Effect of Pressure on Change of State

A gas cylinder in the kitchen holds LPG as a sloshing liquid, not a gas — yet nobody cooled it down. A block of dry ice on a lab table quietly turns to fog and vanishes, leaving no puddle behind. Both scenes are answered by the same missing half of the "change of state" story: pressure. Temperature is the lever most students learn first, changing how much kinetic energy particles carry, but squeezing a substance — raising the pressure on it — changes something else: how close together its particles are forced to sit. Push hard enough, especially while also keeping the temperature down, and even a gas that would never liquefy on its own at room conditions is squeezed into a liquid, or a liquid into a solid. This article stays qualitative, exactly as the NCERT Class 9 syllabus does, and builds the LPG and dry-ice examples into a working intuition, no gas laws required.

The core insight

Increasing pressure forces particles closer together, so pressure — acting together with temperature — can push a substance toward a more closely packed state, which is how gases are liquefied (LPG) and why solids under released pressure can sublime (dry ice).

The core idea

A change of state is decided by a tug-of-war between two things: the inter-particle force of attraction, which pulls particles toward each other into a compact arrangement, and the kinetic energy of the particles, which pushes them apart and lets them move freely. Whichever side wins decides whether a substance behaves as a solid, a liquid, or a gas. Temperature only ever touches one side of that tug-of-war — it raises or lowers kinetic energy. Pressure touches the other side. Squeezing a substance does not change how energetically its particles are moving; instead, it forcibly reduces the space between them. With particles pressed closer, the same inter-particle force of attraction — unchanged in strength — has a much shorter distance to act over, so its pull becomes relatively more effective at holding the particles together. That is why compressing a gas can tip the tug-of-war in favour of attraction even without any cooling at all. The two levers work best in combination:

Increasing pressure on a gas squeezes its particles closer together, shrinking inter-particle space.
Decreasing temperature on a gas lowers particle kinetic energy, weakening the outward push.

Apply both together and a gas can be liquefied — or a liquid solidified — at a temperature well above the point where it would change state under pressure alone. This is precisely how industrial and household gases are stored as compact liquids or solids without needing the very low temperatures that pressure alone would demand.

\text{high pressure} + \text{low temperature} \;\longrightarrow\; \text{closer packing} \;\longrightarrow\; \text{gas} \to \text{liquid (or solid)}

where: "closer packing" means the average distance between particles has shrunk, tipping the balance from kinetic energy (which favours spreading out) toward inter-particle force (which favours staying together); no numerical pressure or temperature values are demanded at this level — the relationship is qualitative.

Figure

Two side by side particle boxes, gas particles spread far apart at low pressure on the left and the same particles squeezed close together into a liquid like cluster at high pressure on the right

Why it works

There is no numerical formula at this level, so the reasoning itself carries the weight of a derivation: the same inter-particle force, brought into shorter range by pressure, wins the tug-of-war against kinetic energy.

Step 1. Start from a gas at ordinary atmospheric pressure and room temperature. Its particles carry enough kinetic energy, relative to the (weak, over that large distance) inter-particle force, to stay spread apart and move freely — the defining behaviour of a gas.

Step 2. Apply a large increase in pressure, for instance by compressing the gas inside a sealed steel cylinder. The particles are forced into a much smaller volume, so the average distance between any two neighbouring particles drops sharply.

Step 3. Inter-particle force of attraction grows stronger as distance shrinks, even though nothing about the force itself has changed — it is simply acting over a shorter range now. At the new, much smaller spacing, that force becomes strong enough, relative to the particles' (unchanged) kinetic energy, to hold them together in a fixed-volume, closely packed arrangement.

Step 4. The substance has therefore changed from gas to liquid without any drop in temperature — pressure alone tipped the balance. Cooling is not strictly required, though combining pressure with lower temperature (which reduces kinetic energy too) makes liquefaction happen even more readily and is the usual industrial practice.

Step 5. The reverse also holds: drop the pressure back down, and the inter-particle force weakens (relatively) as particles are allowed to spread out again, so the liquid reverts to gas. This reversibility is exactly what a cylinder valve exploits.

Worked example

Question: LPG (liquefied petroleum gas) is a gas at atmospheric pressure and room temperature, yet it is stored and transported as a liquid inside a sealed steel cylinder without refrigeration. Explain why, and describe what happens the moment the cylinder valve is opened.

