Gases may be distinguished from other forms of matter by their :

1. Classification and Particulate Nature of Matter (basic)

At its most fundamental level, matter is not a continuous block but is composed of extremely small particles that are in constant motion. The way these particles behave—specifically how close they are and how strongly they attract one another—defines the state of matter we observe. This is known as the particulate nature of matter Science, Class VIII. NCERT (2025 ed.), Chapter 7, p.113.

The transition between solids, liquids, and gases is governed by the tug-of-war between kinetic energy (which makes particles move apart) and interparticle forces (which pull them together). In solids, the attraction is so strong that particles only vibrate in fixed positions. In liquids, the forces are weaker, allowing particles to slide past each other while staying close. However, in the gaseous state, particles have enough energy to completely overcome these attractive forces, moving freely in all directions Science, Class VIII. NCERT (2025 ed.), Chapter 7, p.112.

Feature	Solids	Liquids	Gases
Interparticle Attraction	Strongest	Moderate	Negligible
Interparticle Space	Minimum	Small	Maximum
Movement	Fixed position (vibration)	Within a limited space	Free in all directions

A critical concept to master is the free surface. Because liquids have a definite volume and are held together by cohesive forces, they settle and form a distinct top surface when poured into a container. Gases, however, possess negligible interparticle attraction Science, Class VIII. NCERT (2025 ed.), Chapter 7, p.113. This causes them to expand indefinitely to fill any container, meaning they cannot form a free surface. This total lack of a stable boundary is what truly sets gases apart from liquids and solids Science, Class VIII. NCERT (2025 ed.), Chapter 7, p.115.

Sources: Science, Class VIII. NCERT (Revised ed 2025), Chapter 7: Particulate Nature of Matter, p.112; Science, Class VIII. NCERT (Revised ed 2025), Chapter 7: Particulate Nature of Matter, p.113; Science, Class VIII. NCERT (Revised ed 2025), Chapter 7: Particulate Nature of Matter, p.115

2. Properties of Solids and Liquids: Shape and Volume (basic)

Welcome back! In our journey to master matter, we must first understand why a rock stays a rock while water flows. Everything around us is made of matter, which is composed of extremely small particles held together by interparticle forces of attraction. The strength of these forces determines whether something is a solid, a liquid, or a gas Science, Class VIII. NCERT (Revised ed 2025), Chapter 7: Particulate Nature of Matter, p.113.

Solids are characterized by having both a definite shape and a definite volume. Think of an iron nail or a stone; no matter which container you put them in, they look the same and occupy the same amount of space. This happens because their particles are tightly packed with very strong interparticle attractions, leaving almost no room for the particles to move from their fixed positions Science, Class VIII. NCERT (Revised ed 2025), Chapter 7: Particulate Nature of Matter, p.102. Even if you try hammering a solid like wood or iron, the particles resist moving apart because they are so strongly held together.

Liquids, on the other hand, are a bit more flexible. They have a definite volume but no fixed shape. If you take 200 mL of water and pour it from a tall glass into a flat bowl, the volume remains exactly 200 mL, but the water changes its shape to fit the bowl Science, Class VIII. NCERT (Revised ed 2025), Chapter 7: Particulate Nature of Matter, p.104. This is because the interparticle forces in liquids are slightly weaker than in solids, allowing the particles to slide and move past one another. Interestingly, liquids also form a free surface (the top layer) when poured into a container, which is a key distinction from gases that simply expand to fill everything.

Property	Solids	Liquids
Shape	Fixed/Definite	Takes the shape of the container
Volume	Fixed/Definite	Fixed/Definite
Particle Movement	Stationary (vibrate only)	Can move past each other
Interparticle Space	Minimum	Slightly more than solids

Sources: Science, Class VIII. NCERT (Revised ed 2025), Chapter 7: Particulate Nature of Matter, p.102, 104, 113

3. Surface Tension and Cohesive Forces in Liquids (intermediate)

To understand how liquids behave, we must look at the interparticle forces of attraction that act like a microscopic glue. In any substance, these particles are constantly pulling on one another. When this attraction occurs between similar molecules (like water molecule to water molecule), we call it a cohesive force. In solids, these forces are extremely strong, locking particles into a fixed shape. However, in liquids, the forces are slightly weaker, allowing particles to move within a defined space, giving liquids a definite volume but no fixed shape Science, Class VIII NCERT (2025), Chapter 7: Particulate Nature of Matter, p.113.

