When ice melts, its

1. States of Matter and Phase Transitions (basic)

Welcome to your first step in mastering Thermal Physics! To understand how the world around us responds to heat, we must first look at the very building blocks of matter. Matter typically exists in three states: solid, liquid, and gas. The primary difference between these lies in how their constituent particles are arranged. In a solid, particles are closely packed with strong interparticle interactions, giving them a fixed shape and volume. In contrast, liquids have particles that are still close but can move past each other, allowing them to flow while maintaining a fixed volume Science Class VIII NCERT, Particulate Nature of Matter, p.113.

A fascinating phenomenon occurs during a phase transition (like melting or boiling). If you heat a block of ice, its temperature rises until it reaches 0 °C. At this point, even if you keep adding heat, the temperature does not change until all the ice has turned into water. This "hidden" heat is called Latent Heat Physical Geography by PMF IAS, Vertical Distribution of Temperature, p.294. Instead of raising the temperature, this energy is used to break the bonds holding the particles in their rigid solid structure. When ice melts, it absorbs the latent heat of fusion; when water evaporates, it absorbs the latent heat of vaporization Physical Geography by PMF IAS, Vertical Distribution of Temperature, p.295.

While most substances expand when they melt, water is a unique rebel. Due to its hydrogen-bonding network, ice forms an open, hexagonal crystalline structure that takes up more space than liquid water. When ice at 0 °C melts into water at 0 °C, this structure collapses. The result? The mass remains constant (conserving the total amount of matter), but the volume decreases by about 9%. This explains why ice is less dense than water and why it floats—a critical fact for Earth's geography and biology!

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

2. Thermal Expansion and Contraction (basic)

At the heart of thermal physics lies a simple rule: matter is made of tiny particles that are constantly in motion. When we add heat, we increase the thermal energy of these particles. In a solid state, particles are closely packed and held by strong attractive forces, limiting them to small vibrations (Science, Class VIII NCERT, Particulate Nature of Matter, p.112). As temperature rises, these particles vibrate more vigorously and push further apart, leading to thermal expansion. This is why air parcels rise when heated—they expand, become less dense, and grow lighter than the surrounding air (Physical Geography by PMF IAS, Vertical Distribution of Temperature, p.297). Conversely, cooling usually causes particles to lose energy and huddle closer together, resulting in contraction.

While most substances expand when they transition from a solid to a liquid, water is a fascinating exception to this rule. In its solid form (ice), water molecules arrange themselves into a rigid, open crystalline structure held together by hydrogen bonds. This "caged" structure actually takes up more space than liquid water. When ice reaches its melting point of 0 °C (Science, Class VIII NCERT, Particulate Nature of Matter, p.103), the thermal energy collapses this cage. The molecules move past each other and actually pack closer together (Science, Class VIII NCERT, Particulate Nature of Matter, p.113). Consequently, when ice melts, its volume decreases by about 9%.

It is vital to distinguish between volume and mass here. Throughout this expansion or contraction, the mass remains constant. Matter is neither created nor destroyed; only the space it occupies (volume) and how tightly it is packed (density) change. This explains why ice floats on water: because it occupies more volume for the same mass, it is less dense than the liquid it originated from.

Process	Mass	Volume	Density
Most Solids Melting	Constant	Increases	Decreases
Ice Melting (0 °C)	Constant	Decreases	Increases

Sources: Science, Class VIII NCERT, Particulate Nature of Matter, p.112; Physical Geography by PMF IAS, Vertical Distribution of Temperature, p.297; Science, Class VIII NCERT, Particulate Nature of Matter, p.103; Science, Class VIII NCERT, Particulate Nature of Matter, p.113

3. The Principle of Conservation of Mass (basic)

In our journey through thermal physics, we must first ground ourselves in the fundamental nature of matter. In everyday language, we often use the terms mass and weight interchangeably, but as a civil services aspirant, you must distinguish them clearly. Mass is the total quantity of matter present in an object, whereas weight is the gravitational force acting upon that mass Science, Class VIII, p.142. While your weight might change if you move from Earth to the Moon, your mass remains identical because the amount of "stuff" you are made of hasn't changed.

This leads us to the Principle of Conservation of Mass. Originally established in the context of chemistry, this law states that mass can neither be created nor destroyed Science, Class X, p.3. When we apply this to thermal physics—specifically during a phase transition like melting—it means that the total number of molecules stays exactly the same. Whether water is in a solid block of ice or a pool of liquid, the individual H₂O molecules are simply being rearranged; none are lost or gained in the process.

