Heating, Cooling and Change of State
In 1898, James Dewar liquefied hydrogen at −253 °C, showing that adding heat energy can hold a temperature flat for minutes while particles rearrange — not just make things hotter.
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● Know
- Matter exists in three common states: solid, liquid, gas.
- Heating or cooling a substance can change its state — these are all physical changes.
- Each state change has a specific name and direction.
● Understand
- Energy is the driver — adding it pushes particles apart, removing it lets them settle.
- Pure substances melt and boil at fixed temperatures (e.g. ice at 0 °C, water at 100 °C).
- Sublimation skips the liquid state — solid goes straight to gas (e.g. dry ice).
● Can do
- Name the six common state changes (melting, freezing, boiling, condensing, evaporating, sublimating).
- Explain a state change in terms of particle behaviour and energy.
- Predict what state a substance is in at a given temperature.
Put a glass of water in the freezer overnight and in the morning you find solid ice — a completely different texture from clear liquid. Leave that same ice on the bench and within 20 minutes it is liquid again. That same water, same substance, shifting between visible states — all driven by energy. There are six common changes of state that every student of science must know. Melting turns a solid into a liquid, while freezing does the reverse. Boiling and evaporating both turn liquid into gas, but boiling happens throughout the liquid at a fixed temperature, whereas evaporation only occurs at the surface and can happen at any temperature. Condensation is the reverse of evaporation, turning gas back into liquid. Finally, sublimation lets a solid skip the liquid stage entirely and become a gas directly.
At the particle level, adding thermal energy makes particles move faster and spread further apart. Removing energy slows them down and pulls them closer together. The state of a substance depends on the balance between the kinetic energy of its particles and the interparticle forces holding them together.
Place an ice cube on a bench on a warm day. It absorbs heat from the surroundings, the particles vibrate faster, and the rigid lattice breaks down into flowing water. If you heat that water in a kettle, bubbles of steam form throughout the liquid at 100 °C — that is boiling.
The Bureau of Meteorology monitors phase changes across Australia every day. When forecasters predict frost formation or dew on your morning windscreen, they are applying the same physics of condensation and freezing that you learn in the classroom.
Many students think that boiling simply means "very hot." That is not true. Liquid nitrogen boils at −196 °C, which is far colder than any Australian freezer. Boiling is defined by reaching a specific temperature where the vapour pressure equals atmospheric pressure, not by how warm it feels.
When you heat a solid, its temperature rises until it reaches the melting point. At this temperature, something surprising happens: the temperature stops rising even though you keep adding energy. The incoming thermal energy is used to break the bonds between particles in the solid lattice, not to make them move faster. Once all the solid has melted, the temperature rises again until the boiling point is reached, where another flat section appears.
These flat sections on a heating curve are the signature of a change of state. Scientists call the energy involved latent heat — hidden heat that changes the arrangement of particles without changing their temperature. The same flat sections appear in reverse when a substance cools down.
Water boils at 100 °C at sea level, but on the summit of Mount Kosciuszko the air pressure is lower, so water boils closer to 93 °C. A cup of tea made at the top of Australia's highest mountain takes longer to brew because the boiling water is cooler.
The Australian Antarctic Division studies ice melting under extreme conditions. In some regions, ice melts at temperatures below 0 °C because the immense pressure of glaciers lowers the melting point — a phenomenon that helps scientists understand how ice sheets move and change.
A common belief is that solids are always denser than liquids. Water is the famous exception: ice floats because it is less dense than liquid water. This unusual property is vital for aquatic life in Australian rivers and lakes, because ice forms on the surface and insulates the water below.
Understanding the difference between evaporation and boiling is essential. Evaporation is a surface phenomenon that happens at any temperature — puddles dry up on a cool morning because the fastest-moving water particles at the surface escape into the air. Boiling, by contrast, happens throughout the entire liquid at a specific temperature called the boiling point.
