ChemistryChapter 19: Chromatography
Running the Chromatogram
Stand the strip so only the bottom 5-6 mm sits in the solvent.
Learning objectives
- State the main idea of running the chromatogram in clear words.
- Explain why running the chromatogram is useful in chemistry.
- Apply the correct method, observation, equation, or comparison.
- Avoid the common mistakes and write a complete exam answer.
Definition
Stand the strip so only the bottom 5-6 mm sits in the solvent. The solvent must stay BELOW the spots. Hang the paper so it is not touching the walls. Keep the cylinder closed. Stop when the front is about 1-2 cm from the top. Mark the front immediately, then dry.
Important terms
- 5-6 mm
- Stand the strip so only the bottom 5-6 mm sits in the solvent.
- BELOW
- Stand the strip so only the bottom 5-6 mm sits in the solvent.
- not touching the walls
- Stand the strip so only the bottom 5-6 mm sits in the solvent.
- closed
- Stand the strip so only the bottom 5-6 mm sits in the solvent.
- 1-2 cm from the top
- Stand the strip so only the bottom 5-6 mm sits in the solvent.
- Mark the front immediately
- Stand the strip so only the bottom 5-6 mm sits in the solvent.
Overview
Start rules and stop rules:
Starting
Dip only 5-6 mm of paper. If solvent covers the spots, the dyes dissolve into the bulk liquid and the run is wrecked. Leave a gap around the strip so it does not touch the walls, or the solvent will creep sideways. Lid on.
Examples Solvent below the baseline · Strip hangs free · Lid closed
Stopping
Do not wait forever. When the solvent front is about 1-2 cm from the top, lift the paper. Mark the front immediately: after it dries you cannot see where it stopped. Then let the strip dry before you measure.
Examples Front near the top · Pencil tick on the front · Dry, then measure
WHAT — Core idea
Stand the strip so only the bottom 5-6 mm sits in the solvent. The solvent must stay BELOW the spots. Hang the paper so it is not touching the walls. Keep the cylinder closed. Stop when the front is about 1-2 cm from the top. Mark the front immediately, then dry.
WHY — Why it matters
A closed cylinder keeps the vapour saturating the paper. An open cylinder lets solvent evaporate from the front, so the front may stop or smear. Hang straight: a lean against the glass makes a crooked front.
HOW — Method, evidence, or comparison
1. 1. STARTING: Dip only 5-6 mm of paper. If solvent covers the spots, the dyes dissolve into the bulk liquid and the run is wrecked. Leave a gap around the strip so it does not touch the walls, or the solvent will creep sideways. Lid on. 2. 2. STOPPING: Do not wait forever. When the solvent front is about 1-2 cm from the top, lift the paper. Mark the front immediately: after it dries you cannot see where it stopped. Then let the strip dry before you measure.
HOW — Answer in an exam
Solvent BELOW the spots. Mark the front at once. If the front reaches the top, Rf cannot be measured fairly because the true front is lost.
Explanation
A closed cylinder keeps the vapour saturating the paper. An open cylinder lets solvent evaporate from the front, so the front may stop or smear. Hang straight: a lean against the glass makes a crooked front.
In simple words
Solvent BELOW the spots. Mark the front at once. If the front reaches the top, Rf cannot be measured fairly because the true front is lost.
Key points
- 5-6 mm
- BELOW
- not touching the walls
- closed
- covers the spots
Exam preparation
Exam Focus
Quick revision
Revision Zone
Review the key facts, formulas and checks before you move on.Running the Chromatogram
Sticky Notes · Quick Revision
WHAT
- Stand the strip so only the bottom 5-6 mm sits in the solvent.
Key Points
- 5-6 mm
- BELOW
- not touching the walls
- closed
- 1-2 cm from the top
HOW
- Dip only 5-6 mm of paper.
- Do not wait forever.
Example
- Solvent below the baseline
- Strip hangs free
- Lid closed
Common Mistake
- The solvent level must start below the sample spots.
Exam Tip
- Solvent BELOW the spots.
- Mark the front at once.
60-second revision
- Stand the strip so only the bottom 5-6 mm sits in the solvent.
- The solvent must stay BELOW the spots.
- Dip only 5-6 mm of paper.
- Do not wait forever.
- Solvent BELOW the spots.
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