However, water is famously "anomalous." Most substances expand when they melt, but water does the opposite. Due to a unique hydrogen-bonding network, ice forms an open, cage-like crystalline structure that takes up more space than liquid water. When ice melts at 0 °C, this rigid structure collapses, and the molecules actually pack closer together, leading to a volume reduction of about 9%. It is vital to remember: the density increases and the volume decreases, but the mass remains constant. Even if the "size" of the substance looks different to your eyes, the scale would show that the quantity of matter has not shifted.

Sources: Science, Class VIII (NCERT 2025 ed.), The Amazing World of Solutes, Solvents, and Solutions, p.142; Science, Class X (NCERT 2025 ed.), Chemical Reactions and Equations, p.3

4. Archimedes' Principle and Buoyancy (intermediate)

When we submerge an object in a fluid, whether it is a pebble in a glass or a massive ship in the ocean, it experiences an upward force called buoyant force. According to Archimedes' Principle, this upward force is exactly equal to the weight of the fluid that the object displaces. This principle determines whether an object will sink or float: if the object’s weight is greater than the weight of the displaced fluid, it sinks; if they are equal, the object floats Science, Class VIII, Exploring Forces, p.76.

In the context of thermal physics, water behaves in a very peculiar way compared to almost every other substance. Most materials become denser as they cool and transition from liquid to solid. However, water reaches its maximum density at 4 °C. As it cools further and freezes into ice at 0 °C, its molecules arrange themselves into an open, hexagonal crystalline structure held together by hydrogen bonds. This structure takes up more space than the liquid state, meaning water expands upon freezing. Since the mass remains the same but the volume increases, the density of ice is lower than that of liquid water (approximately 9% less dense), which is why ice floats Science, Class VIII, The Amazing World of Solutes, Solvents, and Solutions, p.148.

This density difference is the reason why massive icebergs in the ocean have only about one-ninth of their mass visible above the water line, while the rest remains hidden beneath the surface Certificate Physical and Human Geography, Landforms of Glaciation, p.58. When this ice eventually melts back into liquid water, the rigid crystalline structure collapses. The molecules move closer together, leading to a decrease in volume while the total mass remains conserved. This unique thermal property is vital for life; it ensures that lakes freeze from the top down rather than the bottom up, insulating the aquatic life below.

State of H₂O	Structure	Density Change
Liquid (at 4 °C)	Closely packed molecules	Highest Density
Solid Ice (at 0 °C)	Open crystalline lattice	Lower Density (Expansion)

Sources: Science, Class VIII NCERT, Exploring Forces, p.76, 79; Science, Class VIII NCERT, The Amazing World of Solutes, Solvents, and Solutions, p.148; Certificate Physical and Human Geography, GC Leong, Landforms of Glaciation, p.58

5. Latent Heat and Isothermal Phase Changes (intermediate)

In thermal physics, Latent Heat (derived from the Latin latere, meaning 'to lie hidden') is the energy absorbed or released by a substance during a change in its physical state that occurs without any change in temperature. When you heat a block of ice at 0 °C, the thermometer won't budge until every crystal has turned to liquid. This is because the energy supplied isn't increasing the kinetic energy (temperature) of the molecules; instead, it is being used to break the molecular bonds holding the solid together Physical Geography by PMF IAS, Vertical Distribution of Temperature, p.294.

While most substances expand when they melt, water is a fascinating exception. In its solid state (ice), water molecules form an open crystalline structure held together by hydrogen bonds. This lattice has a lot of empty space, making ice less dense than liquid water. When ice reaches its melting point (0 °C), the addition of the latent heat of fusion causes this structure to collapse. The molecules actually move closer together, leading to a decrease in volume of about 9% while the mass remains constant Physical Geography by PMF IAS, Vertical Distribution of Temperature, p.295. This is why ice floats and why liquid water is denser than ice.

This concept is vital in Geography and Meteorology. For instance, when water vapor in a rising air parcel condenses into clouds, it releases latent heat of condensation. This 'internal' heat source warms the air parcel from within, causing it to cool down much more slowly than dry air as it rises—a phenomenon known as the wet adiabatic lapse rate Physical Geography by PMF IAS, Vertical Distribution of Temperature, p.299. Without this energy transfer, our weather systems and storms would lack their primary fuel source.

