ChemistryCore20 min read

States of Matter

The particle model, changes of state, and diffusion

This topic appears in:

9th Class Chemistry · ch.110th Class Chemistry · ch.1O Level Chemistry (5070) · ch.1IGCSE Chemistry (0620) · ch.1A Level Chemistry (9701) · ch.4

Practise the full question bank:

9th Class Chemistry — MCQs & exam Qs10th Class Chemistry — MCQs & exam QsO Level Chemistry (5070) — MCQs & exam QsIGCSE Chemistry (0620) — MCQs & exam QsA Level Chemistry (9701) — MCQs & exam Qs
01

Everything is particles, arranged three ways

Solids, liquids and gases are the same particles with different amounts of energy. The particle model explains every physical property from just two things: how the particles are arranged and how much they move.

In a solid the particles are packed in a fixed, regular pattern, vibrating in place — so a solid keeps its shape and volume. In a liquid they touch but can slide past each other — fixed volume, but it flows to fit the container. In a gas they are far apart and move freely at speed — no fixed shape or volume, and easily compressed because of all the empty space.

StateArrangementMovementShape / volume
Solidregular, packedvibrate in placefixed shape, fixed volume
Liquidclose, disorderedslide past each otherfits container, fixed volume
Gasfar apartfast, random, freefills container, compressible
02

Changes of state are energy in or out

Heating gives particles energy to break free of their neighbours; cooling lets forces pull them back. Melting (solid→liquid) and boiling (liquid→gas) take energy in; condensing and freezing give it out. Sublimation goes straight from solid to gas.

The subtle part: while a substance is changing state, its temperature does not rise, even though you keep heating. The energy is going into breaking the forces between particles, not speeding them up — which is why a heating curve has flat plateaus at the melting and boiling points.

Heat steadily and watch the temperature climb, then stall at the melting point, climb again, then stall at the boiling point. Each plateau is energy going into the change of state, not into temperature.

03

Temperature and pressure

Temperature is a measure of the average kinetic energy of the particles — hotter means faster. That is why a gas exerts more pressure when heated in a fixed container: faster particles hit the walls harder and more often. Squeeze a gas into a smaller volume and the collisions become more frequent, so pressure rises too.

Why gases compress and solids do not

A gas is mostly empty space between particles, so there is room to push them closer — it compresses easily. In a solid or liquid the particles are already touching, so there is almost nothing to squeeze out.

04

Diffusion

Diffusion is the spreading of particles from where they are crowded to where they are sparse, purely from their own random motion — no stirring needed. It is why you smell perfume across a room and why a drop of dye colours a whole beaker.

It is faster when hotter (particles move quicker) and faster for lighter particles (less mass, higher speed at the same temperature). Diffusion is fast in gases, slow in liquids, and essentially absent in solids, where particles cannot move past each other.

Worked example

Cotton wool soaked in ammonia (light) and in hydrochloric acid (heavier) is put at opposite ends of a tube. A white ring forms where they meet. Nearer which end, and why?

  1. Both gases diffuse toward each other and react to form a white solid (ammonium chloride).Diffusion carries each gas along the tube from its own random motion.
  2. Ammonia particles are lighter, so they move faster and travel further in the same time.At the same temperature, lighter particles have a higher average speed.
  3. The ring forms nearer the hydrochloric-acid end.The faster ammonia covers more of the tube before they meet.

Nearer the HCl end — the lighter, faster ammonia diffuses further before the two gases meet.

05

Heating and cooling curves

Heat a solid steadily and its temperature does not simply rise in a straight line. It climbs, then pauses at the melting point while the solid turns to liquid, climbs again, then pauses at the boiling point while the liquid turns to gas. During each plateau the temperature is constant even though heat is still going in.

That is because the energy is being used to overcome the forces between particles (breaking them apart), not to make the particles move faster. Cooling reverses the whole picture, with plateaus as the substance releases energy while it condenses and freezes.

Why the plateau?

At a change of state, added energy goes into breaking the forces between particles rather than raising their kinetic energy — so the thermometer holds steady until the change is complete.

