Compressibility of states

Compressibility is the property of a substance that describes how much its volume can be reduced when squeezed. It is not a fixed number you calculate at this level — it is a qualitative comparison across the three states, and the comparison is decided entirely by interparticle space, the tiny gaps between neighbouring particles.

• Gases are compressible. Gas particles sit far apart, so a gas is mostly empty space between particles rather than particles themselves. Pushing on a gas simply crowds the particles into that available space, and the volume drops noticeably — which is exactly what makes a bicycle pump or a syringe full of air so easy to push.
• Liquids are practically incompressible. Liquid particles are already close together, so there is only a thin margin of interparticle space left. Pushing on a liquid finds almost nowhere to send the particles, so the volume barely changes — the reason hydraulic brake fluid and water-filled pipes transmit force instead of simply squashing.
• Solids resist compression even more strongly. Solid particles are packed tighter still, with the smallest interparticle spaces of the three states, and are locked together by strong interparticle attraction. There is essentially no spare room and no easy give, so solids do not noticeably compress under ordinary pressure either.

The order of compressibility — gas much greater than liquid, liquid roughly equal to (or very slightly greater than) solid — mirrors the order of interparticle space exactly. Nothing here needs an equation: the chapter's whole argument is "more gap to squeeze equals more compressible."

Goal. This section explains, without any formula, why the same push produces a large volume change in a gas but a negligible one in a liquid or solid, by tracing the pushing force down to what it can actually do to the interparticle space in each state — the reasoning a Class 8 student is expected to reproduce, not just the labelled conclusion.

Step 1. Picture what applying pressure physically does: it pushes neighbouring particles closer together. The push itself does not change how large the particles are or destroy any particles — it only reduces the gaps between them. So the amount by which a substance can shrink is capped by how much gap exists to begin with.

Step 2. In a gas, the particles are spread far apart with huge amounts of empty interparticle space between them, and the attraction between particles is too weak to hold them at a fixed separation. Pushing the plunger of an air-filled syringe therefore has plenty of "room" to work with — the particles slide closer together and the occupied volume falls sharply, exactly as seen when 30 mL of air shrinks to about 15 mL under moderate force.

Step 3. In a liquid, the particles start out close together, so the available interparticle space is small from the outset. The same push that easily closed a large gas gap now meets particles that are already almost touching their neighbours — there is simply very little space left to remove, so the volume can only fall by a tiny, often unmeasurable amount.

Step 4. In a solid, the interparticle space is smaller still, and the particles are additionally held in place by strong interparticle attraction that resists any rearrangement. Both the shortage of space and the strength of the bonds work against compression, which is why solids resist a squeeze at least as strongly as liquids do, even though both are described as "practically incompressible" at this level.

Step 5. Put together, the size of the volume change under a given push is not a coincidence — it directly tracks the amount of interparticle space present before the push. Rank the states by that space (gas, then liquid, then solid, decreasing) and you have automatically ranked them by compressibility (gas, then liquid, then solid, decreasing).

Question. A student fills one syringe with 30 mL of air and another identical syringe with 30 mL of water, seals each tip with a finger, and pushes both plungers with the same moderate force. The air-filled syringe reads 15 mL afterward; the water-filled syringe reads 29 mL. The student then repeats the water trial but presses twice as hard — the reading still stays close to 29 mL. Explain both results.

Step 1. For the air-filled syringe, the large interparticle space between gas particles gives the pushing force plenty of gap to close, so the volume falls substantially, from 30 mL to about 15 mL — a drop of half the original volume.

Step 2. For the water-filled syringe under moderate force, the interparticle space is already small, so even though the same force is applied, there is very little gap available to remove — the volume drops only slightly, from 30 mL to about 29 mL.

Step 3. Pressing twice as hard on the water does not change this outcome, because the limitation is not the size of the force but the shortage of interparticle space itself; doubling a force that has almost nothing to compress still finds almost nothing to compress, so the reading stays close to 29 mL rather than falling further.

Answer: the gas compresses noticeably (30 mL to about 15 mL) because of its large interparticle space, while the liquid stays practically incompressible (about 29 mL) regardless of how hard it is pushed, because its interparticle space is too small to give the force any real room to act.

Competitive stretch

An exam question may ask which single change would make the water-filled syringe compress noticeably. The correct answer is not "push harder" but "give the particles more space to move into" — for instance, heating the water toward its boiling point so it starts turning to vapour, since only a genuine increase in interparticle space (not a bigger force) can change how compressible a substance is at this level.

• Saying liquids "cannot be compressed at all" — the accurate phrase is practically incompressible; the tiny remaining interparticle space still allows an extremely small change, it is just too small to matter here.
• Assuming a bigger push will eventually compress a liquid the way it compresses a gas — pushing harder does not create new interparticle space, so it does not meaningfully change the result (see the worked example, Step 3).
• Explaining compressibility purely through interparticle attraction — the chapter's key variable is the amount of interparticle space available to shrink; attraction in solids only adds extra resistance on top of that.
• Confusing compressibility with a fixed shape or fixed volume — compressibility is about volume change under pressure, which is a separate property from whether a state keeps a fixed shape.

Ask the tutor: want to see the syringe activity worked through step by step for air versus water, with the interparticle-space picture drawn out at each stage?