This internal cohesion leads to a fascinating phenomenon called surface tension. Imagine a molecule in the middle of a glass of water; it is pulled in every direction by its neighbors, resulting in a balanced state. However, a molecule at the surface has no water molecules above it. It feels a net inward pull toward the bulk of the liquid. This imbalance causes the surface to contract and behave like a stretched elastic membrane. This is why small droplets of water tend to be spherical—the shape that has the minimum surface area for a given volume.

The most critical distinction between liquids and gases lies in these cohesive forces. Because the interparticle attractions in gases are negligible, gas particles are completely free to move in all directions Science, Class VIII NCERT (2025), Chapter 7: Particulate Nature of Matter, p.113. Consequently, gases will expand indefinitely to fill any container and cannot form a free surface. Liquids, on the other hand, maintain a stable top boundary (a free surface) because their cohesive forces keep the particles held together even when the container isn't full Science, Class VIII NCERT (2025), Chapter 7: Particulate Nature of Matter, p.115.

Property	Liquids	Gases
Interparticle Attraction	Strong enough to maintain volume	Negligible; particles move freely
Free Surface	Forms a stable top surface	None; expands to fill space
Volume	Definite	Indefinite

Sources: Science, Class VIII NCERT (2025), Chapter 7: Particulate Nature of Matter, p.113; Science, Class VIII NCERT (2025), Chapter 7: Particulate Nature of Matter, p.115

4. Fluid Mechanics: Buoyancy and Archimedes' Principle (intermediate)

When we drop an object into a fluid—be it a liquid or a gas—it enters a tug-of-war between two opposing forces. On one side, gravity pulls the object downward toward the Earth's center. On the other side, the fluid exerts an upward force known as buoyancy or upthrust Science, Class VIII. NCERT (2025), Exploring Forces, p.77. This upward force is the reason you feel lighter when you step into a swimming pool; the water is literally pushing up against your weight.

The magnitude of this force is governed by Archimedes' Principle. Archimedes discovered that the upward buoyant force acting on an object, whether it is fully or partially submerged, is exactly equal to the weight of the fluid that the object displaces Science, Class VIII. NCERT (2025), Exploring Forces, p.76. This principle applies to all fluids because their particles, unlike those in a solid, are free to move and flow around an intruding object Science, Class VIII. NCERT (2025), Particulate Nature of Matter, p.113. When an object enters the fluid, it must push some fluid particles out of the way; those displaced particles essentially 'push back' with a force equal to their own weight.

Whether an object sinks or floats depends entirely on the balance between its own weight and this displaced fluid's weight. If the object is denser than the fluid, it may not be able to displace enough fluid to match its own weight before it is fully submerged, leading it to sink. We can summarize the conditions for floating and sinking as follows:

Scenario	Comparison of Forces	Result
Sinking	Weight of object > Weight of displaced liquid	Object moves downward p.76
Floating	Weight of object = Weight of displaced liquid	Object stays at the surface p.76

Sources: Science, Class VIII. NCERT (2025), Exploring Forces, p.76; Science, Class VIII. NCERT (2025), Exploring Forces, p.77; Science, Class VIII. NCERT (2025), Particulate Nature of Matter, p.113

5. Gas Laws and Kinetic Behavior (exam-level)

To understand gases, we must look at the Kinetic Molecular Theory, which tells us that gas particles are in a state of constant, rapid, and random motion. Unlike solids, where particles are locked in place, or liquids, where they slide past one another while staying close, gas particles have negligible interparticle attractions. This lack of 'stickiness' means that gas particles move freely in all directions, occupying every cubic centimeter of any container they inhabit. As a result, gases possess neither a definite shape nor a fixed volume, and they will expand indefinitely unless confined Science, Class VIII. NCERT (Revised ed 2025), Chapter 7: Particulate Nature of Matter, p. 106.

A critical distinction between gases and liquids lies in the concept of a free surface. If you pour water into a glass, gravity and cohesive forces pull the molecules together to form a stable top surface. However, because gas particles move with such high kinetic energy and weak attraction, they never settle to form a stable top boundary; they simply fill the entire available space. This distinguishes gases from other 'fluids' (like liquids), which do possess a free surface and a definite volume Science, Class VIII. NCERT (Revised ed 2025), Chapter 7: Particulate Nature of Matter, p. 115.

