A sample of carbon dioxide that undergoes a transformation from solid to liquid and then to gas would undergo

1. Classification of Matter: Solids, Liquids, and Gases (basic)

To understand the foundation of Thermal Physics, we must first define what we are studying: Matter. Simply put, matter is anything that has mass and occupies space. However, it is equally important to recognize what is not matter. While light, heat, and electricity are essential to our world, they lack mass and volume and are therefore forms of energy, not matter Science, Class VIII, Nature of Matter: Elements, Compounds, and Mixtures, p.130. All matter is composed of tiny constituent particles that are constantly in motion and are held together by interparticle attractions.

The physical state of a substance—whether it is a solid, liquid, or gas—is primarily determined by the balance between the kinetic energy of these particles and the strength of the forces pulling them together. As the distance between particles increases, even slightly, these attractive forces decrease drastically Science, Class VIII, Particulate Nature of Matter, p.101. This is the fundamental reason why solids have a definite shape while gases expand to fill any container: the particles in a gas are so far apart that their mutual attraction is almost negligible.

Feature	Solid	Liquid	Gas
Shape & Volume	Fixed shape and volume	Fixed volume; takes shape of container	No fixed shape or volume
Interparticle Distance	Very small (tightly packed)	Moderate	Very large
Compressibility	Negligible	Low	Very High

Crucially, when a substance changes from one state to another (like ice melting into water), it undergoes a physical change. The chemical identity remains identical—for example, CO₂ remains CO₂ whether it is solid "dry ice" or a gas. While the mass remains constant during these transitions, the density changes significantly because the volume increases as particles move further apart Science, Class VII, Changes Around Us, p.59. Understanding these transitions is the first step in mastering how heat energy interacts with the physical world.

Sources: Science, Class VIII (NCERT Revised 2025), Nature of Matter: Elements, Compounds, and Mixtures, p.130; Science, Class VIII (NCERT Revised 2025), Particulate Nature of Matter, p.101, 107; Science, Class VII (NCERT Revised 2025), Changes Around Us: Physical and Chemical, p.59

2. Physical vs. Chemical Changes (basic)

To understand Thermal Physics, we must first distinguish between how matter changes its appearance versus how it changes its identity. A physical change is one where a substance alters its physical properties—such as its shape, size, or state of matter—but no new chemical substance is created Science-Class VII, Chapter 5, p. 59. Think of grinding a piece of chalk into a fine powder; while the stick is gone, every tiny speck is still chemically chalk Science-Class VIII, Chapter 7, p. 99. These changes are often (though not always) reversible, like melting ice back into water.

In the context of thermal physics, phase changes (like boiling or freezing) are classic physical changes. Take Carbon Dioxide (CO₂) as an example. When solid 'dry ice' turns into CO₂ gas, its chemical formula remains exactly the same. However, its density changes drastically. While the mass of the sample stays constant, the volume it occupies increases as the molecules move much further apart in the gaseous state. Because density is mass per unit volume (D = M/V), the increase in volume leads to a significant decrease in density, even though the chemical identity of the CO₂ is untouched.

Conversely, a chemical change involves a chemical reaction where one or more new substances are formed with entirely different properties Science-Class VII, Chapter 5, p. 68. In these cases, the internal molecular bonds are broken and rearranged. Common examples include combustion (burning), where heat and light are released as a new oxide is formed, or the curdling of milk, where the chemical structure of the proteins is permanently altered Science-Class VII, Chapter 5, p. 70.

Feature	Physical Change	Chemical Change
New Substance	No new substance formed.	New substance(s) created.
Properties	Changes in shape, size, or state.	Changes in chemical composition.
Example	Melting wax, erosion of rocks.	Rusting of iron, cooking food.

Sources: Science-Class VII . NCERT(Revised ed 2025), Changes Around Us: Physical and Chemical, p.59, 68, 70; Science, Class VIII . NCERT(Revised ed 2025), Particulate Nature of Matter, p.99

3. Defining Mass, Volume, and Density (basic)

To understand thermal physics, we must first master the three fundamental pillars of matter: Mass, Volume, and Density. At its simplest, matter is anything that has mass and occupies space. Mass represents the quantity of matter in an object, while Volume is the three-dimensional space that matter occupies Science, Class VIII . NCERT(Revised ed 2025), The Amazing World of Solutes, Solvents, and Solutions, p.140. In thermal physics, a key rule to remember is that during physical changes—like a block of ice melting into water—the mass remains constant because the number of molecules doesn't change, even though the space they occupy might.

Density is the bridge between mass and volume. It is defined as the mass present in a unit volume of a substance, expressed by the formula: Density = Mass / Volume. Think of density as a measure of how "tightly packed" the particles are. For example, if you have an aluminium block weighing 27 g with a volume of 10 cm³, its density is 2.7 g/cm³ Science, Class VIII . NCERT(Revised ed 2025), The Amazing World of Solutes, Solvents, and Solutions, p.141. Because the density of water is approximately 1 g/cm³, we say aluminium is 2.7 times denser than water, a concept known as relative density.

