States of Matter

La méthode, étape par étape

1. 1

   Describe the particle arrangement in each state

   solid: fixed lattice · liquid: touching but mobile · gas: far apart, fast

   Every property follows from arrangement. A solid holds its shape because its particles cannot move past each other; a gas fills its container because nothing holds its particles together at all.

2. 2

   Explain properties from that arrangement

   gases compress because there is space between particles

   Solids and liquids barely compress because their particles already touch. Gases compress easily because most of a gas is empty space. Exam answers gain marks for the particle explanation, not just the observation.

3. 3

   Name the changes of state in both directions

   melting/freezing · boiling/condensing · sublimation/deposition

   Each change has a reverse. Energy is absorbed going towards gas and released coming back, which is why evaporation cools a surface and condensation warms one.

4. 4

   Understand why temperature pauses during a change

   energy goes into breaking forces, not into raising temperature

   During melting or boiling, added energy separates particles rather than speeding them up, so the thermometer holds steady until the change is complete. This plateau on a heating curve is examined regularly.

5. 5

   Distinguish evaporation from boiling

   evaporation: surface only, any temperature · boiling: throughout, at boiling point

   Evaporation happens at the surface at any temperature, as the fastest particles escape. Boiling happens throughout the liquid at a specific temperature. Both produce gas and they are not the same process.

Start with the particles

Almost every question in this topic can be answered by describing what the particles are doing. Why does a gas fill its container? Because its particles move freely with no forces holding them in place. Why can a liquid be poured but not compressed? Because its particles can slide past each other but are already in contact.

Students who memorise properties as lists have to recall three separate lists. Students who can picture the arrangement derive all three, and can also answer the unfamiliar questions that lists do not cover.

• Solid — fixed positions, vibrating, holds shape
• Liquid — touching but mobile, takes container's shape
• Gas — far apart, fast, fills container
• Energy in → towards gas. Energy out → towards solid
• Temperature pauses during a change of state

Reading a heating curve

A graph of temperature against time for a substance being heated shows sloping sections where the temperature rises and flat sections where it does not. The flat sections are the changes of state, and the energy going in is separating particles rather than raising temperature.

Interpreting these graphs is a standard question. Identifying which state exists on each section — and what is happening on each plateau — is the skill being assessed, and it follows directly from understanding what the plateau means.

Why evaporation cools

The fastest-moving particles at a liquid's surface are the ones with enough energy to escape. When they leave, the average energy of what remains is lower, so the liquid cools. This is why sweating works and why a wet surface feels cold.

It is a good example of a question that cannot be answered from a memorised list of properties but is straightforward from the particle model. Questions of this type are how examiners distinguish understanding from recall.

How we teach states of matter

We ask students to draw the particles before answering any question in this topic. A quick sketch of three arrangements answers most questions directly and turns recall into reasoning.

We are also careful to separate evaporation from boiling early, because they are commonly merged and the distinction is examined. Establishing it at the start is much easier than correcting it later.