IGCSE Chemistry — Core: What Things Are Made Of and How They Change

First, PARTICLES. Solids, liquids and gases differ in how their particles are arranged and move, which explains diffusion (faster for lighter particles and at higher temperature), evaporation and condensation, and the shape of heating and cooling curves — where a plateau is a change of state at constant temperature while the particles' arrangement, not their kinetic energy, changes. A pure substance has fixed melting and boiling points; an impure one melts over a range and lower than the pure value, and that single fact answers most 'how do you know it is pure' questions. Second, ATOMS AND BONDING. Atomic number is the proton count, mass number protons plus neutrons, isotopes are atoms of the same element with different neutron numbers. Electronic configuration in shells (2, 8, 8 for the first twenty elements) explains the group patterns: metals lose electrons and non-metals gain or share them, giving ionic bonding between metal and non-metal (a giant lattice, hard, high melting, conducting only when molten or dissolved) and covalent bonding between non-metals — simple molecules with low melting points and no conductivity, plus the three giant covalent structures whose properties are examinable by name: diamond, graphite and silicon dioxide. Metallic bonding is a lattice of positive ions in a sea of delocalised electrons, which is why metals conduct and deform rather than shatter. Third, FORMULAS AND EQUATIONS. Writing a formula from charges, balancing a symbol equation, and using relative atomic mass to check conservation of mass; the electrolysis-free practical chemistry of Core is largely identification — limewater for carbon dioxide, a glowing splint for oxygen, the pop test for hydrogen, anhydrous copper sulfate for water, cobalt chloride paper for water, and the flame tests and precipitate colours for the common cations and anions. Fourth, ACIDS: the ion that makes a solution acidic (H⁺ in water), the pH scale as logarithmic in effect if not in name, the products of acid with metal, base, carbonate and ammonia, the difference between strong and concentrated (degree of ionisation against amount of solute), and the practical routes to a salt — titration for the soluble salts, excess solid for the insoluble ones, with the six-step crystallisation procedure named. Fifth, the AIR AND THE PERIODIC TABLE: the composition of clean air, rusting as the reaction of iron with oxygen and water (and the sacrificial protection that prevents it), reactivity series used to predict displacement and extraction, and the group trends for alkali metals and halogens stated as far as they go at this tier.