In this lesson
Drop a spoonful of sugar into a glass of warm water. Stir. The sugar seems to disappear, but it is still there. Taste the water and you can tell.
What you cannot see is the matter spreading through the water. The particle model of matter is the picture scientists use to explain what tiny pieces of matter are doing when they are too small to observe directly.
Particles are always moving
Matter is anything that has mass and takes up space. Air is matter. Water is matter. A desk, a spoon, and a grain of sugar are matter too.
The particle model says that matter is made of particles. These particles are always moving. In water, particles slide past each other. In air, particles fly through the room. Even in ice, particles vibrate in place.
Temperature is connected to that motion. The hotter a substance is, the faster its particles move on average. Cooling a substance slows the particles down.
↓ click anywhere to add a flame and warm the particles ↓
notice something?
The particles never fully stop moving — even when the chamber cools back down, they keep drifting at a baseline pace.
That's because particles always have some kinetic energy. When you add a flame, they get extra energy and zoom faster, but they soon share it with the rest of the chamber and settle back.
Attractive forces pull particles together
Particles do not only move. Nearby particles also pull on each other a little. Scientists call this an attractive force.
When particles move slowly, attractive forces can hold them close together. When particles move faster, their motion can spread them farther apart. Heating and cooling change the balance between motion and attraction.
Three states: solid, liquid, gas
phase change: no phase change yet — try the slider or a button below.
Heat control
Particle count never changes. The freeze and boil points are pinned to water (0 °C and 100 °C). Other substances melt and boil at different temperatures.
Model shows solid matter at -25 degrees Celsius. Particles vibrate in a tight pattern.
The state of matter depends on how the particles are arranged and how they move.
In a solid, particles are packed close together and vibrate in fixed positions. The attractive forces are strong enough to keep the shape steady.
In a liquid, particles are still close together, but they can slide past one another. A liquid keeps its volume but takes the shape of its container.
In a gas, particles are far apart and move freely through the available space. A gas spreads out to fill its container.
Worksheet
These exercises are not graded. Use them to check whether the particle model can explain temperature and states of matter.
Practice · Not graded
SC.6.MAT.1Practice the idea
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Which sample has the fastest-moving particles on average?
Choose which sample has the fastest-moving particles on average.Show common mistakes
Student says
“Particles in a solid do not move.”
What it reveals
Treats solid as completely still instead of fixed in shape.
Targeted response
Particles in a solid vibrate in place. They are not flowing past each other like a liquid, but they are still moving.
Student says
“When ice melts, some of the particles disappear.”
What it reveals
Confuses a change in arrangement with a loss of matter.
Targeted response
Melting changes the arrangement and motion of particles. In a closed sample, the same matter is still present, so the mass stays the same.
Student says
“Temperature is just how warm something feels to my hand.”
What it reveals
Uses a sensory idea where a measurement idea is needed.
Targeted response
Touch can be useful, but temperature is a measurement connected to particle motion. A thermometer gives a more precise reading than your hand.
Going further
The particle model is a model, not a photograph. It is useful because it explains patterns we can observe: sugar spreading through water, ice melting, water boiling, and thermometers changing as substances warm or cool. The next lessons use this same model to explain phase changes, thermometers, expansion, contraction, and why materials behave differently when temperature changes.