water Physical Geography by PMF IAS, Vertical Distribution of Temperature, p.294.
• Latent Heat of Vaporization: The heat required to change a liquid into a gas. This is why boiling water stays at 100°C until it has all evaporated; the extra heat is being carried away by the vapor molecules Physical Geography by PMF IAS, Vertical Distribution of Temperature, p.294.

In Geography and Earth Sciences, this concept is a "powerhouse." When water vapor in the atmosphere condenses into rain, it releases Latent Heat of Condensation. This release of energy actually warms the surrounding air, making it more buoyant and helping fuel massive weather systems like tropical cyclones Physical Geography by PMF IAS, Vertical Distribution of Temperature, p.299. Even deep inside our planet, the solidification of the Earth's core releases latent heat of crystallization, which contributes to the internal heat that drives plate tectonics Physical Geography by PMF IAS, Earths Interior, p.59.

An interesting exception to standard thermal behavior occurs with ice and pressure. While most substances require higher temperatures to melt under pressure, ice is different because water is denser than ice. Applying pressure to ice actually lowers its melting point—a phenomenon called regelation. When you press two ice blocks together, the interface melts slightly; when you release the pressure, the water refreezes, bonding the blocks into one. This unique property allows glaciers to "flow" and explains why we can form snowballs by squeezing shredded ice.

Sources: Physical Geography by PMF IAS, Vertical Distribution of Temperature, p.294; Physical Geography by PMF IAS, Vertical Distribution of Temperature, p.295; Physical Geography by PMF IAS, Earths Interior, p.59; Physical Geography by PMF IAS, Vertical Distribution of Temperature, p.299

3. Anomalous Expansion of Water (intermediate)

In the physical world, most substances follow a predictable rule: they contract (shrink) when cooled and expand when heated. However, water is a fascinating rebel. Between the temperatures of 0°C and 4°C, water exhibits what we call the Anomalous Expansion of Water. While most liquids get denser as they get colder, water reaches its maximum density at exactly 4°C Science, Class VIII NCERT, The Amazing World of Solutes, Solvents, and Solutions, p.148.

To understand this, imagine cooling a beaker of water starting from room temperature. As the temperature drops toward 4°C, the water behaves normally—it contracts and becomes denser. But once it hits 4°C, any further cooling causes it to expand. As it approaches 0°C and begins to freeze, the molecules arrange themselves into a crystalline "open-cage" structure. This structure takes up more space than the liquid state, meaning the same mass of water now occupies a larger volume. Consequently, ice is less dense than liquid water, which is why ice floats Science, Class VIII NCERT, The Amazing World of Solutes, Solvents, and Solutions, p.148.

This anomaly has profound consequences for life on Earth. In cold climates, as a lake cools, the densest water (at 4°C) sinks to the bottom while the lighter, colder water rises to the surface to freeze into ice. This ice layer then acts as an insulator, trapping heat below and allowing aquatic plants and animals to survive in the 4°C liquid water at the bottom, even if the air temperature is -20°C. While we generally learn in geography that cold water is denser and tends to sink Geography, Class XI NCERT, Movements of Ocean Water, p.111, this rule only holds true until water reaches that critical 4°C threshold.

Sources: Science, Class VIII NCERT, The Amazing World of Solutes, Solvents, and Solutions, p.148; Geography, Class XI NCERT, Movements of Ocean Water, p.111

4. Pressure and Boiling Point (The Pressure Cooker Principle) (intermediate)

To understand the Pressure Cooker Principle, we first need to look at what boiling actually is. At a microscopic level, boiling is a "battle" between two opposing forces. On one side, heat makes liquid particles move vigorously, trying to break free from their neighbors. On the other side, the surrounding air (atmospheric pressure) pushes down on the liquid, effectively "pinning" the particles in place. As explained in Science, Class VIII NCERT, Particulate Nature of Matter, p.105, boiling occurs when the internal movement becomes so energetic that particles overcome these interparticle forces and escape into a gaseous state.

The temperature at which this escape happens—the boiling point—is not fixed; it is highly dependent on the ambient pressure. If you increase the pressure pushing down on the liquid, the particles need even more energy (higher temperature) to push back and escape. This is why the boiling point of liquids increases when pressure increases, and decreases when pressure is reduced. For instance, in the early history of Earth, water remained liquid at a staggering 230°C because the atmospheric pressure was over 27 times higher than it is today Physical Geography by PMF IAS, Geological Time Scale, p.43.

