Qualitative Analysis — Identifying Unknown Substances
10 experiments
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.
Anion Identification Tests
Detecting common anions through selective chemical reactions
Identify unknown anions using specific precipitation, acid-decomposition, and redox tests.
Beilstein Test for Halogens
Detecting halogen atoms in organic compounds using a copper wire flame test
Determine whether an organic compound contains halogen atoms (Cl, Br, I) using the Beilstein copper wire test.
Borax Bead Test for Metal Ions
Identifying transition metals by colored glass beads formed in a borax fusion
Identify transition metal ions by the characteristic colors they produce when fused into a borax (sodium tetraborate) glass bead.
Cation Identification by Precipitation
Systematic separation of Group I–V cations using selective reagents
Identify unknown cations in a solution using the classical qualitative analysis scheme with group reagents.
Chromatography of Plant Pigments
Separating chlorophyll and carotenoids using paper chromatography
Separate and identify plant pigments using paper chromatography, and calculate Rf values for each pigment.
Flame Emission Spectroscopy
Quantitative identification of metals using spectroscope and flame
Use a spectroscope to observe and measure the emission line wavelengths of different metal ions in a flame.
Hard Water Testing with Soap Solution
Comparing water hardness by measuring soap lather formation
Determine the relative hardness of water samples by measuring the volume of soap solution required to produce a lasting lather.
pH Indicator Rainbow
Mapping indicator color changes across the full pH range
Observe how different pH indicators change color at specific pH values and understand the concept of indicator transition ranges.
Starch-Iodine Test for Carbohydrates
Detecting starch in food samples using iodine solution
Use the iodine test to identify starch in various food samples and understand the molecular basis of the blue-black color …
Unknown Substance Identification Challenge
Using physical and chemical properties to identify mystery powders
Apply multiple qualitative tests to identify unknown white powders, developing systematic analytical thinking skills.