Kinetic theory of matter

The kinetic theory rests on three postulates about the particles (atoms, molecules or ions) that make up every substance.

Postulate 1 — continuous, random motion. Particles are never at rest. In a solid they vibrate about a fixed position; in a liquid and a gas they move about more freely. This motion is why particles collide constantly with each other and with the walls of any container, and why a gas exerts pressure — the ceaseless bombardment of particles on the container walls.

Postulate 2 — interparticle space. There is a gap between one particle and its neighbour, called the interparticle space. It is tiny in a solid, a little larger in a liquid, and very large in a gas — which is why gases can be compressed easily and solids almost cannot.

Postulate 3 — interparticle attraction. A force of attraction (also called the force of cohesion) pulls neighbouring particles toward each other. It is strongest in a solid, weaker in a liquid, and nearly zero in a gas. These two properties move together but in opposite directions, which is worth stating precisely because exam answers are marked on exactly this trade-off:

where: the double arrow reads "goes together with" — it is a qualitative correspondence, not an equation to calculate with (there is no formula or numerical relationship at this stage; that begins only in Class IX with the gas laws). The mirror statement — small attraction, large space, gas-like behaviour — describes the opposite end of the scale, and a liquid sits in between on both counts. The figure below puts the three postulates side by side so you can see how spacing and motion change while the number of postulates stays the same across all three states.

Putting the postulates together explains the everyday properties of the three states in one pass. A solid has particles packed close with strong attraction, so they can only vibrate about fixed positions — this gives it a definite shape and a definite volume. A liquid has particles a little farther apart with moderate attraction, so they can slip past one another while staying in contact — this gives it a definite volume but lets it flow into the shape of its container. A gas has particles far apart with almost no attraction, so they move independently and rapidly in every direction — this is why a gas has neither a definite shape nor a definite volume and always expands to fill whatever container holds it.

There is no numerical formula at this stage of the kinetic theory — the reasoning is entirely qualitative, and understanding why the three postulates lead to the observed states is what an exam answer is really graded on.

Step 1. Attraction and spacing are not independent; they move opposite each other as a substance is heated or cooled. Supplying heat energy to a substance gives its particles more energy of motion. More energetic particles can pull farther apart against the force of attraction, which is why interparticle space grows and interparticle attraction effectively weakens as a solid is heated toward melting.

Step 2. The rigidity or fluidity of a state follows directly from how free the particles are to change position. If attraction is strong enough to lock particles into fixed positions (allowing only vibration), the substance resists any change of shape — this is the mechanical definition of "solid." If attraction is too weak to hold particles in one place but still keeps them close together, particles can slide past each other without separating — this is "liquid": no fixed shape, but a fixed volume because particles remain in contact.

Step 3. When attraction becomes negligible, nothing restrains a particle's own motion, so it travels in a straight line until it collides with another particle or a wall. Since the particles are now free and moving fast in every direction, the substance spreads to occupy all the space available to it — this is "gas": neither a fixed shape nor a fixed volume.

Step 4. This same reasoning — free, colliding, randomly-moving particles — also explains gas pressure and the spreading of smells. Pressure is simply the cumulative effect of countless particle collisions against the container walls per second; spreading (diffusion, though this term is not required at this level) is simply particles wandering, on their own, into whatever space is unoccupied. No external push is needed for either effect — postulate 1 (continuous random motion) alone accounts for both.

Question: A student places an ice cube in a warm room. Using only the postulates of the kinetic theory of matter, explain what happens to the ice cube as time passes, and why it eventually disappears from the surface it sits on and instead fills the shape of any container it is poured into.

Step 1. In the ice cube, interparticle attraction between the water particles is strong and interparticle space is small, so the particles can only vibrate about fixed positions — this is why the ice keeps a definite shape and volume, sitting as a solid block on the table.

Step 2. As heat from the warm room reaches the ice, the particles absorb this energy and vibrate more vigorously. Beyond a point, the vibration becomes strong enough to partly overcome the interparticle attraction holding particles at fixed positions.

Step 3. The particles can now move past one another rather than staying fixed, though they remain close together — the ice has melted into liquid water. The interparticle space has increased slightly and the interparticle attraction has weakened compared with the solid state, exactly the trade-off the kinetic theory predicts.

Step 4. Because the particles are free to move past each other while remaining close, the liquid water no longer holds its own shape — it flows and takes the shape of whatever container it is poured into, while still occupying the same total volume as before (ignoring the small volume change on melting).

The ice cube's disappearance and change of shape are explained entirely by weakening interparticle attraction and increasing interparticle space as heat energy is absorbed — no new postulate is needed beyond the original three.

• Treating "interparticle space" as empty space inside a particle. It is the gap between separate particles, not empty space within a single atom or molecule — a very common wording slip in answers.
• Assuming solid particles are perfectly still. They vibrate continuously about fixed positions; motion (postulate 1) never stops in any state, only its type and extent change.
• Explaining gas compressibility using attraction instead of space. Gases compress because interparticle space is large, not because of strong attraction — attraction in a gas is close to negligible, the opposite explanation an exam often expects a student to catch.
• Naming all three postulates vaguely instead of the one that actually explains the observation. A strong answer identifies which postulate (motion, space or attraction) is doing the explanatory work for the specific behaviour asked about.

Ask the tutor: Want to work through another everyday observation — like why a balloon shrinks in cold weather or why hot metal expands — using only these three postulates?