In a pressure cooker, we use this principle to our advantage. By sealing the pot, we trap steam inside, which significantly increases the internal pressure. This forces the boiling point of water to rise above the standard 100°C. Because the water is now hotter than usual without turning into steam, it transfers heat to the food much faster, reducing cooking time. Conversely, at high altitudes like the Himalayas, the air is "thinner" and exerts less pressure. As molecules meet less resistance from air molecules, they enter the air more easily Physical Geography by PMF IAS, Geological Time Scale, p.43. Consequently, water might boil at only 90°C, making it notoriously difficult to cook food thoroughly in an open pot because the water disappears as steam before it gets hot enough to cook the meal.

Sources: Science, Class VIII NCERT, Particulate Nature of Matter, p.105; Physical Geography by PMF IAS, Geological Time Scale The Evolution of The Earths Surface, p.43; Physical Geography by PMF IAS, Tropical Cyclones, p.358

5. Heat Transfer: Conduction, Convection, and Radiation (basic)

Heat always flows from a body at a higher temperature to a body at a lower temperature. This transfer of thermal energy happens through three distinct mechanisms: conduction, convection, and radiation. Understanding these is fundamental to both physics and geography, as they govern everything from how we cook food to how the Earth’s atmosphere maintains its temperature.

Conduction is the process of heat transfer where energy is passed from one particle to the next through direct contact, but the particles themselves do not move from their positions Science-Class VII . NCERT, Heat Transfer in Nature, p.91. Imagine a bucket brigade where people stay in line and just pass the bucket along; that is conduction. This is the primary mode of heat transfer in solids. Materials like metals are good conductors because they allow heat to pass through them easily, which is why our cooking utensils are made of copper or stainless steel. Conversely, materials like wood, plastic, and air are poor conductors (insulators) Science-Class VII . NCERT, Heat Transfer in Nature, p.101.

Convection differs because it involves the actual movement of particles. When a fluid (liquid or gas) is heated, the particles near the heat source become less dense and rise, while cooler, denser particles sink to take their place, creating a circulation loop called a convection current Science-Class VII . NCERT, Heat Transfer in Nature, p.102. This is why a room warms up when a heater is on the floor. Finally, Radiation is unique because it does not require any material medium to travel Science-Class VII . NCERT, Heat Transfer in Nature, p.97. It travels through electromagnetic waves, which is how the Sun's heat reaches Earth through the vacuum of space.

Feature	Conduction	Convection	Radiation
Medium Required?	Yes (mostly solids)	Yes (fluids: liquid/gas)	No (can travel in vacuum)
Particle Movement	Vibrate in place; no migration	Actual bulk movement	No particles involved
Example	Heating a metal rod	Boiling water	Feeling heat from a fire

Sources: Science-Class VII . NCERT(Revised ed 2025), Heat Transfer in Nature, p.91; Science-Class VII . NCERT(Revised ed 2025), Heat Transfer in Nature, p.97; Science-Class VII . NCERT(Revised ed 2025), Heat Transfer in Nature, p.101; Science-Class VII . NCERT(Revised ed 2025), Heat Transfer in Nature, p.102; Physical Geography by PMF IAS, Horizontal Distribution of Temperature, p.282

6. Effect of Pressure on Melting Point (exam-level)

To understand how pressure affects the melting point, we must first look at how a substance behaves when it changes state. For the vast majority of substances, the solid form is denser than the liquid form. This means that as they melt, they expand in volume. When you apply pressure to such a substance, you are essentially trying to keep it compressed. Since the substance needs to expand to melt, the external pressure resists this change. Consequently, you need to provide even more heat to overcome this resistance, which increases the melting point. Most materials, like iron or urea, follow this standard rule (Science, Class VIII NCERT, Particulate Nature of Matter, p.103).

However, ice is a fascinating exception. Water is one of the few substances that is more dense as a liquid than as a solid (ice floats because it is less dense). Because ice contracts when it melts, applying pressure actually 'helps' the process. Pressure favors the state that takes up less space. Therefore, when you increase the pressure on ice, its melting point decreases. This means ice can melt at temperatures slightly below 0°C if enough pressure is applied.

