Particle Model of Physical Change
In 1894, Scottish chemist William Ramsay cooled air to −196 °C and separated 78 litres of nitrogen gas — the same water particles inside are still H₂O whether frozen or boiling.
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● Know
- All matter is made of tiny particles — too small to see but always there.
- Solids, liquids and gases differ in how the particles are arranged, not in what they are.
- In a physical change, the particles rearrange but the particles themselves don't change.
● Understand
- Heating particles gives them more kinetic energy (movement); cooling takes it away.
- Melting, boiling and freezing all happen because of changes in particle motion.
- The particle model is a useful simplification — it explains a lot without being a perfect picture.
● Can do
- Draw or describe what particles do during a state change.
- Use the particle model to explain why physical changes are usually reversible.
- Spot common mistakes about how particles behave (size, identity, disappearance).
Place an ice cube on a warm plate and watch: in about 2 minutes the ice has disappeared and a puddle of clear liquid sits in its place. Look closer and nothing visible has changed about the substance — it is still transparent, still water, still tasteless — yet its shape has changed completely. That observation is the entry point to the particle model. The particle model explains that all matter is made of tiny particles that are constantly moving. When a substance changes state, the particles themselves stay exactly the same. Water is H₂O whether it is ice, liquid or steam — the only difference is how those H₂O particles are arranged and how much energy they have.
In a solid, particles vibrate in fixed positions. Add heat and they move enough to slide past each other — that is a liquid. Add more heat and they break free entirely — that is a gas. Because no bonds between atoms are broken, state changes are always physical. The substance is still the same; only the packaging has changed.
When chocolate melts in your mouth, the cocoa particles gain energy and slide past each other, but each particle is still the same cocoa molecule. When the chocolate cools, the particles slow down and lock back into solid positions. The chocolate can be melted and solidified repeatedly because melting is a physical change.
The Bureau of Meteorology uses the particle model to explain why water evaporates from the oceans and condenses into clouds. Understanding that these are physical changes — the H₂O particles simply rearranging — helps meteorologists predict rainfall and manage water resources across drought-prone Australia.
A very common misconception is that particles themselves get bigger when heated. They do not. The particles stay exactly the same size. What changes is the space between them and how fast they move. Heating adds kinetic energy, which makes particles move more vigorously and take up more room.
To melt or boil a substance, you must add energy — usually heat. That energy becomes kinetic energy, making particles move faster until they can escape their neighbours. To freeze or condense a substance, you must remove energy. Slower particles settle into more ordered arrangements.
Temperature is simply a measure of the average kinetic energy of the particles. Hotter means faster particles; colder means slower particles. Water boils at 100°C under normal atmospheric pressure because that is the temperature at which the particles have enough energy to escape the liquid throughout its volume, not just at the surface.
When steam hits a cold bathroom mirror, it loses energy to the cold glass. The water particles slow down and clump together into liquid droplets — condensation. When you heat water in a kettle, the particles gain energy until they can break free as steam — evaporation or boiling.
CSIRO materials scientists study how particle behaviour changes under extreme temperatures to develop better alloys for Australian industry. By understanding exactly how much energy is needed to rearrange metal particles, they help manufacturers produce stronger, lighter materials for buildings, vehicles and renewable energy systems.
Many students confuse evaporation with boiling. Evaporation happens only at the surface, at any temperature, when the fastest particles escape. Boiling happens throughout the entire liquid at a fixed temperature (the boiling point) when bubbles of gas form inside the liquid. Both are physical changes, but they are different processes.
The three states of matter have distinctly different particle arrangements. In a solid, particles are packed tightly in regular patterns and vibrate in fixed positions. This is why solids keep their shape and volume. In a liquid, particles are still close together but can slide past one another, which lets liquids flow and take the shape of their container.
In a gas, particles are far apart and move rapidly in all directions. They fill whatever space is available and can be compressed. These differences in arrangement and movement explain every observable property of matter, from why ice floats to why gases exert pressure on container walls.
