Density. Chapter I review

In Chapter I you learned that matter consists of very small particles — molecules and atoms. A molecule is the smallest particle that keeps a substance’s properties; it is made of one or several atoms (H₂O — 3 atoms, O₂ — 2 atoms, CO₂ — 3 atoms). Molecules move constantly and randomly: this is confirmed by diffusion and Brownian motion, and the motion speeds up as temperature rises. At short distances there are forces of attraction and repulsion between molecules. A gas keeps neither shape nor volume, a liquid keeps its volume, a solid keeps both. Mass is the measure of inertia (kg), density is the ratio of mass to volume ρ = m : V (kg/m³). In problems we use m = ρ · V and V = m : ρ.