Qualitative Analysis — Identifying Unknown Substances
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10个实验
Qualitative analysis is the systematic identification of ions and compounds in unknown samples through a series of chemical tests. This detective-like process uses selective reagents that produce characteristic colors, precipitates, or gases to confirm or eliminate possible identities. The cation and anion analysis schemes remain foundational exercises in undergraduate chemistry education.
Learning Objectives
Learn systematic cation analysis: Group I (Ag+, Pb2+, Hg22+ — precipitated by HCl), Group II (Cu2+, Bi3+ — precipitated by H2S in acid), Group III (Al3+, Fe3+, Ni2+ — precipitated by NH3/H2S), Group IV (Ca2+, Ba2+ — precipitated by carbonate), Group V (Na+, K+ — identified by flame tests). Develop deductive reasoning and lab observation skills.
Key Techniques
Follow systematic separation schemes — add group reagents in order, centrifuge to separate precipitate from supernatant, and perform confirmatory tests on each fraction. Use spot plates for rapid screening. Record all observations meticulously: precipitate color, texture, solubility in acids/bases, and gas evolution.
Equipment Overview
Required: centrifuge (or gravity settling), test tubes, spot plates, reagent bottles (HCl, H2S or thioacetamide, NH3, Na2CO3), wash bottles, Bunsen burner for flame tests, litmus and pH paper.
pH指示剂彩虹
绘制整个pH范围内的指示剂变色图
观察不同pH指示剂在特定pH值下的颜色变化，理解指示剂变色范围的概念。
卤素的贝尔斯坦测试
用铜丝焰色测试检测有机化合物中的卤素原子
用贝尔斯坦铜丝测试判断有机化合物是否含有卤素原子（Cl、Br、I）。
未知物质鉴定挑战
用物理和化学性质鉴定神秘粉末
对未知白色粉末应用多种定性测试，培养系统分析思维能力。
植物色素的色谱分析
用纸色谱法分离叶绿素和类胡萝卜素
用纸色谱法分离鉴定植物色素，并计算各色素的Rf值。
沉淀法阳离子鉴定
用选择性试剂系统分离I~V族阳离子
用经典定性分析方案中的组试剂鉴定溶液中的未知阳离子。
火焰发射光谱分析
用分光镜和火焰定量鉴定金属
用分光镜观察和测量不同金属离子在火焰中的发射谱线波长。
碳水化合物的淀粉-碘测试
用碘液检测食物样品中的淀粉
用碘液鉴定各种食物样品中的淀粉，理解蓝黑色配合物形成的分子机理。
肥皂液硬水测试
通过测量皂沫形成量比较水的硬度
通过测量产生持续皂沫所需的肥皂溶液体积，测定水样的相对硬度。
金属离子的硼砂珠测试
用硼砂熔融形成的有色玻璃珠鉴定过渡金属
通过过渡金属离子融入硼砂（四硼酸钠）玻璃珠时产生的特征颜色来鉴定过渡金属离子。
阴离子鉴定测试
通过选择性化学反应检测常见阴离子
用特定的沉淀、酸分解和氧化还原测试鉴定未知阴离子。