Ice keeps its shape because its water particles are locked in a fixed lattice. When it melts, the particles break free from their positions and slide around, so the water flows to match the shape of its glass. Heat it further and the particles fly apart as steam, expanding to fill the entire room.
Scientists at the Australian Synchrotron use powerful X-ray beams to study how particles arrange themselves in new materials. By understanding particle packing at the atomic level, they design better batteries, solar cells and medical devices that benefit Australian industries and consumers.
Students sometimes believe that particles stop moving when a substance freezes. This is false. Even in solid ice at –20°C, water particles are still vibrating in their fixed positions. Absolute zero (–273°C) is the theoretical temperature where particles would stop moving entirely, but that temperature has never been reached in everyday conditions.
When a solid is heated, its particles gain {1} and begin to {2} more vigorously. At the melting point, the particles break free from their {3} positions and the solid becomes a {4}.
The most persistent mistake is claiming that particles get bigger when heated. They do not. Each individual particle stays the same size no matter what temperature you reach. What increases is the space between particles and the speed at which they move. Thermal expansion is about spacing, not particle growth.
Another common error is confusing boiling with electrolysis. When water boils, H₂O molecules gain energy and escape as steam — they are still H₂O. When water is electrolysed, electricity breaks the chemical bonds inside H₂O molecules to produce hydrogen gas and oxygen gas — brand-new substances. Boiling is physical; electrolysis is chemical.
A balloon left in the sun expands because the air particles inside move faster and push harder against the rubber. The air particles themselves are exactly the same size — there is just more empty space between them. If the particles had actually grown, the balloon would contain the same amount of matter but each particle would be larger, which does not happen.
ANSTO uses sophisticated cooling systems in its nuclear reactors. Engineers must understand that cooling water does not shrink the water particles — it simply reduces the space between them as they slow down. This particle-level understanding is essential for designing safe cooling systems that prevent overheating.
A student claims that water boiling into steam is a chemical change because the steam "looks like a different substance." This is wrong. Steam is still H₂O — the same particles, just with more energy and more space between them. A chemical change would require breaking the bonds inside the H₂O molecule to create new substances such as hydrogen and oxygen.
Here's a student's working. One line has an error — click it.
- A student claims: 'When water boils, the H₂O molecules split into hydrogen and oxygen gas, so boiling is a chemical change.'
The particle model is one of the most powerful ideas in science. It states that all matter is made of tiny particles that are constantly in motion. The arrangement and energy of these particles determine whether a substance is solid, liquid or gas. When energy is added or removed, particles rearrange — but they do not change identity.
State changes are always physical because the same particles are simply re-packaged. Melting, freezing, evaporating, condensing and sublimating all involve the same substance in different arrangements. Understanding this model lets you explain why materials behave the way they do and predict what will happen when you heat or cool them.
Dry ice (solid carbon dioxide) sublimates at room temperature — it changes directly from solid to gas without becoming liquid. The CO₂ particles gain enough energy to break free from their fixed positions and fly apart as gas. Each particle is still CO₂; only the arrangement has changed.
Researchers studying the Great Barrier Reef monitor how rising ocean temperatures affect coral at the particle level. Warmer water means faster-moving particles, which changes how calcium carbonate crystals form in coral skeletons. Understanding these physical changes helps scientists predict how reefs will respond to climate change.
Some students think that state changes are chemical because the substance "becomes something new." Ice, water and steam certainly look and behave differently, but they are all H₂O. The particles themselves are unchanged. A chemical change would require the H₂O molecules to break apart and form different molecules entirely.
At the start of this lesson, you explored how water molecules stay the same when ice melts — they just move further apart as they gain energy.
Now that you've worked through the particle model, think about whether your initial picture of what's happening inside a melting solid was accurate. Did you imagine the particles moving, or did you picture something different happening to them?
1. According to the particle model, what happens to particles when a solid melts?
2. Which state of matter has particles that are farthest apart?
3. What is the correct term for a gas turning into a liquid?
4. Why does a puddle disappear on a hot day?
5. Which statement about particles in a solid is true?