States of Matter: Solids, Liquids and Gases in the Particle Model

All matter, anything that has mass and takes up space, is made of tiny particles: atoms or molecules. The particles are always moving, and attractions between them pull them together. Whether a substance is a solid, a liquid or a gas depends on the balance between those attractions and how much movement energy the particles have.

The particles themselves stay the same in every state. Ice, liquid water and water vapour are all made of the same water molecules; only their spacing, arrangement and movement change.

	Solid	Liquid	Gas
Shape	Fixed	Takes the container's shape	Fills the container
Volume	Fixed	Fixed	Fills the container
Arrangement	Tightly packed, often regular	Close, no pattern	Far apart, no pattern
Movement	Vibrate in place	Slide past each other	Fast and random
Flow	No	Yes	Yes
Squash	Very hard	Very hard	Easy

Every property in the table follows from the particle picture. Gases are easy to squash because there is empty space between the particles. Liquids flow because their particles can slide past one another, but they are almost as hard to squash as solids because the particles are still touching.

Heating gives particles more movement energy. A solid melts, then the liquid evaporates or boils. Cooling reverses the process: a gas condenses and a liquid freezes. A few substances skip the liquid stage: solid carbon dioxide (dry ice) sublimes straight into a gas, and the reverse change is called deposition.

Melting, boiling and sublimation take in energy. Freezing, condensing and deposition give it out. During any change of state the temperature stays constant until the change is complete, which is why a heating curve has flat sections. Changes of state are physical changes, so the total mass stays the same.

Pure water freezes at 0 °C (32 °F) and boils at 100 °C (212 °F) when the air pressure is normal sea-level pressure. A liquid boils when the pressure of its vapour matches the pressure of the air above it, so the boiling point depends on air pressure.

In the mountain town of Leadville, Colorado, in the US, the air pressure is about 68 kPa, roughly two-thirds of the 101.3 kPa at sea level. There, water boils at about 90 °C (194 °F). The water is boiling, but it is not as hot as boiling water at the coast.

Evaporation is different from boiling. It happens at the surface of a liquid at any temperature, as the fastest particles escape, which is why a puddle dries on a sunny day without ever reaching 100 °C.

Print it on A4 or US Letter as a handout, or on A3 or Tabloid for the classroom wall. Ask students to draw a particle diagram for each state beside the poster, then label every arrow between the states with its name and whether energy goes in or out. It pairs with the free LearnBySlides lesson on states of matter, which has speaker notes, a quiz and a worksheet.