Dry ice is used for making cold baths in laboratories by mixing with volatile organic solvents. Identify the form of dry ice from the following

1. Physical States of Matter and Their Characteristics (basic)

To understand the chemistry we encounter every day, we must first define matter. Matter is anything that has mass and occupies space. It is fascinating to note that while the air we breathe and the water we drink are matter, things like light, heat, or even our emotions are not, as they lack physical mass and volume Science, Class VIII . NCERT(Revised ed 2025), Nature of Matter: Elements, Compounds, and Mixtures, p.130. All matter is made up of tiny particles, and the way these particles behave determines whether something is a solid, liquid, or gas.

The primary difference between these states lies in the interparticle forces and the space between particles. In solids, particles are closely packed with very strong attractive forces, giving them a fixed shape and volume. In liquids, particles have enough energy to move past each other, allowing the substance to flow while maintaining a definite volume. Gases, however, have particles that are far apart and move rapidly in all directions Science, Class VIII . NCERT(Revised ed 2025), Particulate Nature of Matter, p.113. These differences are summarized below:

Feature	Solid	Liquid	Gas
Shape & Volume	Fixed shape and volume	Fixed volume; takes shape of container	No fixed shape or volume
Particle Arrangement	Closely packed; fixed positions	Less closely packed; can move past each other	Very far apart; random motion
Interparticle Forces	Very strong	Moderate	Negligible (very weak)

While most substances follow a predictable path—melting from solid to liquid and then evaporating into gas—some substances exhibit a unique behavior called sublimation. A prime example is Dry Ice, which is the solid form of carbon dioxide (CO₂). Unlike water ice, which melts into a liquid at 0 °C, dry ice transitions directly from a solid to a gas at approximately -78.5 °C. This occurs because the atmospheric pressure is too low for liquid CO₂ to be stable. Because it leaves no liquid residue, it is widely used in laboratories to create "cooling baths" with solvents like acetone to maintain extremely low temperatures for sensitive chemical reactions.

Sources: Science, Class VIII . NCERT(Revised ed 2025), Nature of Matter: Elements, Compounds, and Mixtures, p.130; Science, Class VIII . NCERT(Revised ed 2025), Particulate Nature of Matter, p.103, 109, 113

2. Phase Transitions and the Role of Temperature and Pressure (basic)

At the heart of chemistry is the dance between thermal energy and interparticle attraction. Think of particles in a substance as being held together by invisible springs of attraction. In a solid, these springs are tight and the thermal energy is low, so particles only vibrate in place. As we add heat, the thermal energy increases, making the particles vibrate more vigorously until they eventually break free from their fixed positions to become a liquid or a gas. As noted in Science Class VIII, Particulate Nature of Matter, p.112, it is this balance of energy that determines the physical state of matter.

An intriguing phenomenon occurs during this transition: the temperature of the substance stops rising even though you continue to add heat. This is known as Latent Heat. Whether it is ice melting at 0 °C or water boiling at 100 °C, the energy provided is being used entirely to overcome the attractive forces between particles rather than increasing the kinetic energy (temperature) of the substance. This "hidden" energy is called the latent heat of fusion (for melting) or latent heat of vaporization (for boiling), as explained in Physical Geography by PMF IAS, Vertical Distribution of Temperature, p.294. This principle is why steam at 100 °C causes more severe burns than liquid water at the same temperature—it carries that extra "hidden" energy.

While temperature provides the energy to move particles apart, pressure works in the opposite direction by trying to squeeze them together. Gases are highly compressible because their particles are far apart; increasing pressure causes their density to rise significantly. However, liquids and solids are nearly incompressible because their particles are already very close, as mentioned in Science Class VIII, The Amazing World of Solutes, Solvents, and Solutions, p.148. A fascinating result of this temperature-pressure interplay is sublimation—where a substance like Dry Ice (Solid CO₂) skips the liquid phase entirely and turns directly into a gas at atmospheric pressure. Because it transitions at a staggering -78.5 °C without leaving a liquid residue, it is exceptionally useful for creating ultra-cold cooling baths when mixed with solvents like acetone or ethanol.

