CXC CSEC Chemistry

## The particulate theory All matter is made of very small particles (atoms, molecules or ions) that are in constant motion and have spaces between them. The particles attract one another, and the balance between this attraction and the energy of their motion decides whether a substance is a solid, a liquid or a gas at a given temperature. ### Evidence from diffusion Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration, caused by their random motion. It only makes sense if matter is made of moving particles with gaps between them. In a classic demonstration, a cotton wool plug soaked in concentrated ammonia solution is placed at one end of a long, dry, horizontal glass tube and a plug soaked in concentrated hydrochloric acid at the other end. After a few minutes a ring of white solid ammonium chloride forms inside the tube, nearer the hydrochloric acid end: $$\ce{NH3(g) + HCl(g) -> NH4Cl(s)}$$ The ring forms where the two gases meet. It is nearer the hydrochloric acid end because ammonia molecules (relative molecular mass 17) are lighter than hydrogen chloride molecules (36.5) and so move faster and travel further in the same time. The experiment shows that gas particles move through the air, that they move at different speeds, and that lighter particles diffuse faster. A crystal of potassium manganate(VII) dropped into a beaker of still water gives another example. A purple colour slowly spreads through the…