The particle model is just five short rules. Learn these and you can explain melting, boiling, why a gas fills a room, why you can squash a gas but not a liquid — the lot.
The Five Rules
One — all matter is made of tiny particles. Everything: the desk, the air, you. Two — the particles are always moving; even a “still” solid vibrates on the spot. Three — heat is movement: hotter means more energy and more movement. Four — forces hold particles together; adding energy lets them break free, removing it lets the pulls win. Five — the state depends on the balance: lots of pull and little movement → solid; more movement → liquid; enough to break free → gas.
The Three-step Method for Any Question
One — name the state. Read it from the arrangement: fixed pattern, vibrating only = solid; touching but jumbled and sliding = liquid; far apart and flying = gas.
Two — describe the particles. Say three things: their arrangement (regular / jumbled but touching / spread right out), their spacing (close / close / far apart) and their movement (vibrate in place / slide past each other / zoom freely).
Three — explain the change with energy. A change of state is always energy going in or out. Heat in climbs the ladder: solid → liquid is melting, liquid → gas is boiling. Heat out slides back down: gas → liquid is condensing, liquid → solid is freezing.
A Worked One, Slowly
An ice block left on the bench. It's a solid: particles in a fixed pattern, packed close, only vibrating. The warm room hands energy to them, so they vibrate harder and harder until they break out of their pattern and start sliding past each other — it has melted into liquid water, still close and touching but disordered, which is why the puddle flows. Three sentences: name it, describe it, explain the change with energy. That structure earns full marks every time.