Matter and Materials
Matter and Materials tests the particle model, the classification of substances, simple chemical reactions, and the practical techniques used to separate mixtures. Questions ask you to explain properties in terms of particles, interpret word and symbol equations, choose a separation method, and decide whether a change is physical or chemical.
Particles and states
In solids, particles are closely packed and vibrate about fixed positions, giving a fixed shape and volume. In liquids they are close but able to slide, so volume is fixed and shape follows the container. In gases they are far apart and move rapidly, filling any container. Heating raises kinetic energy; melting, boiling, evaporation, condensation, freezing and sublimation are changes of state and are physical, so the substance is unchanged and mass is conserved. Diffusion is faster at higher temperature and for lighter particles.
Elements, compounds and mixtures
An element is one kind of atom; a compound is two or more elements chemically combined in fixed proportions; a mixture contains substances not chemically combined and can be separated by physical means.
Reactions and equations
Signs of a chemical reaction include a new substance, colour change, gas, precipitate or energy change. Balance symbol equations by counting atoms of each element on both sides; never change subscripts to balance. Acids and bases: the pH scale, indicators, and neutralisation giving salt plus water.
Separation techniques
Filtration removes an insoluble solid from a liquid; evaporation crystallises a dissolved solid; distillation recovers the solvent; chromatography separates and identifies coloured substances using Rf values; a magnet removes iron.
Worked example
Burning magnesium ribbon: word equation magnesium + oxygen gives magnesium oxide; symbol equation 2Mg + O2 gives 2MgO. Reasoning: the white product is a new substance with different properties, so this is a chemical change. There are two Mg atoms and two O atoms on each side, so the equation balances. Burned in an open crucible, the mass appears to rise because oxygen from the air has joined the magnesium; burn a hydrocarbon and the mass may appear to fall because carbon dioxide and water escape as gases. In both cases the law of conservation of mass still holds.
Traps that cost marks
Calling melting a chemical change; reading 2H2O as four hydrogen atoms; confusing filtration with distillation; measuring Rf from the wrong starting point; assuming neutralisation always leaves a pH 7 solution even when acid or base is in excess.