Chemistry: Substances, Reactions, and Particles
Substances and how we separate them
First classify: a pure substance has a fixed composition (an element or a compound), while a mixture can be separated without breaking chemical bonds. Filtering removes an insoluble solid, distillation or evaporation separates a solvent from a dissolved solid, recrystallization purifies a solid, and chromatography separates dyes by differing solubility. A chemical change creates new substances with new properties - gas evolution, precipitation, colour change, heat from combustion - and cannot be reversed by physical means.
Atoms, ions, and equations
Matter is particulate: atoms combine into molecules, and ions form when atoms lose or gain electrons (metals lose, nonmetals gain). Write formulas from valencies, then balance equations by adjusting coefficients only, never subscripts. Conservation of mass follows from atom counting, so in a closed vessel the total mass is unchanged by a reaction.
Acids release hydrogen ions and alkalis release hydroxide ions; neutralization gives a salt plus water. Indicators show the direction of pH, and a precipitate identifies an ion pair such as silver chloride.
Traps: confusing "element" with "atom"; forgetting that escaping gas makes an open container lose mass; muddling oxidation (gain of oxygen, as in combustion and rusting) with reduction; assuming every pair of solutions reacts.
Worked example. Carbon burns as C + O₂ → CO₂, a mass ratio of 12 : 32 : 44. Burning 0.60 g of carbon therefore consumes (32/12) × 0.60 = 1.6 g of oxygen and produces 2.2 g of carbon dioxide, so mass is conserved.