- Define and explain Spreading by Diffusion in your own words
- Use key terms such as diffusion accurately
- Apply what you have learned to new examples and questions
- Avoid the common mistakes learners make with this topic
This lesson focuses on Spreading by Diffusion: see how particles spread from crowded places to empty ones.
See how particles spread from crowded places to empty ones.
Key ideas
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.
Arrangement decides the state
In a solid, particles are packed tightly in fixed positions and only vibrate, which is why solids keep their shape. In a liquid, particles are still close but can slide past each other, so liquids flow. In a gas, particles are far apart and move quickly in all directions, so gases spread out to fill any container.
Key term — diffusion: The spreading of particles from an area of high concentration to an area of low concentration.
Why does diffusion happen faster in hot water than in cold water?
Particles move faster when hot, so they spread and mix more quickly.
Answer: Particles move faster when hot, so they spread and mix more quickly.
- 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.
- Confusing melting with dissolving Melting is a solid becoming a liquid when heated, like ice to water. Dissolving is a solid spreading through a liquid, like salt in water — the solid is still there, just invisible.
Practice
Gas particles are far apart and move rapidly in all directions, so they spread until they fill the whole container.
Particles are packed tightly in fixed positions and vibrate; they cannot move around, which is why solids keep their shape.
The fastest water particles escape from the surface into the air as a gas; this is evaporation.
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.
Quick check
Which of these best defines "diffusion"?
A liquid is heated in a sealed, rigid container and the pressure rises. Explain using particles.
- Spreading by Diffusion: see how particles spread from crowded places to empty ones.
- 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.
- particle: An extremely small piece of matter; everything is made of particles that are constantly moving.
- Watch out for: saying particles melt