States of Matter & Changes
Solids, liquids and gases, and what the particles are doing in each.
Everything is made of particles
All matter is made of tiny particles, far too small to see. What state something is in depends entirely on how those particles are arranged and how fast they move.
That single idea explains every property in this lesson. Why a solid holds its shape, why a gas fills a room, why heating changes things.
The three states
Solid: particles packed tightly in a fixed pattern, vibrating in place. Fixed shape, fixed volume.
Liquid: particles still touching but able to slide past each other. Fixed volume, but takes the shape of its container.
Gas: particles far apart, moving quickly in all directions. No fixed shape or volume; it spreads to fill whatever it is in.
Notice the pattern: as you go solid → liquid → gas, particles gain energy, move faster and spread further apart.
The changes of state
Heating gives particles energy; cooling takes it away.
solid --melting--> liquid --boiling--> gasgas --condensing--> liquid --freezing--> solid
Evaporation is different from boiling: it happens at the surface, at any temperature, which is why a puddle dries up without ever boiling.
These are all physical changes, reversible, and no new substance is made. Melted ice is still water.
The plateau while ice melts
Experiment. Heat ice steadily and record the temperature every minute.
What you expect: a steady climb.
What actually happens: the temperature rises to 0°C, then stops and stays there while the ice melts, even though you are still heating it. Only once it has all melted does it start rising again.
Why? The energy is not raising the temperature. It is being used to break the particles out of their fixed positions.
The same plateau appears at 100°C while water boils. This is one of the clearest pieces of evidence that the particle model is right.
Why washing dries faster in some conditions
Evaporation speeds up when particles at the surface can escape more easily. Four things help:
- Higher temperature, particles move faster.
- More surface area: spread the clothes out rather than bunching them.
- Wind: carries escaped particles away so they cannot return.
- Lower humidity: dry air has room to accept more vapour.
This is why washing dries in an hour on a hot windy day and takes all day during humid monsoon weather, even when it is just as warm. The temperature is not the only factor, the air is already full.
The particle model at work
Cooking. Water boils at 100°C and will not get hotter no matter how high the flame, which is why boiling takes a predictable time.
Refrigeration works by making a liquid evaporate, which draws heat away, the same reason sweating cools you.
Earthen water pots keep water cool by letting a little seep through and evaporate from the outside, taking heat with it. A very old application of exactly this science.
Weather. Evaporation and condensation are the whole engine of the water cycle.
Where it usually goes wrong
- "Particles expand when heated." The particles stay the same size. They move further apart.
- "Evaporation and boiling are the same." Boiling happens throughout, at a fixed temperature; evaporation happens at the surface, at any temperature.
- "Melting and dissolving are the same." Sugar in tea dissolves; ice melts.
- "Steam is what you see." Real steam is invisible; the white cloud is condensed droplets.
What to remember
- State depends on particle arrangement and movement.
- Solid fixed · liquid flows · gas fills.
- Melting, boiling, condensing, freezing, all reversible.
- Temperature pauses during a change of state.
Watch one worked through
Here is one done slowly, with the reason for every move. Then one for you.
A glass of cold water is left on a table on a hot day. Drops of water appear on the outside of the glass. Explain where they came from.
- leaked through glass? ✗Rule out the obvious wrong answer first: the water has not leaked through the glass.Glass has no holes, and the drops are on the outside. Saying what is not happening is part of a good explanation, because it is the answer most people give.
- air = gas + invisible water vapourIdentify what is touching the outside of the glass: air, which contains invisible water vapour.Air always holds some water as a gas. You cannot see it, and anything you cannot see is easy to leave out of an explanation.
- cold glass → cools the air touching itThe glass is cold, so it cools the air right next to it.Energy moves from warmer to cooler. The glass does not make cold, it takes heat away from the air touching it.
- vapour (gas) − heat → water (liquid)Cooled water vapour turns back into liquid. This is condensation.A gas becomes a liquid when it loses enough energy. The same change makes mist on a window and dew on grass in the morning.
Answer: Water vapour in the air touched the cold glass, lost energy and condensed into liquid drops.
Wet clothes on a line dry in the sun. What is this change of state called? One word.
Show the answer
evaporation
Practise States of Matter & Changes
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