Process	Phase Change	Energy Status
Fusion (Melting)	Solid to Liquid	Energy Absorbed
Vaporisation	Liquid to Gas	Energy Absorbed
Condensation	Gas to Liquid	Energy Released
Solidification	Liquid to Solid	Energy Released

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, Vertical Distribution of Temperature, p.299

6. Anomalous Expansion of Water (exam-level)

In the study of thermal physics, we generally observe that substances expand when heated and contract when cooled. This is because higher temperatures increase molecular kinetic energy, causing particles to push further apart. However, water is a unique exception to this rule. Between the temperatures of 0 °C and 4 °C, water exhibits what we call Anomalous Expansion. While most liquids continue to contract as they cool, water reaches its maximum density at 4 °C. If you cool it further from 4 °C down to 0 °C, it actually begins to expand, becoming less dense. This is why ice floats on liquid water—a property that is fundamental to Earth's climate and biology. Science, Class VIII NCERT (Revised ed 2025), The Amazing World of Solutes, Solvents, and Solutions, p.141

The reason for this behavior lies at the molecular level. Water molecules are held together by Hydrogen Bonding. In liquid form, these molecules are close together but move around somewhat randomly. As water freezes into ice, the molecules are forced into a rigid, open crystalline structure (a hexagonal lattice) to accommodate the hydrogen bonds. This lattice contains a significant amount of empty space, meaning a fixed mass of ice occupies more volume than the same mass of liquid water. When ice melts at 0 °C, this structure collapses, and the molecules move closer together, leading to a volume reduction of approximately 9%. Even though the state changes, the chemical identity remains the same. Science, Class VIII NCERT (Revised ed 2025), Nature of Matter, p.121

This anomaly has profound ecological consequences. In cold regions, as the surface water of a lake cools toward 4 °C, it becomes denser and sinks, replaced by warmer water from below. However, once the surface water reaches 4 °C, any further cooling makes it less dense, so it remains on the surface until it freezes. This ensures that the densest, 4 °C water stays at the bottom, while the ice forms an insulating layer on top. This prevents entire water bodies from freezing solid, allowing aquatic life to survive in the liquid water below, even when surface temperatures are near 0 °C. Fundamentals of Physical Geography, Geography Class XI (NCERT 2025 ed.), Water (Oceans), p.104

Temperature Range	Effect of Cooling	Density Change
Above 4 °C	Contraction (Normal)	Increases
4 °C to 0 °C	Expansion (Anomalous)	Decreases
At 0 °C (Freezing)	Significant Expansion	Sharp Decrease

Sources: Science, Class VIII NCERT (Revised ed 2025), The Amazing World of Solutes, Solvents, and Solutions, p.141; Science, Class VIII NCERT (Revised ed 2025), Nature of Matter, p.121; Fundamentals of Physical Geography, Geography Class XI (NCERT 2025 ed.), Water (Oceans), p.104

7. Solving the Original PYQ (exam-level)

Now that you have mastered the anomalous expansion of water and the density-volume relationship, this question tests your ability to apply those building blocks to a real-world phase change. While most substances follow the general rule of expanding when melting, water is a critical exception. According to the principles found in NCERT Class XI Physics, the hydrogen-bonded hexagonal lattice of ice is an open structure with significant empty space. When you apply thermal energy to melt the ice, this rigid 'cage' collapses, allowing the molecules to pack more tightly together in the liquid state.

To arrive at the correct answer, you must walk through two logical steps. First, apply the Law of Conservation of Mass: since no matter is added or removed during the melting process, the mass must remain constant. Second, recall that liquid water is denser than ice (which is why ice floats). Since Density = Mass / Volume, if the mass stays the same and the density increases as the ice turns to liquid, the volume must decrease. Therefore, the logical conclusion is that (A) volume decreases is the only choice that fits the physical reality of water's behavior.

UPSC often designs these questions to trap students who rely on generalizations rather than specific properties. Option (B) is a common pitfall because most materials expand when they melt, making an 'increase' seem intuitive. Options (C) and (D) are classic distractors that suggest a change in mass; as a savvy aspirant, you should immediately eliminate these because mass remains unchanged during a physical phase transition. By isolating the anomalous nature of water from the standard laws of thermal expansion, you can navigate these traps with ease.