定义Definition
旁观离子是存在于反应溶液中、但在反应前后没有变化、不参与反应的离子。写离子方程式时把它们从两边删去。
A spectator ion is present in the reaction mixture but is unchanged by the reaction and takes no part in it. Spectator ions are cancelled from both sides when an ionic equation is written.
例Examples
AgNO₃(aq) + NaCl(aq) → AgCl(s) + NaNO₃(aq) 中，Na⁺ 和 NO₃⁻ 是旁观离子，离子方程式为 Ag⁺ + Cl⁻ → AgCl。锌置换硫酸铜中的铜时，SO₄²⁻ 是旁观离子：Zn + Cu²⁺ → Zn²⁺ + Cu。
In AgNO₃(aq) + NaCl(aq) → AgCl(s) + NaNO₃(aq), Na⁺ and NO₃⁻ are spectators and the ionic equation is Ag⁺ + Cl⁻ → AgCl. When zinc displaces copper from copper(II) sulfate, SO₄²⁻ is a spectator: Zn + Cu²⁺ → Zn²⁺ + Cu.
判断方法How to identify them
先把可溶的强电解质拆成离子，比较方程式两边：形式和状态都没有改变的离子就是旁观离子。注意生成沉淀、气体或水的离子不是旁观离子。
Write the soluble strong electrolytes as ions and compare the two sides: ions that appear unchanged, in the same form and state, are spectators. Ions that end up in a precipitate, a gas or water are not.