Step 1. Inside the sealed cylinder, LPG is held under very high pressure. This forces its particles much closer together than they would ever be at atmospheric pressure.

Step 2. At that reduced spacing, the inter-particle force of attraction becomes strong enough, relative to the particles' kinetic energy at cylinder temperature, to hold them in the closely packed, fixed-volume arrangement characteristic of a liquid — so the LPG exists as liquid inside the cylinder.

Step 3. Storing it this way is efficient precisely because liquid LPG occupies far less volume than the same mass would occupy as gas at atmospheric pressure; a cylinder small enough to carry holds a useful quantity of fuel.

Step 4. When the valve is opened, gas escapes and the pressure inside, near the valve, drops toward atmospheric pressure. With that pressure drop, particles are no longer confined so close together, and the inter-particle force can no longer hold them in the liquid arrangement, so liquid LPG converts back to gas, ready to burn.

Final answer: pressure alone — not cooling — is what keeps LPG liquid inside the cylinder, and releasing that pressure at the valve is what turns it back into burnable gas.

Competitive stretch

Question: Solid carbon dioxide (dry ice) is left in an open tray at room temperature and atmospheric pressure. It shrinks steadily, giving off a white gaseous mist, without ever forming a liquid puddle. Explain.

Step 1. Solid CO $_2$ is manufactured and kept stable only under high pressure; under those conditions CO $_2$ can be solid even at a temperature where, at atmospheric pressure, it would normally be a gas.

Step 2. Placed in the open tray, the dry ice is now exposed to ordinary atmospheric pressure — far below the pressure that stabilised it as a solid, and also far below any pressure at which liquid CO $_2$ could exist at room temperature.

Step 3. Because there is no pressure window in which liquid CO $_2$ is stable at room temperature and atmospheric pressure, the inter-particle force cannot hold a closely packed liquid arrangement there. The solid has only one route available to it — straight to gas.

Final answer: the solid converts directly to gas — it sublimes — because atmospheric pressure at room temperature offers no stable liquid phase for CO₂, which is also why dry ice is prized for leaving no wet residue.

Common traps

Treating pressure and temperature as acting on separate, unrelated properties — both act on the same tug-of-war between inter-particle force and kinetic energy, and industrial liquefaction typically uses both together.
Assuming pressure alone can liquefy any gas at any temperature — NCERT's point is that a combination of raised pressure and lowered temperature is the general method; some gases need very high pressure or noticeably lower temperature or both.
Calling dry ice's disappearance "evaporation" — evaporation is liquid turning to gas; dry ice goes solid directly to gas, which is sublimation.
Reaching for $PV = nRT$ or Boyle's law — this NCERT topic is deliberately qualitative; no gas-law numericals belong here.

Ask the tutor: curious why some gases liquefy more easily under pressure than others, or want to see a few more pressure-and-state examples worked through?

Why does opening the LPG cylinder valve turn liquid fuel back into gas we can burn?

Inside the sealed cylinder, high pressure keeps LPG particles squeezed close enough for inter-particle force to hold them as a liquid. Opening the valve lets gas escape near the opening, and the pressure there drops toward atmospheric. With that lower pressure, particles are no longer forced close together, so inter-particle force can no longer hold the liquid arrangement, and it converts back to gas — which is exactly the form needed for burning at the stove.

Is dry ice cold because it is under pressure, like an LPG cylinder?

Not quite the same idea. Dry ice is solid carbon dioxide, and it stays solid because it is kept at low temperature and/or under pressure during manufacture. Once released to ordinary atmospheric pressure and room temperature, there is no stable liquid CO2 phase available, so it goes straight from solid to gas — sublimation — absorbing heat from its surroundings as it does, which is why it feels intensely cold and is useful for packaging.

Can any gas be turned into a liquid just by squeezing it hard enough, without any cooling?

Often yes for many everyday gases like LPG, but NCERT's safer statement is that the general method combines increased pressure with decreased temperature. Pressure alone can be enough for some substances at room temperature, but others need to be cooled somewhat as well before the inter-particle force can win out over kinetic energy — the syllabus asks you to know that both levers work together, not to calculate exactly how much of each is needed.