ChemistryChapter 19: Chromatography
Rf Value
Rf is the retardation factor.
Learning objectives
- State the main idea of rf value in clear words.
- Explain why rf value is useful in chemistry.
- Apply the correct method, observation, equation, or comparison.
- Avoid the common mistakes and write a complete exam answer.
What this grid covers
Rf is the retardation factor. Rf = distance spot / distance solvent front. Measure from the baseline to the spot centre and from the baseline to the front. Rf sits from 0 to 1. The highest Rf moved farthest. Same Rf under the same conditions is evidence, not absolute proof.
Important terms
- Rf
- Rf is the retardation factor.
- retardation factor
- Rf is the retardation factor.
- distance spot / distance solvent front
- Rf is the retardation factor.
- spot centre
- Rf is the retardation factor.
- 0 to 1
- Rf is the retardation factor.
- highest Rf
- Rf is the retardation factor.
WHAT — Core idea
Rf is the retardation factor. Rf = distance spot / distance solvent front. Measure from the baseline to the spot centre and from the baseline to the front. Rf sits from 0 to 1. The highest Rf moved farthest. Same Rf under the same conditions is evidence, not absolute proof.
WHY — Why it matters
Rf = 810 = 0.8, 610 = 0.6, 210 = 0.2. Always use the same units (cm). A value above 1 means you measured past the front or swapped the two distances. Same Rf is evidence of identity, not absolute proof: two different dyes can share an Rf by chance.
HOW — Method, evidence, or comparison
A dye 8 cm up → 8 cm (front 10 cm) → Rf = 0.8; Purple spot → 6 cm → Rf = 0.6; Red spot → 2 cm → Rf = 0.2
HOW — Answer in an exam
Rf = 810 = 0.8, 610 = 0.6, 210 = 0.2. Always use the same units (cm). A value above 1 means you measured past the front or swapped the two distances. Same Rf is evidence of identity, not absolute proof: two different dyes can share an Rf by chance.
Worked Rf values with front 10 cm from the baseline:
| Spot | Distance from baseline | Rf |
|---|---|---|
| 1. A dye 8 cm up | 8 cm (front 10 cm) | Rf = 0.8 |
| 2. Purple spot | 6 cm | Rf = 0.6 |
| 3. Red spot | 2 cm | Rf = 0.2 |
In simple words
Rf = 810 = 0.8, 610 = 0.6, 210 = 0.2. Always use the same units (cm). A value above 1 means you measured past the front or swapped the two distances. Same Rf is evidence of identity, not absolute proof: two different dyes can share an Rf by chance.
Key points
- Rf
- retardation factor
- distance spot / distance solvent front
- spot centre
Exam preparation
Exam Focus
Quick revision
Revision Zone
Review the key facts, formulas and checks before you move on.Rf Value
Sticky Notes · Quick Revision
WHAT
- Rf is the retardation factor.
Key Points
- Rf
- retardation factor
- distance spot / distance solvent front
- spot centre
- 0 to 1
HOW
- Rf = 810 = 0.8, 610 = 0.6, 210 = 0.2.
- Always use the same units (cm).
Example
- A dye 8 cm up — 8 cm (front 10 cm) — Rf = 0.8
- Purple spot — 6 cm — Rf = 0.6
- Red spot — 2 cm — Rf = 0.2
Common Mistake
- Measure both distances from the baseline, not from the bottom of the paper.
Exam Tip
- Rf = 810 = 0.8, 610 = 0.6, 210 = 0.2.
- Always use the same units (cm).
60-second revision
- Rf is the retardation factor.
- Rf = distance spot / distance solvent front.
- Rf = 810 = 0.8, 610 = 0.6, 210 = 0.2.
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