Key Terms
- Rf: Distance substance / distance solvent front (0–1, no units)
- Stationary phase: Does not move (e.g. paper)
- Mobile phase: Solvent that moves
- Line spectrum: Unique pattern of wavelengths per element
- Flame emission spectroscopy: Identifies metals by line spectra
Must-Know Facts
- Rf always between 0 and 1 (no units)
- 1 spot = pure; multiple spots = mixture
- Instrumental methods: accurate, sensitive, fast
- Li⁺ = crimson, Na⁺ = yellow, K⁺ = lilac, Ca²⁺ = orange-red, Cu²⁺ = green/blue-green
- Line spectra unique to each element
Rf Calculation
- Rf = distance moved by substance ÷ distance moved by solvent front
- Example: substance 4.8 cm, solvent front 6.0 cm → Rf = 4.8/6.0 = 0.80
- Compare Rf of unknown to Rf of reference to identify
Instrumental vs Traditional
- More accurate (precise measurements)
- More sensitive (detects tiny amounts)
- Faster (results in seconds/minutes)
- BUT: more expensive, needs specialist operators
Common Mistakes
- Saying Rf values have units: Rf is a ratio — it has NO units and is always between 0 and 1
- Saying one spot on chromatography proves purity: One spot suggests a pure substance but does NOT prove it — a different solvent might separate components that appear as one spot
- Saying flame emission spectroscopy identifies compounds: It identifies ELEMENTS (metals) only — it gives no information about what compounds or anions are present
- Forgetting instrumental methods have disadvantages: Instrumental methods are more accurate and sensitive, but they are also more expensive and require trained specialist operators — examiners want both sides
This topic summary covers Knowledge Organiser: Instrumental Analysis within Instrumental Analysis for GCSE Chemistry. Revise Instrumental Analysis in Chemical Analysis for GCSE Chemistry with 6 exam-style questions and 6 flashcards. This topic appears regularly enough that it should still be part of a steady revision cycle. It is section 12 of 12 in this topic. Use this topic summary to connect the idea to the wider topic before moving on to questions and flashcards.
Practice questions for Instrumental Analysis
Which formula correctly calculates the Rf value in chromatography?
State two advantages of using instrumental methods of analysis compared with traditional chemical tests.
Quick recall flashcards
In paper chromatography, what does the number of spots on a chromatogram tell you about purity?
One spot → pure substance (single component)
Multiple spots → mixture / impure sample (multiple components, each with different Rf)
What is the formula for calculating the Rf value in chromatography?
Rf = distance moved by substance ÷ distance moved by solvent front
Always between 0 and 1. No units.