Chemistry: Properties of Matter, Chemical Changes, Atoms and Molecules, Ions and Reactions
Properties of matter
The exam tests particle models, density and the difference between physical and chemical change. In solids, particles vibrate in fixed positions; in liquids they slide past each other; in gases they move freely and fill the container. Heating increases particle speed. Density = mass ÷ volume. Melting, dissolving and distillation are physical changes - no new substance forms. Burning, rusting and neutralisation are chemical changes and produce new substances.
Atoms and molecules
An atom has protons and neutrons in the nucleus, with electrons in shells. Atomic number = number of protons; mass number = protons + neutrons. Isotopes have the same number of protons but different numbers of neutrons. Elements contain one kind of atom; compounds contain atoms chemically bonded in fixed ratios. For main-group elements the group number gives outer-shell electrons and the period number gives the number of shells.
Ions and reactions
Metal atoms lose electrons to form positive ions; non-metals gain electrons to form negative ions. Ionic compounds are electrically neutral overall: Mg²⁺ with Cl⁻ gives MgCl₂. In precipitation reactions, soluble ions combine to form an insoluble solid. Acids release H⁺ ions, alkalis release OH⁻ ions, and neutralisation gives a salt plus water.
Worked example
Magnesium burns: 2Mg + O₂ → 2MgO. 4.8 g of Mg is 4.8 ÷ 24 = 0.2 mol. The equation shows 2 mol Mg gives 2 mol MgO, so 0.2 mol MgO forms. Mass = 0.2 × 40 = 8.0 g. The extra mass is the oxygen that combined, so total mass is conserved.
Traps
Burning in open air can look like mass loss because products escape as gas. Balance equations before using mole ratios. Do not confuse atoms with molecules - O₂ is a molecule of an element. Simplify ionic subscripts after balancing charges. Evaporation is physical, not chemical.