Unit 04 · Chromatography & Chemical Analysis
Chromatography lets an analyst pull a mixture apart to see what it’s made of. This unit covers how a mobile phase carries components across a stationary phase at different rates, how the Rf value turns that separation into a repeatable number, how ink and dye can be separated to compare a questioned sample against a known, and the crucial difference between presumptive and confirmatory testing. Mastery means you can separate an unknown systematically and state exactly what each test can and cannot establish.
| Criterion | Not yet | Approaching | Mastered |
|---|---|---|---|
| Chromatography principles & the Rf value | Cannot explain what makes components separate. | Describes the setup but cannot compute or interpret an Rf value. | Explains mobile and stationary phases and calculates a reproducible Rf value for each component. |
| Ink & dye separation | Runs a plate but reads nothing from the result. | Separates a sample but cannot compare it to a known. | Separates an unknown ink or dye and compares its pattern and Rf values against a known standard. |
| Presumptive vs. confirmatory testing | Treats a presumptive result as a final identification. | Names the two test types but confuses what each can establish. | Distinguishes presumptive from confirmatory tests and states what each can and cannot prove. |
| Analyzing an unknown systematically | Guesses the identity of a sample without a procedure. | Runs tests but in no logical order and without narrowing possibilities. | Works an unknown through an ordered scheme, narrowing possibilities before confirming. |
| Technique, controls & documentation | Omits controls or keeps no record of the run. | Runs the test but handles controls or documentation inconsistently. | Runs proper controls, documents each step, and reports results with their limits. |
| Integration (cross-domain) | Treats chemical analysis as isolated facts; makes no cross-domain connection. | Names a link from chemical analysis to history, reading, or writing but cannot defend why it matters. | Connects chromatography and chemical analysis to its anchor across History · Reading · Writing (plus chosen electives) and defends why the connection matters. |
“The questioned ink separated into the same three bands as the pen from the desk, with Rf values that line up, so I can report them as consistent. But that’s a presumptive comparison — I’d need a confirmatory test before I’d say more, and even then I’d report what the evidence supports, not a certainty.”
“The colors spread out, so it’s the same pen. The presumptive test turned positive, so that proves what it is.”
You demonstrate this unit through a chromatography investigation — separating a questioned ink or dye, calculating Rf values, and comparing against knowns aloud — not a multiple-choice test. A criterion counts as mastered only when you can run the separation and state honestly what a presumptive result does and does not establish. Mastery is demonstrated, not awarded.
A 5-page clipboard packet — unit overview, key terms, the mastery rubric, anchor examples, and a score sheet you can print and grade against.