B.1.1—Molecular states

Particle view

Matter is made of particles in continuous random motion. The state depends mainly on how closely particles are packed, how freely they move and how strongly intermolecular forces hold them together.

Compare the three states

State	Arrangement and separation	Motion	Macroscopic consequence
Solid	closely packed, ordered or locally fixed	vibrate about fixed positions	fixed shape and volume
Liquid	close together but not fixed in a lattice	move and slide past neighbours	fixed volume, takes container shape
Gas	widely separated	move freely between collisions	no fixed shape or volume

Use temperature carefully

At the same temperature, particles have the same average kinetic energy in the kinetic-theory model. The different states are then distinguished by separation and intermolecular forces, not by claiming that one state automatically has hotter particles.

Common trap

Do not describe a solid as having motionless particles. “Fixed position” means the particles oscillate about equilibrium positions; it does not mean their kinetic energy is zero.

How it is assessed

The evidence shows structured comparison and application: compare solid and gas at the same temperature, or connect the anomalous density of water to why lakes can remain liquid below surface ice.

Command terms

Compare / Discuss / Describe

What earns marks

For compare/discuss questions, award each distinct physical comparison explicitly: solid has stronger intermolecular forces, smaller separations and vibration about fixed positions; gas has weaker effective forces, larger separations and freer motion. At the same temperature, state that the average particle kinetic energy is the same. For the water-density application, link water at 4 °C sinking and surface ice to insulation of liquid below.

Watch for

Saying particles in a solid are motionless, or listing properties without explicitly comparing the states.

Representative question

[Maximum number: 3]

Compare the molecular conditions of the solid phase and the gas phase at the same temperature.

Microscopic story

Matter contains moving particles. Temperature tracks average random kinetic energy, while internal energy also includes intermolecular potential energy. Phase changes alter particle behaviour at constant temperature.

Transfer story

A temperature difference gives the net direction of thermal energy transfer. Conduction transfers energy through local interactions, convection through moving fluids, and radiation through electromagnetic waves.

Equation map

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

$Q=mc\Delta T,\quad Q=mL$

$\frac{\Delta Q}{\Delta t}=\frac{kA\Delta T}{\Delta x}$

$L=\sigma AT^4$

$b=\frac{L}{4\pi d^2}$

$\lambda_{\max}T=2.9\times10^{-3}\,\mathrm{m\,K}$

Question strategy

Identify whether the question concerns a state property, a transfer mechanism or a rate.
Convert to SI units and use kelvin whenever an absolute temperature appears.
Check whether temperature changes, remains constant during a phase change, or enters a fourth-power/inverse-square relation.
State the physical reason, not only the numerical substitution.

B.1.1—Molecular states

Explain Solids, Liquids and Gases

B.1.1 Exam Analysis

Assessment in practice

Synthesize B.1 Thermal Energy Transfers

Question 1