Assertion (A): It is possible to fuse two ice cubes by pressing them together. Reason (R): Pressure affects the melting point of a solid

1. Basics of Phase Changes: Melting and Freezing (basic)

Welcome to your first step in mastering Thermal Physics! To understand phase changes like melting (solid to liquid) and freezing (liquid to solid), we must look at the microscopic world. All matter is composed of tiny particles held together by interparticle forces of attraction. In a solid, these particles are packed tightly and vibrate only slightly because their thermal energy is low Science, Class VIII NCERT, Particulate Nature of Matter, p.112. When we add heat, we increase this thermal energy until the particles gain enough strength to overcome those attractive forces and slide past one another—this is the point of melting.

A fascinating observation during this process is that the temperature remains constant while the substance is changing state. You might wonder: If we are still heating the pot, why isn't the thermometer rising? This is because the heat is being consumed as Latent Heat of Fusion. Instead of increasing the kinetic energy (temperature), the energy is busy "breaking the chains" of the solid structure to turn it into a liquid Physical Geography by PMF IAS, Vertical Distribution of Temperature, p.295. Different materials have vastly different melting points based on how strong their internal "glue" is:

Material	Melting Point (°C)	Nature of Force
Ice (H₂O)	0 °C	Relatively Weak
Urea	133 °C	Moderate
Iron (Fe)	1538 °C	Extremely Strong

Finally, we must touch upon a unique phenomenon called Regelation. Generally, increasing pressure raises the melting point of a substance. However, ice is an anomaly because it is less dense than liquid water. When you press two ice cubes together, the high pressure at the contact point lowers the melting point, causing a tiny bit of ice to melt into water. Once you release the pressure, the melting point jumps back to 0 °C, and that thin film of water immediately refreezes, welding the two cubes together. This is why you can form a snowball by squeezing loose snow!

Sources: Science, Class VIII NCERT, Particulate Nature of Matter, p.103, 112; Physical Geography by PMF IAS, Vertical Distribution of Temperature, p.295

2. Latent Heat of Fusion (basic)

When we heat a block of ice at 0 °C, a curious thing happens: even though we are providing energy, a thermometer stuck in the ice won't show a rise in temperature until all the ice has turned into water. This "hidden" energy is what we call Latent Heat. Specifically, the Latent Heat of Fusion is the energy required to change a substance from a solid to a liquid state without any change in its temperature Physical Geography by PMF IAS, Vertical Distribution of Temperature, p.294.

To understand why the temperature doesn't rise, we have to look at the molecular level. In a solid like ice, the constituent particles are closely packed and held together by strong interparticle forces of attraction Science, Class VIII NCERT, Particulate Nature of Matter, p.113. When heat is added during melting, that energy isn't used to make the particles move faster (which would raise the temperature); instead, it is consumed entirely to overcome or weaken these attractive forces, allowing the particles to move past each other and transition into a liquid state Science, Class VIII NCERT, Particulate Nature of Matter, p.113.

An fascinating application of this concept is Regelation. Most substances expand when they melt, but ice is an exception—it is less dense than liquid water. Because of this, when you apply high pressure to ice, its melting point actually drops below 0 °C. If you press two ice cubes together, a thin layer of ice at the contact point melts into water. Once you release the pressure, the melting point returns to normal, and that thin film of water refreezes, effectively "welding" the two cubes together. This reversible process of melting under pressure and refreezing once pressure is reduced is a classic demonstration of how phase changes interact with external forces.

Process	State Change	Energy Action
Fusion (Melting)	Solid → Liquid	Heat is Absorbed
Solidification (Freezing)	Liquid → Solid	Heat is Released

Sources: Physical Geography by PMF IAS, Vertical Distribution of Temperature, p.294-295; Science, Class VIII NCERT, Particulate Nature of Matter, p.113

3. Anomalous Expansion of Water (intermediate)

In the world of physics, most substances follow a predictable pattern: they expand when heated and contract when cooled. However, water is a 'rebel' with a unique property called Anomalous Expansion. Between the temperatures of 0°C and 4°C, water behaves exactly the opposite of what you would expect. While heating most liquids causes them to expand, heating water from 0°C to 4°C actually causes it to contract. Conversely, cooling water from 4°C down to 0°C causes it to expand.

