States of matter

Matter is already known to exist in three familiar physical states — solid, liquid and gas — a topic taken up in much greater depth later in Unit 3 (Chapter 10). In a solid, the constituent particles (atoms, ions or molecules) are held tightly together in a highly ordered arrangement, which is why solids have both a definite shape and a definite volume. In a liquid, the particles remain close to one another but are free to move around within the body of the liquid, so a liquid takes the shape of its container while keeping a fixed volume. In a gas, the particles are spread far apart compared to a liquid or solid, letting a gas expand to fill whatever container it occupies. Importantly, these three states are not fixed for a given substance — they are interconvertible simply by …