06

Gas pressure and the kinetic model

Gas particles move fast in all directions and collide with the container walls; the total force of those collisions over the wall area is the pressure. Heat the gas and the particles move faster, hitting the walls harder and more often, so the pressure rises (at fixed volume). Squeeze the gas into a smaller volume and the collisions become more frequent, so the pressure rises too.

This same picture — many tiny particles in constant random motion — explains diffusion, evaporation and why gases fill their container completely.

Practice questions

6 questions · 22 marks · full working on every one

Try each one on paper first, then open the working. The marks are shown where they are actually awarded, because that is where they are actually lost.

Short questions

4 · 10 marks

Two marks each, in the style of the short-question section of the paper. Answer in two or three lines.

SQ1[2 marks]
Describe the arrangement and movement of particles in a solid.
Model answer

Particles are closely packed in a regular, fixed arrangement and can only vibrate about fixed positions.

Examiner tip. Solid = fixed, regular, vibrating; liquid = close but random and moving; gas = far apart and fast.

SQ2[2 marks]
Explain, in terms of particles, why a gas exerts pressure on the walls of its container.
Model answer

Gas particles move rapidly in all directions and collide with the walls; the force of these collisions per unit area is the pressure.

Examiner tip. Pressure comes from countless particle–wall collisions.

SQ3[3 marks]
Explain what happens to the particles as a solid is heated until it melts and then boils.
Model answer

Heating gives particles more kinetic energy so they vibrate more; at the melting point the forces holding the lattice break and particles move past each other (liquid); at the boiling point particles gain enough energy to escape completely (gas).

Examiner tip. Energy goes into overcoming forces between particles at each change of state.

SQ4[3 marks]
A student heats a pure solid until it melts. Sketch-describe what happens to the temperature before, during and after melting, and explain the shape.
Model answer

Before melting the temperature rises as particles gain kinetic energy. During melting it stays constant (a plateau) because the energy overcomes the forces holding the lattice. After melting it rises again as the liquid warms.

Examiner tip. A plateau on a heating curve always marks a change of state.

Long questions

2 · 12 marks

Theory and numerical together, as they appear in the long-question section.

LQ1[6 marks]
A student investigates diffusion using gas jars.
  1. Define diffusion. [2]
  2. Explain why bromine vapour spreads more slowly than hydrogen gas at the same temperature. [2]
  3. State and explain the effect of raising the temperature on the rate of diffusion. [2]
Mark scheme
  1. Diffusion is the movement of particles from a region of higher concentration to lower concentration[1]
  2. ...until they are evenly spread / down a concentration gradient[1]
  3. Bromine molecules are heavier / have greater mass than hydrogen molecules[1]
  4. At the same temperature heavier particles move more slowly, so diffuse more slowly[1]
  5. Raising temperature increases the rate of diffusion[1]
  6. Because particles gain kinetic energy and move faster[1]

Diffusion = spreading down a concentration gradient; lighter particles diffuse faster; higher temperature → faster diffusion.

Examiner tip. At a given temperature, lighter particles move faster (same average kinetic energy, smaller mass).

LQ2[6 marks]
A gas is sealed in a container.
  1. Explain, using the kinetic particle model, how the gas exerts pressure on the walls. [2]
  2. Explain what happens to the pressure if the gas is heated at constant volume. [2]
  3. Explain what happens to the pressure if the volume is halved at constant temperature. [2]
Mark scheme
  1. Particles move rapidly and randomly, colliding with the walls[1]
  2. the force of these collisions per unit area is the pressure[1]
  3. Heating gives particles more kinetic energy so they move faster[1]
  4. collisions are harder and more frequent, so pressure rises[1]
  5. Halving the volume means the same particles are in a smaller space[1]
  6. so collisions with the walls are twice as frequent and the pressure doubles[1]

Pressure = wall collisions per area; heating raises it (faster, more frequent collisions); halving the volume doubles it (more frequent collisions).

Examiner tip. Relate every pressure change back to the frequency and force of particle–wall collisions.