CIE iGCSE Co-ordinated Sciences-C12.3 Chromatography- Study Notes- New Syllabus
CIE iGCSE Co-ordinated Sciences-C12.3 Chromatography – Study Notes
CIE iGCSE Co-ordinated Sciences-C12.3 Chromatography – Study Notes -CIE iGCSE Co-ordinated Sciences – per latest Syllabus.
Key Concepts:
Core
- Describe how paper chromatography is used to separate mixtures of soluble coloured substances, using a suitable solvent
- Interpret simple chromatograms to identify:
(a) unknown substances by comparison with known substances
(b) pure and impure substances
Supplement
- State and use the equation for Rf: Rf = distance travelled by substance ÷ distance travelled by solvent
CIE iGCSE Co-Ordinated Sciences-Concise Summary Notes- All Topics
Paper Chromatography
Paper chromatography is an experimental method used to separate mixtures of soluble coloured substances using a suitable solvent.
Method:
- Draw a pencil line near the bottom of a piece of chromatography paper.
- Place small spots of the mixture (e.g. inks or dyes) on the line.
- Dip the paper into a suitable solvent (e.g. water or ethanol), ensuring the solvent level is below the spots.
- The solvent moves up the paper by capillary action, carrying the soluble substances with it.
- Different substances travel at different speeds, so they separate into distinct spots.
- When the solvent front has nearly reached the top, remove the paper and mark the solvent line with a pencil.
Explanation:
- Substances that are more soluble in the solvent travel further up the paper.
- Substances that are less soluble or more strongly attracted to the paper travel shorter distances.
- This produces a chromatogram showing the separated spots.
Key Uses:
- To identify substances in a mixture.
- To check the purity of a substance (a pure substance shows only one spot).
- To compare an unknown sample with known reference substances.
Example :
A student uses paper chromatography to separate a sample of black ink. Explain why the pencil line must be drawn instead of a pen line.
▶️ Answer/Explanation
Step 1: Ink from a pen would dissolve in the solvent.
Step 2: This would interfere with the separation of the coloured substances.
Step 3: Pencil is insoluble in the solvent, so it does not affect the chromatogram.
Final Answer: The line is drawn in pencil because pencil does not dissolve in the solvent.
Interpreting Chromatograms
Paper chromatography can be used to identify substances and check their purity by examining the chromatogram (the pattern of separated spots).
(a) Identifying Unknown Substances by Comparison
- Run the unknown sample alongside known reference substances on the same chromatography paper.
- If the unknown produces a spot at the same height (same Rf value) as one of the known substances, then the unknown contains that substance.
- If the spots are at different heights, the unknown substance is different from the reference substances.
(b) Identifying Pure and Impure Substances
- A pure substance produces only one spot on the chromatogram.
- An impure substance produces two or more spots, showing that it is a mixture of substances.
Example :
A chromatogram shows that a black ink sample produces three spots. Two of these spots line up with known food dyes A and C, while one spot does not match any known dye. Is the black ink pure or impure, and what does it contain?
▶️ Answer/Explanation
Step 1: Since the black ink produces more than one spot, it is impure.
Step 2: The spots at the same level as A and C show that the ink contains food dyes A and C.
Step 3: The unmatched spot shows that the ink also contains another unidentified dye.
Final Answer: The black ink is impure and contains dyes A, C, and an unknown dye.
Rf Value in Chromatography
The Rf value (retention factor) is a ratio used in chromatography to identify substances by comparing how far they move relative to the solvent front.
\(\mathrm{Rf = \dfrac{distance\ travelled\ by\ substance}{distance\ travelled\ by\ solvent}}\)
Explanation:
- The distance travelled by the substance is measured from the baseline (where the spot started) to the centre of the spot.
- The distance travelled by the solvent is measured from the baseline to the solvent front (highest point reached by the solvent).
- The Rf value is always between 0 and 1.
- Different substances usually have different Rf values under the same conditions, allowing identification.
Example :
In a chromatography experiment, a dye travelled 4.5 cm while the solvent travelled 6.0 cm. Calculate the Rf value of the dye.
▶️ Answer/Explanation
Step 1: Use the formula \(\mathrm{Rf = \dfrac{distance\ travelled\ by\ substance}{distance\ travelled\ by\ solvent}}\).
Step 2: \(\mathrm{Rf = \dfrac{4.5}{6.0}}\).
Step 3: \(\mathrm{Rf = 0.75}\).
Final Answer: The Rf value = 0.75.