Chemistry and Matter
Particles and states
Use the particle model to explain solids, liquids and gases, and the changes of state between them. Melting and boiling happen at fixed temperatures for a pure substance; evaporation occurs at any temperature from the surface. Particles in all states move, so diffusion happens in liquids and gases.
Substances and separation
Distinguish elements, compounds and mixtures. A pure substance contains only one substance - it does not mean "clean". Separate mixtures by filtration (insoluble solid), crystallisation (soluble salt), simple distillation (solvent from solution), fractional distillation (liquids with different boiling points) and chromatography (soluble coloured substances).
Atoms and bonding
Atoms have protons and neutrons in the nucleus and electrons in shells. Group number gives outer electrons; period number gives shells. Metals lose electrons to form positive ions, non-metals gain electrons to form negative ions; ionic compounds have giant lattices with high melting points, while simple covalent molecules have weak forces between molecules.
Reactions
Balance equations by changing coefficients only, never subscripts, and check that mass is conserved. Learn the general reactions: acid + metal → salt + hydrogen; acid + carbonate → salt + water + carbon dioxide; acid + base → salt + water. Neutralisation moves pH towards 7. Exothermic reactions release energy to the surroundings; endothermic ones absorb it.
Traps
"Boiling" confused with evaporation; mass apparently lost in an open system; unbalanced charges in formulae; thinking higher pH always means stronger acid; muddling exothermic with endothermic.
Worked example
Balance CH₄ + O₂ → CO₂ + H₂O. Carbon and hydrogen need two water molecules, giving four oxygen atoms on the right, so write 2O₂: CH₄ + 2O₂ → CO₂ + 2H₂O.