Lesson 13
Changes in Particles with States of Matter
Learners use the particulate nature of matter to explain why solids, liquids, and gases behave so differently. The key is not just particle arrangement, but the strength of attraction between particles and the role of temperature.

Key Ideas
  • The difference between solids, liquids, and gases comes down to how strongly particles attract each other. In a solid, attraction is strong enough that particles lock in place. In a liquid, attraction is weaker and particles can slip past each other. In a gas, there is no meaningful attraction and particles fly freely in all directions.
  • Particles are always moving. Even in a solid sitting perfectly still, the particles inside are vibrating constantly. They just cannot go anywhere because the attraction holds them in place.
  • Temperature tips the balance between attraction and jiggling. At low temperatures, jiggling is slight and attraction wins, giving a solid. As temperature rises, jiggling overcomes attraction and the substance becomes a liquid. At high enough temperatures, particles break free entirely and become a gas.
  • Weight does not change when a substance changes state. The particles rearrange, but none disappear. The same number of particles in ice becomes the same number of particles in water. Only the space they take up changes.
  • Water is the most familiar example of state change, but many substances do the same thing. Rock melts into lava and solidifies when cooled. Iron becomes molten liquid when heated and solidifies when cooled. Butter melts on a warm pan and hardens in the refrigerator.
  • Particle jiggling and mass movement are two different things. Particle jiggling is the constant internal vibration of particles. Mass movement is a whole object or substance moving from one place to another. When you feel wind on your face, that is mass movement, not particle jiggling.

Optional Home Activities


Vocabulary
  • Attraction: The pulling force between particles that holds them together. It is strongest in solids, weaker in liquids, and absent in gases.
  • Particle vibration: The constant internal movement of particles in all matter. It increases with temperature.
  • Diffusion: The spreading of particles from an area of higher concentration to an area of lower concentration.
  • State change: The shift of a substance from one state of matter to another, caused by a change in temperature.
  • Mass movement: The movement of a whole object or substance from one place to another. This is different from particle vibration.

Discussion Questions
  • If weight stays the same when state changes, why does a full glass of water overflow when you freeze it?
  • If you could somehow slow particles down to nearly zero movement, what state of matter would you expect every substance to be in?
  • A smell spreads faster in a warm room than a cold one. How does the particle model explain this?

Hands-On Activity: Diffusion Experiment

Supply List
  • Three identical clear glasses or jars
  • Hot water (teacher pours; not boiling)
  • Cold water (add ice first, then remove ice before adding food coloring)
  • Room temperature water
  • Food coloring (one color)
  • A dropper or spoon
  • Notebook and pencil

Instructions
  • Fill each glass with one type of water: one hot, one room temperature, one cold. Label them.
  • At the same time, add one drop of food coloring to each glass. Do not stir.
  • Observe all three glasses and record what you see every 30 seconds for about three minutes.
  • Discuss: in which glass did the color spread fastest? In which did it spread slowest? Why?
  • Connect to particles: in hot water, particles are moving faster and with more energy, so the food coloring particles spread and mix quickly. In cold water, slower-moving particles mean slower mixing.
  • Write your conclusion: what does the speed of diffusion tell us about how temperature affects particle movement?

Sources

L13: Changes in Particles with States of Matter by Selene

Phet State of Matter Basics Simulation

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