Process	Phase Change	Energy Involvement
Melting / Fusion	Solid to Liquid	Absorbs Latent Heat
Vaporization	Liquid to Gas	Absorbs Latent Heat
Condensation	Gas to Liquid	Releases Latent Heat
Sublimation	Solid to Gas	Absorbs Energy directly

Sources: Science Class VIII, NCERT, Particulate Nature of Matter, p.112; Physical Geography by PMF IAS, Vertical Distribution of Temperature, p.294-295; Science Class VIII, NCERT, The Amazing World of Solutes, Solvents, and Solutions, p.148

3. The Phenomenon of Sublimation (intermediate)

In our daily lives, we are accustomed to seeing matter change states in a specific order: ice melts into water, and water boils into steam. However, sublimation is a fascinating "shortcut" in chemistry where a substance transitions directly from a solid state to a gaseous state without ever becoming a liquid. This occurs when the vapor pressure of the substance is high enough that it bypasses the liquid phase at a given atmospheric pressure. As noted in geographical contexts, this is a key part of the water cycle, though the reverse process—gas turning directly into solid—is specifically called desublimation or deposition. Physical Geography by PMF IAS, Hydrological Cycle (Water Cycle), p.329

To understand why this happens, we must look at the particulate nature of matter. In a solid, particles are tightly packed. When we heat a substance like camphor, we are adding thermal energy. For most substances, this energy first weakens the bonds to allow flow (liquefaction), but for sublimable substances, the energy is sufficient to overcome all attractive forces simultaneously, allowing the particles to escape directly into the air as a gas. This is why the fragrance of camphor can reach all corners of a room almost instantly when heated. Science Class VIII NCERT, Particulate Nature of Matter, p.114

The most famous industrial application of this phenomenon is Dry Ice, which is the solid form of carbon dioxide (CO₂). It is called "dry" because it leaves no liquid residue; at standard atmospheric pressure, it sublimes at a bone-chilling -78.5 °C. Because it stays so cold and transitions cleanly to gas, it is often mixed with volatile organic solvents like acetone or ethanol to create extremely stable cooling baths in laboratories. These baths are essential for maintaining the precise, low-temperature environments needed for sensitive chemical reactions.

Feature	Melting (Standard)	Sublimation
Phase Sequence	Solid → Liquid → Gas	Solid → Gas (Skips Liquid)
Residue	Leaves liquid (e.g., wet water)	No liquid residue (e.g., dry ice)
Examples	Ice, Iron, Wax	Dry Ice, Camphor, Naphthalene balls

Sources: Physical Geography by PMF IAS, Hydrological Cycle (Water Cycle), p.329; Science Class VIII NCERT, Particulate Nature of Matter, p.114

4. Carbon Dioxide (CO₂): Atmospheric Role and Global Warming (intermediate)

Carbon dioxide (CO₂) is often discussed as a pollutant, but from a first-principles perspective, it is a vital atmospheric component that regulates Earth's temperature. Under normal conditions, the greenhouse effect is a natural, life-sustaining process. It works like this: short-wave solar radiation from the Sun passes through our atmosphere and warms the Earth's surface. The Earth then attempts to shed this heat by radiating it back toward space as long-wavelength thermal radiation (infrared). CO₂ molecules act like a thermal blanket, absorbing this outgoing heat and re-radiating it in all directions, including back toward the surface Environment and Ecology, Majid Hussain, Environmental Degradation and Management, p.7. Without this natural warming, Earth would be too cold for most life forms.

However, the rapid burning of fossil fuels has increased CO₂ concentrations, making it the largest single climate forcing agent—meaning it is the primary driver of the enhanced greenhouse effect since the industrial era Environment, Shankar IAS Academy, Climate Change, p.259. To measure the impact of different gases, scientists use Global Warming Potential (GWP). CO₂ is the baseline for this scale, assigned a GWP of 1. Other gases are compared against it based on how much energy they absorb and how long they stay in the atmosphere Environment, Shankar IAS Academy, Climate Change, p.260.

Gas	Global Warming Potential (100-yr)	Atmospheric Lifetime
Carbon Dioxide (CO₂)	1 (The Baseline)	Variable (Approx. 100 years)
Methane (CH₄)	21	~12 years
Nitrous Oxide (N₂O)	310	~120 years

In applied laboratory settings, CO₂ takes on a different but equally important role in its solid state, known as dry ice. Unlike water ice, dry ice does not melt into a liquid; instead, it sublimes, transitioning directly from a solid to a gas at atmospheric pressure at a temperature of -78.5 °C. Because it leaves no liquid residue, it is termed "dry." Chemists often mix dry ice with volatile organic solvents like acetone or ethanol to create extremely cold cooling baths. These baths are essential for maintaining stable, ultra-low temperatures during sensitive chemical reactions or for the rapid preservation of biological samples.

