Three Simple Statements Explain Every State of Matter

They can, and that is what makes the kinetic molecular theory worth learning rather than memorising.

The theory says only three things: matter is made of tiny particles, there is space between them, and they are in constant motion while attracting one another. Change nothing but the size of the space and the strength of the attraction, and the same three statements produce a solid, a liquid or a gas.

A good theory does not describe each case separately — it explains all of them from one set of rules and then predicts what you have not yet tested. This page covers the first part of the ICSE Class 8 Chemistry chapter on matter: how the states are classified, what the theory claims, and the experiments that back it up.

By checking four properties: shape, volume, compressibility and density.

Solid — has a definite shape and a definite volume, is almost incompressible, and has the highest density of the three states of the same substance. A brick, a coin, a piece of ice.

Liquid — has no definite shape but a definite volume, is almost incompressible, and has a moderate density. It takes the shape of its container while keeping a level free surface. Water, milk, kerosene.

Gas — has neither definite shape nor definite volume, is highly compressible, and has the lowest density by a wide margin. It fills its container completely. Air, oxygen, cooking gas.

Why volume is the discriminating property. Shape alone cannot separate a liquid from a gas — both take the shape of the container. Only volume does: pour $500\text{mL}$ of water into a $2\text{L}$ bottle and you still have $500\text{mL}$ of water, but release $500\text{mL}$ of gas into the same bottle and it immediately occupies all $2\text{L}$.

How large the density gap really is. Compare a substance in its liquid and gaseous forms. Liquid oxygen has a density of about $1141{\text{kg/m}}^3$, while oxygen gas at ordinary conditions is about $1.43{\text{kg/m}}^3$. The ratio is

$$\frac{1141}{1.43}\approx 800$$

The same molecules, spread over roughly eight hundred times the volume. Nothing about the oxygen molecule changed — only the space between molecules did, which is precisely what the theory will explain.

A boundary case to be careful about. Some substances do not fit neatly. Wet mud, toothpaste and glass have properties of more than one state, and borderline cases like these are why the four properties are checked together rather than relying on any single one.

Three statements, each of which takes a different value in each state.

Postulate 1 — intermolecular space. Matter is made of extremely small particles called molecules, and there is empty space between them. This gap is the intermolecular space.

Postulate 2 — intermolecular force. Molecules attract one another. This attraction is the intermolecular force, and it becomes rapidly weaker as the space grows larger.

Postulate 3 — molecular motion. Molecules are in constant motion, and so possess kinetic energy. That energy increases with temperature.

How the three read in each state:

- Solid. Space: smallest. Force: strongest. Motion: molecules only vibrate about fixed positions — they cannot travel.
- Liquid. Space: larger. Force: moderate. Motion: molecules slide and roll past one another within the body of the liquid.
- Gas. Space: very large. Force: negligible. Motion: molecules move freely and rapidly in all directions.

Now read the properties off the postulates. This is the part worth practising, because the theory is what turns the classification table into something you can reconstruct rather than recall.

- A solid keeps its shape because its molecules cannot leave their fixed positions.
- A liquid keeps its volume but not its shape because its molecules can move past each other but cannot escape the attraction.
- A gas fills its container because the attraction is too weak to hold the molecules anywhere.
- A gas is compressible and a solid is not, because a gas is mostly empty space and a solid has almost none to remove.
- A gas has the lowest density because the same mass of molecules is spread over a far greater volume.

The inverse relation to hold on to. Larger space always means weaker force. Writing the two as independent — large space and strong force — is the single commonest error in this chapter, and it makes every prediction come out backwards.

Going from solid to liquid to gas, intermolecular space increases and intermolecular force decreases at every step.

And the driver is kinetic energy. Heating a substance raises the molecular kinetic energy, and once it is large enough to overcome the attraction, the molecules move further apart — so the force weakens further, which allows still more separation.

Melting — solid to liquid. Heat makes the molecules vibrate more violently until they break out of their fixed positions. The space widens a little, the force weakens a little, and the molecules can now slide.

Vaporisation — liquid to gas. Further heating gives the molecules enough energy to escape the attraction of their neighbours altogether. The space widens enormously and the force becomes negligible.

And in reverse. Cooling removes kinetic energy and the force reasserts itself — a gas condenses and a liquid freezes. Compressing a gas achieves the same thing by shortening the space directly, which is how cooking gas is stored as a liquid in a steel cylinder.

Ranking the three states:

- Intermolecular space: solid $<$ liquid $<$ gas
- Intermolecular force: solid $>$ liquid $>$ gas
- Molecular kinetic energy: solid $<$ liquid $<$ gas
- Density: solid $>$ liquid $>$ gas
- Compressibility: solid $<$ liquid $<$ gas

The essential point about what does not change. The molecules themselves are identical in all three states. Ice, water and steam are all ${\text{H}}_2\text{O}$ — the same molecule, differently spaced and differently energetic. This is why every change of state is a physical change and is reversible: nothing has been made or destroyed, so nothing needs to be unmade.

One genuine exception to the density ranking. Water is denser as a liquid than as a solid, which is why ice floats. That is a property of water in particular, not a fault in the theory.

Two simple ones, each testing a different postulate.

Dilution of potassium permanganate — proving the particles are extremely small.

Dissolve a single small crystal in $1000\text{mL}$ of water. The whole beaker turns purple. Now take just $1\text{mL}$ of that coloured solution and add it to a fresh $1000\text{mL}$ of water. It is still visibly coloured.

Each step dilutes the original by a factor of $1000$, so after two steps the dilution is

$$1000\times 1000=10^6$$

For the final beaker to still show colour, the original crystal must have contained at least a million separate particles — and in truth far more, since the colour is still plain. A crystal you could barely see has been divided a million times over.

Diffusion — proving the particles move on their own.

Open a bottle of scent or light an agarbatti at one end of a still room, and the smell reaches the far end without any draught. Nobody carried it; the molecules travelled there themselves. This spreading of one substance through another is diffusion.

Diffusion is fastest in gases, slower in liquids and negligible in solids — exactly the order of molecular freedom the theory predicts. Drop a crystal of copper sulphate into a tall jar of still water and the blue colour creeps upward over hours; the same substance as a gas would fill the jar in seconds.

Dissolving — proving there is space between particles.

Fill a glass to the brim with water and stir in a spoonful of sugar. The sugar disappears and the water does not overflow. The sugar molecules have settled into the gaps between the water molecules.

The same effect appears with two liquids: mix $50\text{mL}$ of water with $50\text{mL}$ of alcohol and the total is noticeably less than $100\text{mL}$, because each liquid's molecules occupy the other's spaces.

And a demonstration of the force. Two lead spheres with freshly cut flat faces, pressed hard together, stick. Mercury gathers into beads rather than spreading out. Both show the attraction between molecules acting directly.

Why these experiments matter more than the statements. A theory that only restates what you already knew is worth little. The value here is that the postulates were tested — and each experiment could have failed. The sugar could have overflowed the glass, and the scent could have stayed in its bottle.