含义Meaning
化学反应中，参加反应的各物质质量总和等于生成的各物质质量总和。
In a chemical reaction the total mass of the reactants that react equals the total mass of the products formed.
符号说明Symbols
| 符号Symbol | 含义 Meaning | 单位 Unit |
|---|---|---|
| \(m\) | 质量mass | g, kg |
推导Derivation
化学反应的本质是原子的重新组合：反应前后原子的种类和数目都不变。
A reaction only regroups atoms: the kinds and numbers of atoms are unchanged.
每种原子的质量不变，所以各原子质量之和（即总质量）不变。
Each atom keeps its mass, so the sum over all atoms — the total mass — is unchanged.
拉瓦锡在密闭容器中加热锡、汞等金属，称量证实了这一点（1774–1789）。
Lavoisier confirmed it by weighing metals heated in sealed vessels (1774–1789).
适用条件与局限Assumptions & limitations
只适用于化学变化；必须把气体等所有参与物质都计入（敞口容器中生成的气体逸出会使称量“减少”）。
Count every participating substance, including gases; open vessels lose gaseous products.
严格地说，反应放热时体系质量按 \(\Delta m = \Delta E/c^2\) 有极微小减少，化学反应中约 \(10^{-9}\) 量级，可忽略；核反应中不可忽略。
Strictly, released energy lowers mass by \(\Delta m = \Delta E/c^2\) — about \(10^{-9}\) of the mass in chemistry (negligible) but significant in nuclear reactions.
例题Worked example
12 g 碳在 32 g 氧气中完全燃烧，生成 CO₂ 的质量为 12 + 32 = 44 g。
12 g of carbon burning completely in 32 g of oxygen gives 12 + 32 = 44 g of CO₂.