Sources: Environment and Ecology, Majid Hussain, Environmental Degradation and Management, p.7; Environment, Shankar IAS Academy, Climate Change, p.254, 259, 260; Environment, Shankar IAS Academy, Environment Issues and Health Effects, p.425

5. Carbonation and Ocean Acidification (intermediate)

At its simplest level, carbonation is the process of dissolving carbon dioxide (CO₂) in water. While we often encounter this in fizzy drinks, it is a fundamental chemical process occurring on a global scale in our oceans. When CO₂ from the atmosphere enters the water, it doesn't just sit there as a gas; it undergoes a chemical reaction with water molecules to form carbonic acid (H₂CO₃). This acid then breaks down (dissociates) into hydrogen ions (H⁺) and bicarbonate ions (HCO₃⁻) Environment Shankar IAS Academy, Ocean Acidification, p.264. It is important to note that the ability of water to hold these gases is sensitive to environment: the solubility of gases generally decreases as the temperature of the liquid increases Science Class VIII NCERT, Chapter 9, p.139.

The term Ocean Acidification describes the ongoing decrease in the pH of the Earth's oceans. Since the Industrial Revolution, the pH of ocean surface waters has dropped by about 0.1 units. While that sounds small, because the pH scale is logarithmic, this represents a massive 60% increase in hydrogen ion concentration Environment Shankar IAS Academy, Ocean Acidification, p.263. You might wonder why we call it 'acidification' if the ocean's pH is still around 8.1 (which is technically basic/alkaline). We use this term because 'acidification' refers to the direction of travel—the water is moving toward the acidic end of the scale, even if it hasn't crossed the neutral threshold of 7.0 yet.

The real 'biological crisis' of acidification lies in a secondary reaction. As hydrogen ions (H⁺) increase, they react with carbonate ions (CO₃²⁻) to form more bicarbonate. This effectively 'steals' the carbonate ions that marine organisms like corals, mollusks, and crustaceans need to build their shells and skeletons out of calcium carbonate (CaCO₃) Environment Shankar IAS Academy, Ocean Acidification, p.264. When carbonate ions become scarce, these organisms must spend more energy to build their structures, or worse, their existing shells can begin to dissolve, similar to how metal carbonates react and dissolve when exposed to acids Science Class X NCERT, Acids, Bases and Salts, p.21.

Sources: Environment Shankar IAS Academy, Ocean Acidification, p.263-264; Science Class VIII NCERT, The Amazing World of Solutes, Solvents, and Solutions, p.139; Science Class X NCERT, Acids, Bases and Salts, p.21

6. Industrial Applications of CO₂ and Compressed Gases (intermediate)

To understand the industrial utility of Carbon Dioxide (CO₂), we must first look at its unique physical property: sublimation. Unlike water ice, which melts into a liquid, solid CO₂—commonly known as Dry Ice—transitions directly into a gas at atmospheric pressure. This occurs at a bone-chilling temperature of approximately -78.5 °C. In industrial and laboratory settings, dry ice is often combined with volatile organic solvents like acetone to create cooling baths, which are essential for maintaining stable, ultra-low temperatures for chemical reactions or the safe transport of biological samples. Beyond cooling, CO₂ is a cornerstone of fire safety technology. In a classic soda-acid fire extinguisher, a reaction is triggered between an acid (like sulfuric acid) and a metal hydrogencarbonate (like sodium hydrogencarbonate). As noted in Science, class X (NCERT 2025 ed.), Acids, Bases and Salts, p.36, this chemical reaction rapidly generates CO₂ gas. Because CO₂ is non-combustible and heavier than air, it blankets the fire, cutting off the oxygen supply and effectively extinguishing the flames. This same gas can be identified in labs by passing it through calcium hydroxide (limewater), which turns milky due to the formation of calcium carbonate Science, class X (NCERT 2025 ed.), Acids, Bases and Salts, p.20. In the modern context of climate change, the industrial management of CO₂ has evolved into Carbon Capture and Storage (CCS). This involves three critical stages: trapping the gas from industrial emissions, transporting it via pipelines, and storing it deep underground or in the ocean to prevent it from entering the atmosphere Environment, Shankar IAS Academy (10th ed.), Mitigation Strategies, p.281. To measure the impact of various industrial gases, scientists use Global Warming Potential (GWP), which converts the heat-trapping ability of different gases into a 'CO₂ equivalent' value Environment, Shankar IAS Academy (10th ed.), Environment Issues and Health Effects, p.425.