This constant motion also explains why gases exert pressure. Pressure is essentially the cumulative force of billions of tiny gas particles colliding with the walls of their container. Just like liquids, gases exert this pressure in all directions — upwards, downwards, and sideways Science, Class VIII. NCERT (Revised ed 2025), Pressure, Winds, Storms, and Cyclones, p. 85. In our atmosphere, gravity prevents these high-energy particles from escaping into space, creating a dense layer of gases like Nitrogen (78%) and Oxygen (21%) near the surface Physical Geography by PMF IAS, Manjunath Thamminidi, Earth's Atmosphere, p. 271.

Property	Liquid	Gas
Volume	Definite	Indefinite (fills container)
Free Surface	Yes (top surface)	No
Interparticle Force	Strong enough to hold volume	Negligible

Sources: Science, Class VIII. NCERT (Revised ed 2025), Chapter 7: Particulate Nature of Matter, p.106; Science, Class VIII. NCERT (Revised ed 2025), Chapter 7: Particulate Nature of Matter, p.115; Science, Class VIII. NCERT (Revised ed 2025), Pressure, Winds, Storms, and Cyclones, p.85; Physical Geography by PMF IAS, Manjunath Thamminidi, Earth's Atmosphere, p.271

6. Distinguishing Gases: Expansion and Free Surfaces (exam-level)

To understand why matter behaves differently in its various states, we must look at how particles interact. Both liquids and gases are classified as fluids because they have the ability to flow and take the shape of the vessel containing them. However, a critical distinction lies in their volume and surface behavior. A liquid has a definite volume; if you pour 100ml of water into a large bucket, it stays as 100ml and forms a distinct boundary at the top called a free surface. This happens because the cohesive forces between liquid molecules are strong enough to keep them together against gravity, but weak enough to allow them to slide past one another. In contrast, gases are characterized by their indefinite expansion. Because the interparticle attractions in a gas are negligible, the particles move rapidly in all directions. As seen in experiments with smoke jars, a gas will not settle at the bottom or form a top layer; instead, it spreads to occupy the entire available volume of its container Science, Class VIII, NCERT (Revised ed 2025), Chapter 7: Particulate Nature of Matter, p. 106. This is why a gas cannot form a free surface. Whether in a small balloon or a large room, the gas expands until it hits the boundaries of its enclosure, exerting pressure in all directions—up, down, and sideways Science, Class VIII, NCERT (Revised ed 2025), Chapter 7: Pressure, Winds, Storms, and Cyclones, p. 86.

Feature	Liquids	Gases
Volume	Fixed/Definite	Indefinite (fills container)
Free Surface	Forms a stable top surface	No free surface possible
Particle Motion	Slide over each other	Move freely in all directions

While other properties like buoyancy (the upward force exerted by a fluid) apply to both liquids and gases according to Archimedes' principle, the fundamental structural difference remains the gas's inability to maintain a boundary. This lack of a free surface is what truly separates the gaseous state from the liquid state in a physical environment.

Sources: Science, Class VIII. NCERT (Revised ed 2025), Chapter 7: Particulate Nature of Matter, p.106; Science, Class VIII. NCERT (Revised ed 2025), Chapter 7: Pressure, Winds, Storms, and Cyclones, p.86

7. Solving the Original PYQ (exam-level)

Now that you have mastered the particulate nature of matter, this question tests your ability to apply those microscopic behaviors to macroscopic observations. You learned that while solids have strong interparticle attractions and liquids have enough to keep particles close, gas particles possess high kinetic energy and negligible interparticle attractions. This fundamental building block is the key to understanding why gases behave differently in a container. According to Science, Class VIII. NCERT (Revised ed 2025), because gas particles move freely in all directions, they lack a fixed volume and will expand indefinitely to occupy every available corner of their vessel.

To arrive at the correct answer, think like a scientist observing a container. A solid holds its shape, and a liquid, while taking the shape of the bottom of the container, settles to form a distinct horizontal free surface due to gravity and cohesion. However, because a gas expands to fill the entire volume provided, it can never settle to form such a boundary. Therefore, the (C) inability to form free surfaces is the only property listed that uniquely distinguishes gases from both solids and liquids. This is a classic conceptual check on the fluidity and compressibility differences you just studied.

UPSC often includes "partially true" options to act as traps. Option (B) ability to flow and option (D) ability to exert a buoyant force are properties of fluids—a category that encompasses both liquids and gases—meaning they cannot be used to distinguish one from the other. Similarly, option (A) is a distracter because colorlessness is not a universal property of gases (e.g., Chlorine is greenish-yellow) nor is it exclusive to them, as many liquids and solids are also transparent or colorless. Always look for the defining characteristic that remains true for all substances in that state of matter.