This unique behavior means that water reaches its maximum density at exactly 4°C Science, Class VIII, The Amazing World of Solutes, Solvents, and Solutions, p.148. Below 4°C, the water molecules begin to arrange themselves into a structured, open hexagonal lattice. By the time water freezes at 0°C, this structure takes up more space than the liquid phase did. This is why ice is less dense than liquid water and why it floats on the surface of ponds and glasses alike Science, Class VIII, The Amazing World of Solutes, Solvents, and Solutions, p.148.

The ecological significance of this phenomenon cannot be overstated. In cold climates, as a lake cools, the 4°C water (being the densest) sinks to the bottom. The water that continues to cool toward 0°C becomes lighter and stays at the surface until it freezes into ice Science, Class VII, Heat Transfer in Nature, p.98. This floating ice acts as an insulating blanket, preventing the water below from freezing solid and allowing aquatic life to survive in the liquid 4°C environment at the bottom.

Temperature Change	Effect on Volume	Effect on Density
Heating from 0°C to 4°C	Decreases (Contraction)	Increases
Heating above 4°C	Increases (Expansion)	Decreases
Cooling from 4°C to 0°C	Increases (Expansion)	Decreases

Sources: Science, Class VIII (NCERT 2025), The Amazing World of Solutes, Solvents, and Solutions, p.148; Science, Class VII (NCERT 2025), Heat Transfer in Nature, p.98

4. Pressure and Boiling Points (Phase Change Context) (intermediate)

When we think of boiling water, we usually think of 100°C. However, this temperature is not a fixed universal constant; it is simply the temperature at which water boils at standard atmospheric pressure. Boiling is a physical struggle between the internal energy of liquid molecules and the external pressure pushing down on them. For a liquid to turn into a gas, its molecules must move vigorously enough to overcome the interparticle forces of attraction and escape into the air Science, Class VIII. NCERT(Revised ed 2025), Particulate Nature of Matter, p.105.

The relationship is direct: the higher the ambient pressure, the higher the boiling point. Conversely, if you reduce the pressure, the boiling point drops. This is why it is possible to boil water at room temperature in a vacuum chamber. As pressure decreases, evaporating molecules meet less resistance from the surrounding air and can escape the liquid state much more easily Physical Geography by PMF IAS, Manjunath Thamminidi, PMF IAS (1st ed.), Geological Time Scale, p.43. This concept explains why pressure cookers are so effective; by trapping steam to increase internal pressure, they allow water to reach temperatures well above 100°C without boiling away, which cooks food much faster Understanding Economic Development. Class X . NCERT(Revised ed 2025), CONSUMER RIGHTS, p.78.

Pressure also influences the melting point of substances, though the effect depends on the substance's density. For most materials, increased pressure raises the melting point because it forces particles closer together into a solid structure. However, water is a fascinating exception. Because ice is less dense than liquid water, applying pressure actually lowers its melting point. This phenomenon, known as regelation, allows you to fuse two ice cubes by pressing them together; the pressure melts a thin film of water at the interface, which then refreezes the moment the pressure is released.

Scenario	Pressure Change	Effect on Boiling Point	Practical Example
High Altitude	Lower Pressure	Decreases	Water boils at ~70°C on Mt. Everest.
Pressure Cooker	Higher Pressure	Increases	Food cooks faster at temperatures >100°C.

Sources: Science, Class VIII. NCERT(Revised ed 2025), Particulate Nature of Matter, p.105; Physical Geography by PMF IAS, Manjunath Thamminidi, PMF IAS (1st ed.), Geological Time Scale, p.43; Understanding Economic Development. Class X . NCERT(Revised ed 2025), CONSUMER RIGHTS, p.78

5. Pressure Effects on Melting: The Clausius-Clapeyron Principle (exam-level)

To understand how pressure affects melting, we must first look at how most substances behave. Usually, when a solid melts into a liquid, its molecules spread out and its volume increases. If you apply crushing pressure to such a substance, you are essentially trying to keep those molecules packed tightly together, making it much harder for them to transition into a liquid state. Consequently, for the vast majority of substances, increasing pressure raises the melting point. A perfect example of this is the Earth's inner core: even though temperatures reach a staggering 6000°C, the immense gravitational pressure prevents the iron from melting, keeping it solid Physical Geography by PMF IAS, Earths Interior, p.56.

