Chemistry - 物質の構成・化学変化・イオン・化学反応とエネルギー
物質の構成
A pure substance has a fixed composition; a mixture can be separated by physical means such as filtration, distillation or chromatography. Elements contain one kind of atom; compounds contain bonded atoms of different kinds. Know the atomic model (protons, neutrons, electrons), common symbols and formulas, and the difference between an atom, a molecule and an ion.
化学変化
Combination, decomposition, oxidation and reduction are the core reaction types. The law of conservation of mass says the total mass of reactants equals that of products, so balance every equation before calculating. Oxidation is combination with oxygen; reduction is removal of oxygen (or gain of electrons).
イオン
Atoms that lose or gain electrons become ions; ionic compounds conduct electricity when molten or dissolved. Learn precipitation reactions, electrolysis and neutralisation: H⁺ + OH⁻ → H₂O.
化学反応とエネルギー
Exothermic reactions (combustion, neutralisation) release heat and warm the surroundings; endothermic reactions (thermal decomposition, photosynthesis) absorb heat and cool them. Batteries and fuel cells convert chemical energy into electrical energy.
Worked example. 2Mg + O₂ → 2MgO. Burning 2.4 g of magnesium gives 4.0 g of magnesium oxide, so 1.6 g of oxygen was used - consistent with the 48 : 32 : 80 (3 : 2 : 5) mass ratio.
Trap. Mass apparently "lost" when a gas escapes in an open container does not break conservation of mass.