7.2 The Three States of Matter
If everything is made of particles, why is iron hard, water runny, and air invisible and free? The answer is how strongly the particles pull on each other.
Interparticle Attraction The constituent particles of matter are held together by attractive forces called interparticle attractions. Their strength depends on the substance and on the distance between particles — even a small increase in distance weakens them sharply. This strength is what decides whether a substance is a solid, liquid or gas.
Solids
Activity 7.3 — Testing solids Examine an iron nail, rock salt, a stone, a key, wood, aluminium. They all keep a definite shape and volume, and resist hammering. In solids, the particles are tightly packed with very strong interparticle attractions, locked in fixed positions. They can only vibrate about their spots — they cannot move past one another.
Liquids
Activity 7.4 — Water changes shape, not amount Pour 200 mL of water from container A into differently shaped containers B and C. The water takes the shape of each container, but its volume stays 200 mL. So liquids have no fixed shape but a fixed volume. Their particles can move about, but only within a limited space — the attractions are a little weaker than in solids, yet still strong enough to keep particles close (you can push a finger through water and it closes right back up).
Gases
Activity 7.5 — Smoke fills the jar Trap smoke in one gas jar, then connect it to an empty jar: the smoke spreads to fill both completely. Gases have no fixed shape and no fixed volume — their particles move freely in all directions, with negligible interparticle attraction. (The same is seen with iodine vapour filling a jar.)
Fluids Both liquids and gases flow and don’t keep a fixed shape, so together they are called fluids. Solids, which hold their shape, are not fluids.
Sort each statement below into the state it describes — Solid, Liquid, or Gas.