States of Matter

Every particle in a substance experiences two competing influences at once.

Inter-particle force of attraction pulls neighbouring particles toward each other and, if strong enough, locks them into fixed positions relative to one another.

Kinetic energy of particles is the energy of their constant, restless motion — vibrating, sliding, or flying — and it works to spread particles apart and let them move independently. Which influence wins decides the state. In a solid, the inter-particle force dominates almost completely: particles are packed as closely as possible and can only vibrate about fixed mean positions. This gives a solid its fixed shape, fixed volume, rigidity, and near-zero compressibility — there is essentially no spare space between particles to squeeze out. In a liquid, the two influences are more evenly matched: the force is still strong enough to hold particles close together (fixed volume) but weak enough to let them slide past one another (no fixed shape, it flows and takes the shape of its container). In a gas, kinetic energy dominates so completely that the inter-particle force becomes almost negligible: particles fly apart, fill whatever container they are given, diffuse rapidly through other gases, and can be compressed a great deal because there is so much empty space between them to begin with. This ordering of particle spacing directly decides density, since the same mass is squeezed into very different volumes:

where: $\rho$ is density, $m$ is the mass of the sample, and $V$ is the volume it occupies. Because gas particles occupy far more volume than the same mass of liquid or solid particles, $\rho_{\text{gas}} \ll \rho_{\text{liquid}} < \rho_{\text{solid}}$ holds in general — density is really just a scorecard for how tightly particles are packed.

The claim that a single tug-of-war between inter-particle force and kinetic energy can explain every listed property of solids, liquids, and gases sounds almost too convenient, so it is worth checking each property against the underlying particle picture rather than accepting it on faith.

Step 1. Start with shape and volume. Both properties are really questions about how free particles are to change their relative positions. If the force is strong enough to lock particles into fixed positions (a solid), neither shape nor volume can change — there is no freedom to rearrange. If the force is weak enough to let particles slide past each other but still strong enough to keep them touching (a liquid), the collection can flow into any shape while its total volume stays essentially unchanged, since the particles remain in contact. If kinetic energy overwhelms the force almost entirely (a gas), particles separate as far as the container allows, so neither shape nor volume is fixed.

Step 2. Move to compressibility. This property asks how much empty space exists between particles that could still be squeezed out. Solids and liquids already have particles touching or nearly touching, so there is negligible or very little spare space, hence negligible or very low compressibility. Gases have enormous empty space relative to particle size, so pushing particles closer together is easy, giving high compressibility.

Step 3. Consider rate of diffusion. Diffusion is simply how quickly particles of one substance mix into another by moving through the gaps between them. Gas particles move fastest and have the most open space to move through, so gases diffuse quickest. Liquid particles move more slowly and have less open space, so liquids diffuse more slowly. Solid particles barely move from their fixed positions at all, so diffusion in solids is extremely slow.

Step 4. Finally, density falls out of the same picture without any new assumption: for a fixed mass of a substance, tighter particle packing means smaller volume, and $\rho = m/V$ then guarantees a larger density. Since particle spacing increases from solid to liquid to gas, density decreases in that same order. Every property in the comparison table, then, traces back to exactly one cause: the balance between force and kinetic energy, and the particle spacing that balance produces.

Question: A solid block of wax is rigid and keeps its shape on a table. When heated, it becomes liquid wax that can be poured into a mould, and if heated much further it would boil away as gas. Explain the two changes using inter-particle force and kinetic energy, and state what happens to the wax's density at each stage.

Step 1. In solid wax at room temperature, the inter-particle force holds particles in fixed positions where they can only vibrate, not travel — this gives the block its fixed shape, fixed volume, and rigidity, and its density is the highest of the three states.

Step 2. As the wax is heated, particles absorb energy and vibrate more vigorously; kinetic energy is rising while the inter-particle force stays roughly the same.

Step 3. Once heating passes the melting point, kinetic energy becomes large enough to partly overcome the inter-particle force. Particles can now move past one another, though they remain close together — this is the liquid state, so the wax flows and takes the shape of the mould, but its volume (and hence density) changes only slightly from the solid.

Step 4. If heating continued past the boiling point, kinetic energy would overwhelm the inter-particle force almost entirely, particles would fly far apart, and the wax vapour would occupy a much larger volume for the same mass. Melting lowers density only slightly because particles stay in contact, but boiling into a gas drops density sharply because particles separate by large distances.

• Saying a liquid has no fixed shape and no fixed volume — a liquid does have a fixed volume; only its shape changes with the container. Only gases lack both a fixed shape and a fixed volume.
• Treating rigidity, fluidity, and compressibility as separate unrelated facts to memorise — all three are direct, derivable consequences of one thing: how much inter-particle space is available and how strong the inter-particle force is.
• Assuming every property of a state must be recalled by rote rather than reasoned from the force-versus-kinetic-energy balance — if a question asks about an unfamiliar property, work it out from particle spacing rather than guessing.
• Mixing up the density order — always check that gas density is the lowest and solid density is generally the highest, since it is particle spacing, not particle mass, that changes across states.

Ask the tutor: want a quick round of practice questions on classifying an unfamiliar substance's state from a short description of its properties — ask your AI tutor to generate a few.