Chemistry: Substances and Chemical Changes
Substance questions test whether you can classify matter, then describe a change with the precise vocabulary of 化学変化 rather than everyday words.
Separating and identifying
- A 純物質 has a fixed 融点 and 沸点; a 混合物 melts and boils over a range. This is the standard test.
- ろ過 removes an insoluble solid; 蒸留 separates a liquid from dissolved solids or from liquids of different boiling point; 再結晶 purifies a solid whose solubility changes steeply with temperature.
- 単体 contains one element; 化合物 contains two or more elements chemically combined; 混合物 is neither.
Types of chemical change
- 化合 (A + B → AB), 分解 (AB → A + B), 酸化 (combining with oxygen), 還元 (removing oxygen), 中和.
- 質量保存の法則: total mass is unchanged. If mass seems to fall, a gas escaped; if it seems to rise, oxygen was taken from the air.
- 発熱反応: combustion, neutralisation, adding water to 酸化カルシウム. 吸熱反応: heating 炭酸水素ナトリウム, mixing 塩化アンモニウム with 水酸化バリウム.
Worked example. Copper combines with oxygen in the mass ratio 4 : 1. Heating 1.2 g of copper fully gives 1.2 + 0.3 = 1.5 g of 酸化銅. If 3.0 g of that oxide is heated with excess carbon, it contains 3.0 × 4/5 = 2.4 g of copper, so 2.4 g is recovered and the oxygen leaves as CO₂.
Traps. Calling melting, boiling or dissolving a 化学変化; claiming the 質量保存の法則 fails when a gas escapes; assuming a 混合物 has a sharp melting point; mixing up 還元 with 分解; forgetting that combustion always needs oxygen from the air, not from the fuel alone.