Matter changes state, melting, freezing, boiling, condensation and sublimation, when heat is absorbed or released. During a change of state the temperature stays constant because the heat energy is used to overcome or form the forces of attraction between particles, not to change their kinetic energy.
This page covers a single Form 4 content standard: changes in the state of matter. It builds directly on the kinetic theory and is examined every year through heating and cooling curves, which reward you for explaining why the temperature behaves as it does. Learn the reasoning here and these questions become some of the most reliable marks in SPM Chemistry.
The interconversion of states
Matter can be changed from one state to another by heating or cooling. Each change has a name you must use correctly:
- Melting, solid to liquid (heat absorbed).
- Freezing (solidifying), liquid to solid (heat released).
- Boiling / evaporation, liquid to gas (heat absorbed).
- Condensation, gas to liquid (heat released).
- Sublimation, solid straight to gas without passing through the liquid state (for example, iodine and dry ice); deposition is the reverse, gas straight to solid.
Heating supplies energy that lets particles move more and break away from one another; cooling removes energy so that forces of attraction pull the particles back into a more ordered state.
Why temperature stays constant during a change of state
This is the key idea the exam tests. When you heat a pure solid, its temperature rises until it reaches the melting point. While the solid is melting, the temperature stays constant even though heat is still being supplied. The reason: the absorbed heat energy is used to overcome the forces of attraction between the particles (increasing their potential energy), not to increase their kinetic energy. Because kinetic energy does not change, the temperature, which depends on average kinetic energy, stays the same. Once all the solid has melted, the temperature rises again until the boiling point, where it stays constant a second time while the liquid boils.
On cooling, the mirror image happens: at the freezing point the temperature stays constant while the liquid freezes, because heat is released as the forces of attraction re-form. This released energy keeps the temperature steady until the change of state is complete.
Definitions. The melting point is the constant temperature at which a solid turns into a liquid; the boiling point is the constant temperature at which a liquid turns into a gas; the freezing point equals the melting point for a pure substance. A flat region (plateau) on a heating or cooling curve marks a change of state, where solid and liquid (or liquid and gas) exist together.
Worked example
Question. Solid naphthalene is heated until it melts and then allowed to cool. On the cooling curve, the temperature falls, then stays constant at 80 °C for a few minutes, then falls again. Explain, in terms of particles, what is happening in the flat region at 80 °C.
Step 1, Identify the change of state. The substance is cooling, and the flat region is at the freezing point, so at 80 °C the liquid naphthalene is freezing (solidifying) into solid naphthalene.
Step 2, Explain why the temperature is constant. During freezing, the particles are getting closer and forming an orderly arrangement, so forces of attraction are being formed and heat energy is released to the surroundings.
Step 3, Link energy to temperature. The heat released exactly balances the heat lost to the surroundings, so the average kinetic energy of the particles does not change and the temperature stays constant at 80 °C until all the liquid has become solid.
Answer. In the flat region the liquid is freezing; the temperature stays constant at 80 °C (the freezing point) because heat energy released as the forces of attraction form keeps the temperature steady while the change of state is completed.
Practice question
Question. A pure solid is heated steadily. Its temperature rises to 53 °C and stays there for four minutes before rising again. State the name of the process taking place during the four minutes, name the temperature 53 °C, and explain why the temperature does not rise even though heating continues.
Answer. The process is melting, and 53 °C is the melting point of the solid. The temperature does not rise because the heat energy supplied is used to overcome the forces of attraction between the particles so that they can move apart (increasing their potential energy), not to increase their kinetic energy; since the average kinetic energy is unchanged, the temperature stays constant until all the solid has melted.
Exam tip
For heating- and cooling-curve questions, first read off whether the substance is being heated (temperature generally rising, energy absorbed) or cooled (temperature generally falling, energy released), this tells you whether a plateau is melting/boiling or freezing/condensing. Then always give the reason for a constant temperature in the same words: the heat energy is used to overcome (or is released as) the forces of attraction between particles, so the kinetic energy, and therefore the temperature, does not change. A frequent lost mark is writing that the substance “stops absorbing heat”; it does not, the energy is still flowing, it is simply changing potential energy, not kinetic energy.
Evaporation versus boiling
Questions often test the difference between evaporation and boiling, so know both clearly. Evaporation happens only at the surface of a liquid, can occur at any temperature below the boiling point, and is slow; only the surface particles with enough kinetic energy escape as vapour. Boiling happens at a fixed boiling point, throughout the whole liquid (bubbles of vapour form inside the liquid, not just at the surface), and is rapid. Both absorb heat, but only boiling occurs at one particular constant temperature. Understanding this lets you answer questions about clothes drying (evaporation) versus water bubbling in a pot (boiling) with the correct terms.
For sublimation, remember the commonly examined examples: iodine and dry ice (solid carbon dioxide) change straight from solid to gas on heating, without passing through the liquid state, useful in purification techniques and often worth an easy mark when you name a correct example.
Where this fits
This standard follows on from matter and the kinetic theory and sits within the Matter and the Atomic Structure chapter. Revise it alongside the rest of the chapter with the revision notes, and practise reading graphs with the worked examples. In our online 1-to-1 SPM Chemistry lessons, taught in English, from RM50/hr, our teachers make sure your curve explanations always link the constant temperature to the forces of attraction, because that single sentence is what separates a partial answer from a full one.
Quick recap
- Melting, freezing, boiling, condensation and sublimation are the changes of state; know which absorb and which release heat.
- Temperature stays constant during a change of state because energy overcomes or forms the forces of attraction, not the kinetic energy.
- A plateau on a heating/cooling curve marks a change of state where two states coexist.
- Always explain constant temperature in terms of forces of attraction and unchanged kinetic energy.
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