In the context of heat, these properties behave in very specific ways. Generally, when you heat a substance, its particles move faster and spread apart, causing the volume to increase. Since the mass stays the same but the volume grows, the density decreases Science, Class VIII . NCERT(Revised ed 2025), The Amazing World of Solutes, Solvents, and Solutions, p.147. This is why hot air rises—it is less dense than the cooler air around it. While pressure also affects density (especially in gases by squeezing particles closer together), its effect on solids and liquids is usually negligible because they are nearly incompressible.

A classic example of these principles in action is a phase change, such as solid CO₂ (dry ice) turning into gas. While the chemical identity (CO₂) and mass remain identical throughout the process, the volume expands drastically as it becomes a gas, leading to a sharp drop in density Science, Class VII . NCERT(Revised ed 2025), Changes Around Us: Physical and Chemical, p. 59.

Factor	Effect of Heating	Effect of Increasing Pressure
Mass	No Change	No Change
Volume	Increases (Particles spread out)	Decreases (Significant in gases)
Density	Decreases	Increases

Sources: Science, Class VIII . NCERT(Revised ed 2025), The Amazing World of Solutes, Solvents, and Solutions, p.140-141, 147-148; Science, Class VII . NCERT(Revised ed 2025), Changes Around Us: Physical and Chemical, p.59

4. Sublimation and the Unique Properties of CO₂ (intermediate)

To understand the unique behavior of Carbon Dioxide (CO₂), we must first look at how matter changes state. Typically, we expect a substance to transition from a solid to a liquid (melting) and then to a gas (evaporation). However, CO₂ is famous for sublimation—a process where a substance transitions directly from the solid phase to the gas phase without ever becoming a liquid Physical Geography by PMF IAS, Hydrological Cycle, p.329. This is why solid CO₂ is colloquially known as "Dry Ice"; it vanishes into the air without leaving a puddle behind.

From a thermal physics perspective, this transition is a physical change. While the appearance and state of the substance change completely, its internal molecular structure remains identical. Whether it is a frozen block or a floating gas, the chemical identity remains CO₂ Science-Class VII, NCERT, Changes Around Us, p.59. However, the density of the substance undergoes a massive shift. In the solid state, CO₂ molecules are closely packed in fixed positions Science-Class VIII, NCERT, Particulate Nature of Matter, p.113. As it sublimes into a gas, the molecules move rapidly and spread far apart, leading to a dramatic increase in volume and a corresponding decrease in density (since Density = Mass / Volume).

Interestingly, CO₂'s impact extends beyond simple phase changes. In environmental science, we measure other greenhouse gases by comparing them to the Global Warming Potential (GWP) of CO₂, using a metric called "CO₂ equivalent" Environment, Shankar IAS Academy, Environment Issues, p.425. Furthermore, high concentrations of CO₂ can even affect the structural integrity of natural ice, making it more brittle and prone to cracking—acting almost like a "corroding agent" at the atomic level Environment and Ecology, Majid Hussain, Climate Change, p.12.

Property	Solid CO₂ (Dry Ice)	Gaseous CO₂
Molecular Packing	Closely packed, fixed positions	Far apart, moving rapidly
Volume	Low	Very High
Chemical Identity	Pure CO₂	Pure CO₂

Sources: Physical Geography by PMF IAS, Hydrological Cycle (Water Cycle), p.329; Science-Class VII, NCERT(Revised ed 2025), Changes Around Us: Physical and Chemical, p.59; Science, Class VIII, NCERT(Revised ed 2025), Particulate Nature of Matter, p.113; Environment, Shankar IAS Academy (ed 10th), Environment Issues and Health Effects, p.425; Environment and Ecology, Majid Hussain (Access publishing 3rd ed.), Climate Change, p.12

5. Law of Conservation of Mass (intermediate)

At the heart of thermal physics and chemistry lies a fundamental truth: Mass can neither be created nor destroyed. This principle, known as the Law of Conservation of Mass, implies that in any closed system, the total quantity of matter remains constant, regardless of the transformations occurring within it. Whether you are burning a piece of wood or simply watching an ice cube melt, the total mass before the process must equal the total mass after the process. In the context of chemical reactions, this means the number of atoms of each element remains identical in both the reactants and the products, which is why we must always balance chemical equations Science (Class X), Chemical Reactions and Equations, p.3.

