What Are Changes of State?
When matter is heated or cooled, it can change from one state to another. These changes are called changes of state (or changes of physical state). The substance itself does not change — water is always H₂O whether it is ice, liquid water, or steam — but the way its particles are arranged and how they move changes.
Changes of state are physical changes, not chemical changes. The original substance can always be recovered.
The Six Changes of State
| Change | From → To | Energy | Example |
|---|---|---|---|
| Melting | Solid → Liquid | Added | Ice melting to water |
| Freezing | Liquid → Solid | Removed | Water turning to ice |
| Evaporation/Boiling | Liquid → Gas | Added | Water turning to steam |
| Condensation | Gas → Liquid | Removed | Steam turning back to water |
| Sublimation | Solid → Gas | Added | Dry ice (solid CO₂) becoming gas |
| Deposition | Gas → Solid | Removed | Water vapour forming frost |
During a change of state, the temperature does not change even though heat energy is being added or removed. All the energy is used to change the arrangement of particles, not to make them move faster. For example, water stays at exactly 0 °C while it is melting.
Melting and Freezing
When a solid absorbs heat, its particles vibrate faster and faster. At the melting point, the energy is enough to break the bonds holding particles in fixed positions. The solid turns to a liquid.
The reverse process — freezing — happens when a liquid loses heat. The particles slow down until they lock into fixed positions. The freezing point of a substance is the same temperature as its melting point.
Evaporation and Boiling
Liquid particles at the surface that gain enough energy can escape into the gas phase — this is evaporation and happens at any temperature. Boiling is different: it occurs throughout the liquid at a specific temperature (the boiling point), and bubbles of gas form inside the liquid.
Condensation is the reverse: gas particles lose energy and slow down enough to be pulled together into a liquid. You see this when steam from a kettle hits a cold window and forms water droplets.
Leaving a wet towel to dry is evaporation. The water molecules with the most energy escape from the surface of the fabric into the air. This is why evaporation cools the surface — the highest-energy particles leave, lowering the average energy of what remains.
Sublimation
Some substances change directly from solid to gas without becoming a liquid first. This is called sublimation. Dry ice (solid carbon dioxide) sublimates at −78.5 °C at atmospheric pressure, which is why it produces a dramatic "fog" effect — the cold CO₂ gas condenses water vapour from the air.
Real world: Cooking — water changing state
When you boil a kettle, water (liquid) turns to steam (gas) at 100 °C — evaporation. When steam hits a cold window or mirror, it condenses back to liquid water droplets. When you put water in the freezer it freezes to solid ice at 0 °C. These everyday cooking and kitchen moments show all three main changes of state.
Real world: Weather — the water cycle
The water cycle is driven entirely by changes of state. Liquid water from oceans evaporates (liquid → gas), rises as water vapour, then condenses in cold air to form clouds (gas → liquid). Water falls as rain or snow. In winter, water vapour can deposit as frost (gas → solid) directly onto cold surfaces.
Real world: Dry ice in food delivery
Dry ice (solid CO₂) is used to keep food cold in deliveries. It sublimates at −78.5 °C — going directly from solid to gas without becoming liquid. This is practical because it leaves no wet mess and keeps food frozen longer than regular ice. The visible "fog" is actually water vapour from the air condensing.
Where do you see this in real life? Watch condensation form on a cold glass of water on a warm day. Notice how wet clothes dry in the sun (evaporation). These are changes of state happening all around you every day.
Practice Problems
Name the change of state that occurs when water vapour in the air lands on a cold mirror and forms water droplets.
Show answer
Why does the temperature of melting ice stay at 0 °C even when heat is being added continuously?