However, Ice is a famous exception to this rule due to its unique crystalline structure. Unlike most substances, ice is actually less dense than liquid water—it expands when it freezes. This means that if you apply pressure to ice, you are actually encouraging it to take up less space, which it can only do by turning back into liquid water. This relationship is described by the Clausius-Clapeyron Principle: if a substance contracts upon melting, an increase in pressure will lower its melting point.

Substance Type	Volume change (Solid → Liquid)	Effect of Pressure on Melting Point
Normal (e.g., Iron)	Increases (Expands)	Melting Point Increases
Ice	Decreases (Contracts)	Melting Point Decreases

This principle explains a phenomenon known as regelation. If you take two ice cubes and press them firmly together, the high pressure at the interface lowers the melting point below the ambient temperature (0°C). A tiny layer of ice melts into a film of water. As soon as you release the pressure, the melting point returns to normal, and that thin film of water refreezes instantly, welding the two cubes together. This same process allows glaciers to move; the sheer weight of the ice can cause melting at the base, creating a thin layer of liquid that acts as a lubricant FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI, Landforms and their Evolution, p.56.

Sources: Physical Geography by PMF IAS, Earths Interior, p.56; FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI, Landforms and their Evolution, p.56; Science, Class VIII, NCERT, Particulate Nature of Matter, p.103

6. Regelation: The Physics of Ice Fusion (exam-level)

To understand Regelation, we must first look at the unique relationship between pressure and the state of matter for water. In most substances, increasing pressure raises the melting point because pressure forces molecules together, encouraging a solid state. However, ice is a rare and famous exception. Because ice is less dense than liquid water (it expands when it freezes), applying external pressure actually makes it easier for the molecules to collapse into a liquid state. According to the Clausius-Clapeyron equation, for substances that contract upon melting, an increase in pressure lowers the melting point.

The process of regelation occurs in two distinct steps. First, when you apply pressure to ice—for example, by pressing two ice cubes together or pulling a wire through a block of ice—the melting point at the contact interface drops below 0°C. This causes a thin layer of ice to melt into a film of water, even though the ambient temperature hasn't changed. Second, as soon as that pressure is removed, the melting point of the water film jumps back up to its normal 0°C. Since the surrounding ice is still freezing cold, this film of water refreezes (fuses) almost instantly, welding the two surfaces together.

This principle is not just a laboratory trick; it is fundamental to how the Earth's cryosphere behaves. As noted in FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Landforms and their Evolution, p.56, water plays a vital role in glacial movement. Regelation allows the base of a massive glacier to melt under its own immense weight, creating a thin lubricating layer of water that allows the ice to slide over rocky terrain. This same physics allows us to pack snow into a snowball; the pressure of our hands melts the edges of the snow crystals just enough to create a liquid bond that freezes solid the moment we stop pressing.

Sources: FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Landforms and their Evolution, p.56; Physical Geography by PMF IAS, Earth's Interior, p.56

7. Solving the Original PYQ (exam-level)

This question perfectly synthesizes the concepts of phase changes and the anomalous properties of water that you have just mastered. While most substances contract upon freezing, water expands, meaning ice is less dense than liquid water. According to the Clausius-Clapeyron principle, this unique property implies that increasing pressure will actually lower the melting point of ice. This specific phenomenon is known as regelation, where the application of pressure causes ice to melt at temperatures below 0°C, only to refreeze once that pressure is removed.

To arrive at the Correct Answer: (A), you must evaluate the logical bridge between the two statements. When you press two ice cubes together, the high pressure at the contact point lowers the melting point, creating a thin film of water. As soon as you release the pressure, the melting point jumps back to 0°C, causing that water to instantly refreeze and "weld" the cubes together. Thus, Assertion (A) is a visible result, and Reason (R) provides the underlying physical law that governs it. In the context of Physical Geography by PMF IAS, understanding these pressure-state relationships is crucial for grasping larger geomorphological processes like glacial movement.

UPSC often uses Option (B) as a trap for students who recognize both facts but fail to see the causal link. However, in this case, the "effect" (fusing) is impossible without the "cause" (pressure-induced melting point shift). Another common pitfall is over-generalizing; while (R) states pressure affects the melting point of all solids, the direction of that effect (raising vs. lowering) is what makes the ice cube experiment work. Since both statements are scientifically sound and directly related, options (C) and (D) can be quickly eliminated through systematic verification of the facts.