Substance Type	Volume Change on Melting	Effect of Increased Pressure
Normal Solids (e.g., Iron, Wax)	Expands	Melting Point Increases
Anomalous Solids (e.g., Ice, Antimony)	Contracts	Melting Point Decreases

This phenomenon leads to a process called Regelation. When you press two blocks of ice together, the high pressure at the contact point lowers the melting point, causing a thin layer of ice to melt into water. Once the pressure is released, the melting point returns to its normal 0°C, and that thin layer of water instantly refreezes, fusing the two blocks into one. This same principle allows us to form snowballs by squeezing shredded ice and explains how massive glaciers can slide over rocky terrain by melting slightly at their base due to the immense weight above (Physical Geography by PMF IAS, Vertical Distribution of Temperature, p.294).

Sources: Science, Class VIII NCERT, Particulate Nature of Matter, p.103; Physical Geography by PMF IAS, Vertical Distribution of Temperature, p.294

7. The Phenomenon of Regelation (exam-level)

In the study of thermal physics, water is famously known for its anomalous behavior. Most substances contract when they freeze, but water expands, making ice less dense than liquid water. This unique property leads to the fascinating phenomenon of Regelation. Typically, increasing pressure on a solid raises its melting point; however, because ice is less dense than water, applying pressure actually lowers its melting point. This means that under high pressure, ice can melt into water even if the temperature is exactly at or slightly below 0°C.

The process of regelation follows a distinct cycle. When you press two blocks of ice together, the pressure at the interface lowers the melting point. A thin film of ice melts into water, which acts as a lubricant. Once the pressure is released, the melting point instantly returns to its normal value (0°C). Since the surrounding ice is still at 0°C or colder, this thin layer of water refreezes (solidifies). This refreezing acts as a thermal weld, fusing the two separate blocks into one single mass. This is exactly why you can form a snowball by squeezing loose, shredded snow together.

This principle is a cornerstone of glaciology. Glaciers are massive bodies of ice that move primarily due to gravity, as noted in NCERT Class XI Fundamentals of Physical Geography, Landforms and their Evolution, p.54. However, the reason they can "flow" over rugged terrain is partly due to regelation. The immense weight of the glacier exerts high pressure on the ice at its base. This pressure causes the bottom layer of ice to melt into a thin film of water, which lubricates the contact point between the glacier and the bedrock, allowing the massive ice sheet to slide forward. This phenomenon is also crucial in understanding how the Earth's interior and materials behave under extreme pressure, a concept touched upon in Physical Geography by PMF IAS, Earth's Interior, p.56.

Sources: NCERT Class XI Fundamentals of Physical Geography, Landforms and their Evolution, p.54; Physical Geography by PMF IAS, Earth's Interior, p.56

8. Solving the Original PYQ (exam-level)

You have just mastered the unique relationship between pressure and phase changes, particularly the anomalous behavior of water. While most substances become denser when they freeze, water expands, meaning ice is less dense than liquid water. This specific property is the foundation for the phenomenon of regelation. When you apply pressure by pressing two ice blocks together, you are effectively forcing the molecules into a state that favors the denser phase—liquid water. Consequently, the melting point of ice is lowered with increase in pressure, causing a microscopic layer of ice at the contact point to melt into water, even if the environment is below 0°C.

To solve this, follow the logical sequence: the application of pressure induces melting, but once that pressure is withdrawn, the melting point instantly returns to its normal value. The thin film of water, now surrounded by sub-zero ice, refreezes (solidifies) almost immediately. This refreezing act creates a crystalline bond that fuses the two separate pieces into a single block. Therefore, Option (A) is the only choice that explains the underlying physical mechanism. This principle is not just a laboratory trick; as highlighted in Physical Geography by PMF IAS, it is the same mechanism that allows massive glaciers to slide over rocky terrain.

UPSC frequently uses distractor traps by presenting general rules that don't apply to exceptions. Option (B) is a common trap because it describes the behavior of most other solids, where pressure increases the melting point. Option (C) is factually incorrect as pressure and temperature are thermodynamically linked. Finally, Option (D), while a true statement under standard atmospheric pressure, fails to explain the change occurring during the interaction. Remember, in UPSC Prelims, you are looking for the causal factor, not just a true statement. Always identify if the substance in the question—like ice—follows the standard rule or is a known exception.