States of Matter and Particle Theory

Matter exists as a solid, liquid, or gas. The particle model explains the properties of each state. Solids have fixed shape and volume; liquids flow but have fixed volume; gases fill their container. Changes between states involve energy being transferred.

The Three States of Matter

State	Shape	Volume	Can it flow?	Particle arrangement
Solid	Fixed	Fixed	No	Close together, regular pattern, vibrate in place
Liquid	No fixed shape (takes container shape)	Fixed	Yes	Close together, random, move around each other
Gas	No fixed shape	No fixed volume (fills container)	Yes	Far apart, move fast and randomly

Particle Theory

All matter is made of tiny particles (atoms or molecules). The way these particles behave explains the properties of each state:

In solids: particles are held together by strong forces. They can only vibrate — they cannot move past each other. This is why solids are rigid and keep their shape.
In liquids: particles are close but can slide past each other. This gives liquids their ability to flow and take the shape of a container.
In gases: particles are far apart with little attraction between them. They move rapidly in all directions, filling any space available.

Changing State

When energy is added to matter, particles move faster:

$\text{Solid} \xrightarrow{\text{melting}} \text{Liquid} \xrightarrow{\text{boiling/evaporating}} \text{Gas}$

When energy is removed:

$\text{Gas} \xrightarrow{\text{condensing}} \text{Liquid} \xrightarrow{\text{freezing}} \text{Solid}$

Note

During a change of state (e.g. melting), temperature stays constant even though energy is being added. The energy is used to break the bonds between particles, not to raise the temperature.

Key Temperatures (Water)

Change	Process	Temperature
Solid → Liquid	Melting	0 °C
Liquid → Gas	Boiling	100 °C
Liquid → Solid	Freezing	0 °C
Gas → Liquid	Condensing	100 °C

Real world: Ice cubes in a drink

Drop ice cubes (solid water) into a warm drink. The ice slowly melts — but notice the drink stays close to 0 °C while ice remains in the glass. Energy from the drink is absorbed breaking the bonds between water molecules (changing state), not raising the temperature. Only when all ice has melted does the temperature start to rise.

Real world: Sweating keeps you cool

When you exercise, your body temperature rises. Sweat glands release liquid water onto your skin. As this liquid evaporates (liquid → gas), it absorbs a large amount of energy from your skin — cooling you down. The same principle is used in refrigerators and air conditioners, where a liquid refrigerant evaporates inside the cooling coil.

Real world: Steam burns vs. boiling water burns

Steam at 100 °C causes more severe burns than boiling water at 100 °C, even though they are at the same temperature. When steam condenses on skin (gas → liquid), it releases extra energy (latent heat) — the same energy that was stored when the water boiled. This makes steam burns significantly more dangerous.

Tip Where do you see this in real life? Ice in drinks, sweating, steam irons, refrigerators, and even dew on grass in the morning are all examples of changes of state happening around you every day!

Practice Problems

Why can a liquid flow but a solid cannot?

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In a liquid, particles can slide past each other. In a solid, particles are held in fixed positions and can only vibrate — they cannot move past each other.

A substance is heated and its temperature does not rise for a period of time. What is happening?

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The substance is changing state (e.g. melting or boiling). The energy being added is breaking the bonds between particles, not increasing the temperature.

Why do gases fill their container but liquids do not?

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Gas particles move rapidly in all directions with little attraction between them, so they spread out to fill any space. Liquid particles are attracted to each other and stay together, taking the shape of the container but not expanding to fill it.