Sources: Science, class X (NCERT 2025 ed.), Acids, Bases and Salts, p.36; Science, class X (NCERT 2025 ed.), Acids, Bases and Salts, p.20; Environment, Shankar IAS Academy (10th ed.), Mitigation Strategies, p.281; Environment, Shankar IAS Academy (10th ed.), Environment Issues and Health Effects, p.425

7. Dry Ice: Unique Properties and Laboratory Uses (exam-level)

Dry ice is the solid state of carbon dioxide (CO₂), a substance that behaves quite differently from the water ice we encounter in our daily lives. While water ice melts into a liquid at 0°C, dry ice undergoes a process called sublimation. This means it transitions directly from a solid to a gas at atmospheric pressure without ever becoming a liquid. This transition occurs at the extremely low temperature of approximately -78.5°C. This property is why it is called 'dry'—it leaves behind no wet residue, making it ideal for shipping perishables and for specialized cooling applications where moisture could damage equipment or samples. To measure such extreme temperatures, scientists use specialized laboratory thermometers, often calibrated in the Celsius or Fahrenheit scales Exploring Society: India and Beyond, Understanding the Weather, p.31. In a laboratory environment, dry ice is a fundamental tool for achieving precise, low-temperature conditions. It is frequently used to create cooling baths by mixing it with volatile organic solvents like acetone or ethanol. These mixtures allow chemists to maintain stable temperatures far below the freezing point of water (often down to -78°C) for sensitive chemical reactions or long-term sample preservation. While we know that ice in nature, such as ice floes in the ocean, can lower the surface temperature of surrounding waters Certificate Physical and Human Geography, The Oceans, p.109, dry ice provides a much more drastic and 'clean' cooling effect because it lacks the conductivity and liquid phase of H₂O ice. From a chemical perspective, dry ice remains pure CO₂. If you were to allow a piece of dry ice to sublime and then channel that gas through lime water (calcium hydroxide), the solution would turn milky due to the formation of insoluble calcium carbonate (CaCO₃) Science Class VIII, Nature of Matter, p.119. Interestingly, at the atomic level, high concentrations of CO₂ can actually influence the structural integrity of materials, sometimes making them more brittle or prone to cracking—a factor that materials scientists must consider when using it in pressurized or structural environments Environment and Ecology, Climate Change, p.12.

Feature	Water Ice (H₂O)	Dry Ice (CO₂)
Phase Change	Melting (Solid to Liquid)	Sublimation (Solid to Gas)
Temperature	0°C (32°F)	-78.5°C (-109.3°F)
Residue	Leaves liquid water	Leaves no residue (Dry)

Sources: Exploring Society: India and Beyond, Understanding the Weather, p.31; Certificate Physical and Human Geography, The Oceans, p.109; Science Class VIII, Nature of Matter, p.119; Environment and Ecology, Climate Change, p.12

8. Solving the Original PYQ (exam-level)

This question masterfully connects your recent lessons on states of matter and the specific phase transition known as sublimation. To solve this, you must synthesize the physical properties of molecules with their practical laboratory applications. The core concept here is that certain substances can bypass the liquid state entirely under standard atmospheric pressure; Carbon Dioxide (CO2) is the most prominent example of this in general science. When you see the term "Dry Ice," your mind should immediately link the word dry to the absence of a liquid phase, which is only possible if the substance is in its solid state.

To reach the correct answer, (C) Solid carbon dioxide, follow a logical path: a "cold bath" requires a medium that can maintain a temperature far below the freezing point of water. As discussed in NCERT Class 9 Science, solid CO2 exists at approximately -78.5°C. When mixed with volatile organic solvents, it creates a constant-temperature environment for sensitive chemical reactions. Reasoning through the name itself acts as a major clue—it is called "ice" because it is solid, and "dry" because it sublimes directly into gas without leaving a residue.

UPSC frequently uses nomenclature traps to test your composure. Option (D) Solid hydrogen oxide is a classic example; it is simply a technical term for regular water ice, which would leave a liquid residue and is not "dry." Options (A) and (B) are chemically identical to the correct answer but represent different physical states. Always remember: while CO2 can be compressed into a liquid or exist as the air we exhale, the specific term "Dry Ice" is reserved exclusively for its solid form due to its unique thermal properties.