Changes of State
Section: 1. States of Matter | Syllabus: Cambridge IGCSE Chemistry (0620)
Changes of State
A change of state is a physical change - no new substance is formed, and the change can be reversed.
- Melting (solid → liquid): particles gain enough energy to partly overcome the forces holding them in fixed positions
- Freezing (liquid → solid): particles lose energy and forces of attraction lock them into fixed positions
- Boiling (liquid → gas): particles throughout the liquid gain enough energy to completely overcome the forces of attraction between them; this happens at a fixed temperature (the boiling point) and bubbles form throughout the liquid
- Evaporation (liquid → gas): particles at the surface of a liquid escape into the gas state; this can happen at any temperature below the boiling point
- Condensation (gas → liquid): particles lose energy and forces of attraction pull them close enough together to form a liquid
- Sublimation (solid → gas): particles gain enough energy to go directly from fixed positions to complete freedom, without forming a liquid in between (e.g. solid carbon dioxide, iodine)
The reverse of sublimation (gas → solid) is called deposition.
Six named changes of state connect the three states of matter, each involving energy absorbed or released
Worked Example: Naming Changes of State
- Question: oxygen can exist in different states of matter. Name the change of state for: (a) solid to liquid, (b) gas to liquid. [2]
- Answer: (a) melting [1]; (b) condensing / condensation [1]
Heating and Cooling Curves
When a pure substance is heated at a constant rate, a graph of its temperature against time has a characteristic shape:
- Sloping sections: the substance is entirely solid, liquid, or gas, and its temperature rises as the particles gain kinetic energy
- Flat sections (plateaus): a change of state is taking place - the temperature stays constant even though energy is still being supplied
During a plateau, the energy supplied is used to weaken or break the forces of attraction between particles, not to increase their kinetic energy - this is why the temperature does not rise until the change of state is complete.
Temperature stays constant on the flat sections because the energy supplied is overcoming forces between particles, not increasing their kinetic energy
Worked Example: Interpreting a Heating Curve
- Question: a pure solid is heated at a constant rate from below its melting point until it is entirely gas. Describe what is happening during the two flat sections of the temperature-time graph. [2]
- Answer: the first (lower-temperature) plateau is the melting point - solid and liquid are present together as the solid melts [1]; the second (higher-temperature) plateau is the boiling point - liquid and gas are present together as the liquid boils [1]
Evaporation vs Boiling
| Property | Evaporation | Boiling |
|---|---|---|
| Where it occurs | Only at the surface | Throughout the liquid |
| Temperature | Any temperature below the boiling point | Only at the boiling point |
| Bubbles | No bubbles form | Bubbles form throughout the liquid |
| Speed | Slow | Rapid |
Factors Affecting the Rate of Evaporation
- Temperature: higher temperature gives more particles enough energy to escape the surface
- Surface area: a larger surface area exposes more particles to the possibility of escaping
- Air movement: moving air carries evaporated particles away, stopping them from returning to the liquid
Why Does Evaporation Cause Cooling?
- The particles in a liquid have a range of kinetic energies
- The fastest-moving (highest energy) particles are the ones most likely to escape from the surface
- Once they leave, the average kinetic energy of the particles left behind is lower, so the temperature of the remaining liquid falls
Common Mistakes
- ❌ Thinking evaporation and boiling are the same process because both change a liquid into a gas
- ✓ boiling happens throughout the liquid at one fixed temperature; evaporation happens only at the surface and can occur at any temperature below the boiling point
Energy Changes During State Changes
| Change of state | Energy transfer |
|---|---|
| Melting, boiling, evaporating, subliming | Energy absorbed from the surroundings |
| Freezing, condensing, depositing | Energy released to the surroundings |
Practical Example: Steam Burns
Steam at 100°C causes worse burns than the same mass of water at 100°C, because as the steam condenses on the skin it releases a large amount of extra energy on top of the heat from the water it forms.
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