帮我讲解一下---**Main Topic:** 7. 分步沉淀 (Fractional Precipitation) **Definition:** 同一种沉淀剂可以使溶液中不同离子先后析出的现象叫分步沉淀。(The phenomenon where the same precipitant can cause different ions in a solution to precipitate out sequentially is called fractional precipitation.) **Example Problem:** 例 3.17: 如果在 0.1mol·L⁻¹ 的 Cl⁻ 和 I⁻ 的溶液中加入 AgNO₃ 溶液, 先沉淀何物? Ksp(AgI) = 1.5×10⁻¹⁶; Ksp(AgCl) = 1.56×10⁻¹⁰ (Example 3.17: If AgNO₃ solution is added to a solution containing 0.1 mol·L⁻¹ of Cl⁻ and I⁻, which substance will precipitate first? Ksp(AgI) = 1.5×10⁻¹⁶; Ksp(AgCl) = 1.56×10⁻¹⁰) **Diagrams:** A sequence of three diagrams illustrating fractional precipitation. - **Diagram 1:** A beaker containing a clear solution with labels Cl⁻ and I⁻. An arrow points towards the beaker with the label "加 AgNO₃ 液" (Add AgNO₃ solution), indicating the addition of silver nitrate. - **Diagram 2:** The same beaker now contains some precipitate labeled "AgI" at the bottom, and the solution above it is labeled Cl⁻. An arrow points towards the beaker with the label "加 AgNO₃ 液" (Add AgNO₃ solution), indicating further addition of silver nitrate. - **Diagram 3:** The same beaker now contains precipitate labeled "AgCl" at the bottom, and the solution above it is labeled NO₃⁻. This suggests that after precipitating AgI, adding more AgNO₃ leads to the precipitation of AgCl, with nitrate ions remaining in solution. The diagram illustrates the sequential precipitation of AgI followed by AgCl.