上一片学会用方程式写下反应、并把两边配平。这一片给反应分两个最常见的家族：一种“合起来”，一种“拆开去”。Last leaf we wrote reactions as balanced equations. This leaf sorts the most common ones into two families: joining up, and splitting apart.
第 1 步：化合——多变一Step 1: Combination — many into one
先认识“抱团”的那一类：多个反应物，合成一个生成物。First the “team-up” family: several reactants become a single product.
两块积木拼成一块：氢气燃烧生成水，2H₂ + O₂ →（点燃）2H₂O；生石灰遇水变成熟石灰，CaO + H₂O → Ca(OH)₂。不管原料有几种，产物只剩一种——“多变一”，这就是化合反应。镁条燃烧发出耀眼白光（2Mg + O₂ →（点燃）2MgO）、铁丝在氧气里燃烧（3Fe + 2O₂ →（点燃）Fe₃O₄），也都是化合。Two blocks snap into one: hydrogen burns into water, 2H₂ + O₂ → (ignite) 2H₂O; quicklime meets water and becomes slaked lime, CaO + H₂O → Ca(OH)₂. However many reactants there are, only one product comes out — “many into one”: a combination reaction. Burning magnesium ribbon (2Mg + O₂ → 2MgO) and iron burning in oxygen (3Fe + 2O₂ → Fe₃O₄) belong here too.
反过来呢？一种物质自己“裂开”成好几种，算哪一类？What about the reverse — one substance cracking into several?
第 2 步：分解——一变多Step 2: Decomposition — one into many
过氧化氢在 MnO₂ 催化下咕嘟咕嘟冒泡：2H₂O₂ →（MnO₂）2H₂O + O₂↑，把带火星的木条放上去会复燃，说明放出的正是氧气。水通电也会分解：2H₂O →（通电）2H₂↑ + O₂↑。一种反应物变成不止一种产物——“一变多”，这就是分解反应。With MnO₂ as catalyst, hydrogen peroxide bubbles away: 2H₂O₂ → 2H₂O + O₂↑. Hold a glowing splint over it and it relights — proof the gas is oxygen. Electric current splits water too: 2H₂O → 2H₂↑ + O₂↑. One reactant turns into more than one product — “one into many”: decomposition.
遇到一个新反应，怎么一眼认出它属于哪一类？So how do you tell the family of a new reaction at a glance?
第 3 步：只看两点，一眼判断Step 3: Just two counts settle it
只看两点：反应物几种、生成物几种。烟花为什么又亮又热？里面的硫、镁剧烈燃烧：S + O₂ →（点燃）SO₂ 是化合（多变一）；而石灰石高温煅烧，CaCO₃ →（高温）CaO + CO₂↑ 是分解（一变多）。口诀：“合”是多变一，“分”是一变多——合久必分，反应世界里也一样。Count just two things: how many reactants, how many products. Why are fireworks bright and hot? Sulfur and magnesium inside burn fiercely: S + O₂ → SO₂ is a combination (many into one); while limestone roasted at high temperature, CaCO₃ → CaO + CO₂↑, is a decomposition (one into many). One phrase to keep: “combination joins, decomposition splits” — even reactions live by “meet to part”.
🎮 反应分拣台（把卡片投进正确的筐）🎮 The Sorting Bench (drop cards into the right basket)
下面 4 张方程式卡，数清反应物和生成物的种数，把每张卡投进“化合”或“分解”筐——最后一张有陷阱。Four equation cards below: count the reactants and products, then drop each card into the “Combination” or “Decomposition” basket — the last one is a trap.
一句话记住它：化合 = 多变一，分解 = 一变多；判断只看反应物和生成物的种数。Remember it in one line: combination = many into one, decomposition = one into many; judge only by counting reactants and products.
化合：2H₂ + O₂ →（点燃）2H₂O；镁燃烧、铁丝燃烧也是Combination: 2H₂ + O₂ → 2H₂O; burning magnesium and iron too分解：2H₂O₂ →（MnO₂）2H₂O + O₂↑；水通电分解也是Decomposition: 2H₂O₂ → 2H₂O + O₂↑; electrified water splitting too别被条件骗：高温、催化剂不改变反应类型，数两边种数才关键Don't be fooled by conditions: heat or catalyst never changes the type — count both sides
内容参考 OpenStax 等公开教材，多来源核对 · AI 生成、人工审核 · 发现错误欢迎指正，帮这片叶子长得更好。Based on OpenStax and other open textbooks, cross-checked across sources · AI-generated, human-reviewed · Spotted a mistake? Tell us — help this leaf grow.
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