When we narrow our focus to Phase Changes (a key part of thermal physics), the law holds firm but manifests in interesting ways. For instance, when solid CO₂ (dry ice) turns into a gas, it undergoes a physical change. While its physical properties like state, shape, and density change drastically, its chemical identity and total mass do not change Science (Class VII), Changes Around Us: Physical and Chemical, p.59. Even though the gas occupies a much larger volume than the solid, if you were to weigh the entire system in a sealed container, the scale would not budge by a single milligram.

It is vital to distinguish between mass and density. Density is defined as mass per unit volume (D = M/V). During a phase change, such as water boiling into steam, the mass remains constant, but the volume increases significantly as molecules move further apart. Consequently, the density decreases Science (Class VIII), The Amazing World of Solutes, Solvents, and Solutions, p.141. Even as we supply latent heat to break the molecular bonds during these transitions, that energy does not add "weight" to the substance; it only changes its internal state and temperature Physical Geography by PMF IAS, Vertical Distribution of Temperature, p.295.

Property	During Phase Change	Reasoning
Total Mass	Constant	Law of Conservation of Mass; atoms are only rearranged.
Volume	Increases (usually)	Molecules move further apart (e.g., liquid to gas).
Density	Decreases (usually)	Same mass spread over a larger volume.

Sources: Science (Class X), Chemical Reactions and Equations, p.3; Science (Class VII), Changes Around Us: Physical and Chemical, p.59; Science (Class VIII), The Amazing World of Solutes, Solvents, and Solutions, p.141; Physical Geography by PMF IAS, Vertical Distribution of Temperature, p.295

6. Intermolecular Forces and Density in Phase Transitions (exam-level)

When we look at matter, we see a constant tug-of-war between two opposing forces: Thermal Energy (which tries to push particles apart) and Intermolecular Forces (which try to pull them together). A phase transition—like ice melting or dry ice sublimating into gas—is essentially the point where thermal energy wins or loses this battle. In molecules like Carbon Dioxide (CO₂), while the atoms within the molecule are held by strong covalent bonds, the forces between the molecules are relatively weak. This explains why such substances often have low melting and boiling points Science , class X (NCERT 2025 ed.), Carbon and its Compounds, p.60.

It is crucial to understand that a phase change is a physical change, not a chemical one. Whether CO₂ is a solid (dry ice) or a gas, its chemical identity and mass remain identical. However, its density changes dramatically. Density is defined as the ratio of mass to volume (Density = Mass / Volume). During heating, particles gain kinetic energy and spread out, leading to an increase in volume Science ,Class VIII . NCERT(Revised ed 2025), The Amazing World of Solutes, Solvents, and Solutions, p.147. Since the mass stays the same but the volume increases, the density must decrease.

During the actual transition (like boiling), you might notice the temperature stays constant even though you are adding heat. This energy, known as latent heat, is being used exclusively to overcome those intermolecular forces rather than increasing the kinetic energy (temperature) of the particles Physical Geography by PMF IAS, Vertical Distribution of Temperature, p.295. This shift in molecular spacing is why a small block of solid CO₂ can expand to fill a massive room once it turns into gas.

Phase	Intermolecular Spacing	Relative Density	Molecular Movement
Solid	Very Close	High	Vibration only
Liquid	Close	Medium	Gliding/Flowing
Gas	Very Far Apart	Very Low	Rapid/Random

Sources: Science , class X (NCERT 2025 ed.), Carbon and its Compounds, p.60; Science ,Class VIII . NCERT(Revised ed 2025), The Amazing World of Solutes, Solvents, and Solutions, p.147; Physical Geography by PMF IAS, Vertical Distribution of Temperature, p.295

7. Solving the Original PYQ (exam-level)

This question perfectly synthesizes the concepts of physical changes and the states of matter you have just mastered. To arrive at the correct answer, you must apply the principle that a phase transition—moving from solid to liquid to gas—is a physical process where the chemical identity of the substance remains constant. As noted in Science-Class VII . NCERT (Revised ed 2025), during such changes, the molecules do not rearrange into new substances; a molecule of CO2 remains CO2 whether it is frozen as dry ice or floating as a gas. Therefore, the composition and the total mass of the sample do not change, effectively ruling out options (A) and (C).

The core of the reasoning lies in how those molecules are arranged. While the mass is conserved, the volume the substance occupies changes dramatically as it transitions from a tightly packed solid lattice to a high-energy gaseous state where molecules are far apart. Since density is defined as mass per unit volume, any increase in volume while mass remains constant must result in a change in density. This makes (B) the correct choice. It is a common UPSC tactic to test if you can distinguish between what stays the same (chemical identity) and what must change (spatial arrangement) during a state transition.

Be careful not to fall for the trap in option (D). While the chemical properties are unchanged, the physical properties—such as state, shape, and density—are the very things that define a physical change. Stating there is "no change in physical properties" contradicts the very observation that the substance has transformed from a solid to a gas. Always remember: in a phase change, the identity is the constant, but the physical footprint (density) is the variable.