Two Opposing Quantities Decide Whether Something Is Solid

A tug-of-war between two quantities: the intermolecular force pulling the molecules together, and the kinetic energy of the molecules driving them apart.

Water is the clearest demonstration available. The molecules in ice, in liquid water and in steam are identical — the same ${\text{H}}_2\text{O}$. Nothing about the molecule changes between the three states. Only which side of that tug-of-war is winning changes.

This page covers the whole first part of the ICSE Class 8 Physics chapter on matter: the molecular model, the two competing quantities, how the three states differ, and what happens when you shift the balance.

Every substance is built from molecules that are too small to see, are in constant motion, have space between them, and attract one another.

Those four statements are the whole model, and each one can be demonstrated.

Molecules are extremely small. A crystal of potassium permanganate dropped into water colours the whole beaker. Take a spoonful of that coloured water into a fresh beaker of clear water and it colours that too. The original crystal must have contained an enormous number of pieces for so few of them to still tint a beaker.

Molecules move constantly. Open a bottle of perfume at one end of a room and it is smelled at the other end without any draught. The molecules travelled there themselves. This spreading is called diffusion.

There is space between molecules. Dissolve sugar in a full glass of water and the level does not visibly rise — the sugar molecules settle into gaps between the water molecules.

Molecules attract each other. Two lead spheres with freshly cut flat faces, pressed hard together, stick. Mercury gathers into beads rather than spreading. A steel needle laid gently on water floats even though steel is denser than water.

The force of attraction is called the intermolecular force, and the average gap between molecules is the intermolecular space. These two always move in opposite directions: a larger space means a weaker force, because the attraction falls off sharply with distance.

In a solid the force wins; in a gas the kinetic energy wins; in a liquid neither wins outright.

Solid. Intermolecular space is smallest and the force is strongest. Molecules cannot travel — they only vibrate about fixed positions. So a solid has a fixed shape and a fixed volume, is almost incompressible, and has the highest density of the three states for the same substance.

Liquid. The space is a little larger and the force a little weaker. Molecules can now slide past one another but cannot escape. So a liquid has a fixed volume but no fixed shape — it takes the shape of its container while keeping a level upper surface — and is nearly incompressible.

Gas. The space is very large and the force is negligible. Molecules move freely and rapidly in all directions. So a gas has neither fixed shape nor fixed volume, fills its container completely, is highly compressible, and has by far the lowest density.

Why a gas is so much less dense. Density is mass per unit volume:

$$\rho =\frac{m}{V}$$

Water has a density of about $1000{\text{kg/m}}^3$, while steam at atmospheric pressure is closer to $0.6{\text{kg/m}}^3$. Take $1\text{kg}$ of water:

$$V_{\text{water}}=\frac{1}{1000}=0.001{\text{m}}^3=1\text{L}$$

$$V_{\text{steam}}=\frac{1}{0.6}\approx 1.67{\text{m}}^3$$

The same kilogram of molecules, spread over more than a thousand times the volume. That ratio is the intermolecular space, measured indirectly.

Compressibility follows the same logic. You cannot squeeze a liquid because there is almost no empty space to remove, but a gas is mostly empty space, which is why a bicycle pump works.

Heating raises the kinetic energy; compressing shortens the intermolecular space and so strengthens the force. Each pushes the tug-of-war one way.

Heating a solid. The molecules vibrate harder. Eventually the vibration is violent enough to break out of the fixed positions, and the solid melts. Heat the liquid further and the fastest molecules escape entirely — it boils and becomes a gas.

Cooling a gas. The molecules slow down, the attraction is able to hold them, and the gas condenses to a liquid. Cool further and it freezes.

Compressing a gas. Squeezing the molecules closer brings the intermolecular force back into play. This is why gases can be liquefied by pressure, and why cooking gas is stored as a liquid in a steel cylinder — a large volume of gas is compressed into a few litres.

Direct solid to gas. A few substances skip the liquid state entirely and go straight from solid to gas on heating. This is sublimation, seen in camphor, naphthalene and iodine. Cooling the vapour returns it directly to the solid, which is deposition.

Nothing new is created. Every one of these changes is physical and reversible, because the molecules themselves are untouched. Ice melting and water freezing are the same molecules with different amounts of energy — which is exactly why a change of state is not a chemical change.