Sublimation is less common in daily life but just as important. Solid carbon dioxide (dry ice) sublimes at room temperature, producing thick white fog used in stage effects. Condensation is the reverse of evaporation: when water vapour in the air contacts a cold surface, it loses energy and turns back into liquid droplets.
On a chilly Canberra morning, dew forms on grass blades as water vapour in the air condenses on the cold surfaces. Inside a freezer, frost builds up through deposition — water vapour turning straight to ice crystals, the reverse of sublimation.
CSIRO agricultural scientists study evaporation rates across Australian farming regions. By understanding how temperature, humidity, and wind affect evaporation, they help farmers optimise irrigation and predict crop water needs in an increasingly variable climate.
Some students think the temperature keeps rising while ice melts. It does not. During any change of state, the temperature stays constant because the energy is working to separate particles, not to speed them up. Only after the change is complete does the temperature rise again.
Drawing and interpreting heating curves is a core skill, but students often make the same errors. The most common mistake is drawing a straight diagonal line straight through the melting or boiling point. A correct heating curve must show a flat plateau at each change of state because the temperature stays constant while latent heat is absorbed or released.
Another frequent error is forgetting that pressure affects boiling point. At higher altitudes where atmospheric pressure is lower, water boils at a lower temperature. This means cooking times increase in the mountains, even though the water is bubbling vigorously.
A student plots a heating curve for ice and draws a single straight line from −10 °C to 20 °C. They have missed the flat section at 0 °C where melting occurs. The correct graph rises to 0 °C, stays flat during melting, then rises again as liquid water warms up.
Marine scientists at the Great Barrier Reef Marine Park Authority monitor sea surface temperatures closely. Understanding when and how water changes state helps them model heat transfer in reef ecosystems and predict events such as coral bleaching.
Many students believe melting and freezing happen at different temperatures for a pure substance. They do not. Pure water melts and freezes at exactly 0 °C. The direction of change depends on whether energy is entering or leaving the system, not on the temperature itself.
Here's a student's working. One line has an error — click it.
- Draw a straight diagonal line from -10°C through to 20°C.
- Pass through 0°C without a flat section.
- Claim the temperature rises steadily the whole time.
This card brings together everything you have learned about heating, cooling, and changes of state. Remember that changes of state are physical changes — no new substance forms, only the arrangement and spacing of particles change. Energy drives every transition: add energy to spread particles apart, remove energy to pull them together.
The unusual behaviour of water — expanding when it freezes — is one of the most important facts in Earth science. If ice were denser than liquid water, lakes and oceans would freeze from the bottom up, making aquatic life impossible in cold climates. Australia's native fish and aquatic ecosystems depend on this quirk of chemistry.
Imagine heating a beaker of ice from −10 °C to 120 °C. The temperature rises to 0 °C, stays flat during melting, rises to 100 °C, stays flat during boiling, then rises again as steam heats up. Each flat section represents latent heat breaking or forming interparticle bonds.
Indigenous burning practices across Australia reflect a deep understanding of how heat changes materials. Controlled cool burns manage fuel loads by carefully managing the temperature and rate of combustion, protecting country while respecting the physics of phase change and energy transfer.
Some students think energy stops flowing during a change of state. It does not. Energy continues to enter or leave the system; it simply goes into changing particle spacing rather than temperature. A thermometer held in melting ice will read 0 °C for minutes while energy pours in from the surroundings.
At the start of this lesson, you read about dry ice skipping straight from solid to gas at −89°C without ever becoming a liquid puddle.
Now that you understand sublimation and changes of state using the particle model, how would you explain the dry ice trick to a friend? Compare your explanation now with your first instinct at the start.
1. What happens to the temperature of pure ice while it is melting?
2. Which process is the reverse of condensation?
3. At what temperature does pure water boil at sea level?
4. Which change of state skips the liquid stage?
5. Why does a heating curve have a flat section during melting?