🔁 置换与复分解：换一个，还是换一对🔁 Single & Double Displacement: Swap One, or Swap Pairs

上一片里，反应要么“多合一”，要么“一分多”。可要是两边都想换人呢？⚗️ 小烧瓶摆开实验台，开一场“换位派对”。 Last leaf, reactions either merged many into one or split one apart. But what if both sides want to trade partners? ⚗️ Our flask sets the bench and throws a partner-swap party.

第 1 步：置换反应——单换单Step 1: Single displacement — one for one

先看最简单的换法：一种单质，把化合物里的元素换出来。Start with the simplest swap: one element elbows its way out of a compound.

铁钉浸入硫酸铜溶液，表面镀上红色的铜 — 把铁钉浸进蓝色硫酸铜溶液：铁把铜“挤”出来，自己钻进溶液，铁钉穿上红铜外套，蓝色慢慢变浅。Fe + CuSO₄ → FeSO₄ + Cu——单质换单质、一换一，这叫置换反应。 Dip an iron nail into blue copper sulfate solution: iron shoves copper out and takes its place — the nail gets a copper coat and the blue fades. Fe + CuSO₄ → FeSO₄ + Cu. One element swaps with another, one for one: a single displacement.

如果两边都是“一对”，换法就升级了。When both sides come as pairs, the swap gets fancier.

第 2 步：复分解反应——两队舞伴互换Step 2: Double displacement — two pairs trade partners

复分解：两队舞伴互换，Ag⁺ 与 Cl⁻ 结合成 AgCl — 硝酸银遇上氯化钠：Ag⁺ 牵走 Cl⁻，生出不溶于水的 AgCl 白色沉淀；Na⁺ 只好跟 NO₃⁻ 组队。AgNO₃ + NaCl → AgCl↓ + NaNO₃——两队舞伴互相换人，这叫复分解反应。 Silver nitrate meets sodium chloride: Ag⁺ takes Cl⁻ and forms AgCl — a white solid that won't dissolve — while Na⁺ pairs up with NO₃⁻. AgNO₃ + NaCl → AgCl↓ + NaNO₃. Two pairs trade partners: a double displacement.

沉淀是复分解的“信号”。四大基本反应类型，到这里也凑齐了。That precipitate is the sign. And with it, all four basic reaction types are on the table.

第 3 步：四大类型凑齐，认准“发生的信号”Step 3: All four types — and the sign it happened

试管里的 AgCl 白色沉淀 — 复分解要发生，得生出沉淀、气体或水——三者至少有一个。白色 AgCl 沉淀，就是 AgNO₃ 和 NaCl 换过舞伴的证据。回头数一数：多变一是化合、一变多是分解、单换单是置换、双交换是复分解——四大基本反应类型，到齐！ A double displacement happens only if it makes a precipitate, a gas, or water — at least one of the three. The white AgCl solid is the proof that AgNO₃ and NaCl swapped partners. Now count them all: many-into-one is combination, one-into-many is decomposition, one-for-one is single displacement, pair-for-pair is double displacement — all four basic reaction types, complete.

🎮 你来当换位实验员（60 秒）🎮 Your turn: swap-lab bench (60 seconds)

两场换位实验等你动手：把铁钉放进硫酸铜溶液、把硝酸银滴进食盐水；再认一认反应类型——4 关。Two swap experiments await: dip the nail into copper sulfate, drip silver nitrate into salt water — then name each reaction type. 4 levels.

一句话记住它：置换=单质换单质（铁钉换铜）；复分解=两队互换（出沉淀/气体/水）；跟化合、分解一起，凑齐四大基本反应类型。 Remember it in one line: single displacement swaps one element (iron for copper); double displacement trades two pairs (making a precipitate, gas or water) — together with combination and decomposition, that's all four basic reaction types.

置换反应：A + BC → AC + B，单换单；例：Fe + CuSO₄ → FeSO₄ + CuSingle displacement: A + BC → AC + B, one for one; e.g. Fe + CuSO₄ → FeSO₄ + Cu 复分解反应：AB + CD → AD + CB，双交换；有沉淀/气体/水才发生；例：AgCl↓Double displacement: AB + CD → AD + CB, two pairs trade; needs a precipitate, gas or water; e.g. AgCl↓ 四大类型：化合（多变一）、分解（一变多）、置换（单换单）、复分解（双交换）The four types: combination (many→one), decomposition (one→many), single displacement (one-for-one), double displacement (pair-for-pair)

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