- Define and explain Changes of State in your own words
- Use key terms such as particle accurately
- Apply what you have learned to new examples and questions
- Avoid the common mistakes learners make with this topic
This lesson focuses on Changes of State: explain melting, boiling, condensing and freezing with particles.
Explain melting, boiling, condensing and freezing with particles.
Key ideas
Changes of state are about energy, not new particles
When ice melts, its particles gain energy and break free from their fixed positions — but they are still the same water particles. The temperature stays constant during melting and boiling because the energy goes into loosening the particles rather than making them hotter. That is why a thermometer in boiling water reads 100 °C no matter how fiercely it boils.
Particles are always moving
In the particle model, every substance is made of tiny particles with empty space between them, and the particles are in constant motion. The hotter the substance, the faster its particles move — heating does not make new particles, it makes the existing ones move more. This one idea explains expansion, diffusion and changes of state.
Key term — particle: An extremely small piece of matter; everything is made of particles that are constantly moving.
A thermometer in melting ice reads 0 °C the whole time. Explain why the temperature does not rise.
The energy goes into loosening the particles from their fixed positions rather than raising their speed, so the temperature stays constant until all the ice has melted.
Answer: The energy goes into loosening the particles from their fixed positions rather than raising their speed, so the temperature stays constant until all the ice has melted.
- Saying particles melt Particles do not melt, expand or shrink — the substance does. Heating makes particles move faster and spread out; the particles themselves stay the same size.
- Thinking particles are still in solids Even in a solid, particles vibrate about fixed positions. They only stop moving at absolute zero (−273 °C), a temperature nothing in everyday life reaches.
Practice
Gas particles are far apart and move rapidly in all directions, so they spread until they fill the whole container.
The fastest water particles escape from the surface into the air as a gas; this is evaporation.
Heating makes particles move faster, so they hit the container walls harder and more often, which increases the pressure.
Particles are packed tightly in fixed positions and vibrate; they cannot move around, which is why solids keep their shape.
Quick check
Which of these best defines "particle"?
Why does diffusion happen faster in hot water than in cold water?
- Changes of State: explain melting, boiling, condensing and freezing with particles.
- Changes of state are about energy, not new particles: When ice melts, its particles gain energy and break free from their fixed positions — but they are still the same water particles.
- diffusion: The spreading of particles from an area of high concentration to an area of low concentration.
- Watch out for: saying particles melt