Ice, water and steam are made of exactly the same particles. What changes is how close together they are and how freely they move. One simple model, the kinetic particle theory, explains every property of the three states and every change between them.
In a solid the particles are close together in a regular arrangement and vibrate about fixed positions, so a solid has a fixed shape and volume. In a liquid they are close together but irregular and move around each other, so a liquid has a fixed volume but takes the shape of its container. In a gas they are far apart, randomly arranged and move quickly in all directions, so a gas has no fixed shape or volume and is easily compressed. Heating gives particles more kinetic energy; changes of state happen when the forces between particles are overcome or formed.
Fig. 1: heating curve for water, from ice to steam
Section 01
Three states, one model.
Switch the simulator between solid, liquid and gas and watch how the particles are arranged and how they move. Then heat each state and see the particles speed up.
State
Arrangement
Motion
Properties
solid
close together, regular pattern
vibrate about fixed positions
fixed shape, fixed volume, cannot be compressed
liquid
close together, irregular
move around each other
fixed volume, shape of container, hardly compressed
gas
far apart, random
move quickly in all directions
no fixed shape or volume, easily compressed
Section 02
Changes of state.
Each change of state has its own name. Learn them as pairs: every change can be reversed by heating or cooling.
Melting (solid to liquid) and freezing (liquid to solid) happen at the same temperature, the melting point.
Boiling (liquid to gas) happens throughout the liquid at the boiling point, with bubbles of gas forming inside it.
Evaporation (liquid to gas) happens only at the surface and at any temperature below the boiling point; the fastest-moving particles escape.
Condensation (gas to liquid) happens when a gas is cooled, for example steam on a cold window.
Extended. On a heating curve the temperature stays constant while a substance melts or boils. The energy supplied is used to overcome the forces of attraction between the particles, not to make them move faster. On a cooling curve the same flat sections appear because energy is released as those forces form again.
Examiner trap
A flat section on a heating curve does not mean the heating has stopped. Energy is still going in, but it is being used to overcome the forces between particles, so the kinetic energy and the temperature stay the same until the change of state is complete.
Stage 1 · Learn
Check what the sim just showed you
Four quick checks. Each correct answer earns XP and records this skill on your profile.
Quick check+10 XP
How are the particles arranged and how do they move in a solid?
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Which state has a fixed volume but takes the shape of its container?
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A liquid changes into a gas at its surface at a temperature below its boiling point. This is called:
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Steam touches a cold window and forms droplets of water. This change is called:
Section 03
Gases, temperature and pressure.
Extended. Heat the gas in the simulator and watch the particles hit the walls faster and more often. Then change the pressure and watch the volume respond.
Higher temperature, constant pressure: the particles gain kinetic energy and move faster, so they hit the container walls harder and more often. If the container can expand, such as a gas syringe, the volume increases.
Higher pressure, constant temperature: the particles are pushed closer together, so the volume of the gas decreases. Gases can be compressed because there is so much space between their particles.
Stage 2 · Exam
Exam-style questions
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Why can a gas be compressed much more easily than a liquid?
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(Extended) Solid ice is heated steadily. Why does the temperature stay at 0 °C until all the ice has melted?
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Mercury melts at −39 °C and boils at 357 °C. What is its state at 25 °C?
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(Extended) A gas in a syringe is heated at constant pressure and the plunger moves out. Which explanation is correct?
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Solids, liquids and gases, mastered.
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