6.3.1 Chromatography and
Thin layer chromatography (TLC)
In thin layer chromatography:
 the stationary phase is the chromatography plate; and
 the mobile phase is the liquid solvent.
The more soluble a substance is in the
solvent, the further it travels up the
chromatography plate.
The Rf value of a substance can be
calculated and used to identify the
substance. A particular substance has
the same Rf value in a particular solvent.
Rf = distance travelled by the substance
distance travelled by the solvent
The limitations of thin layer chromatography include:
 Some substances have similar Rf values.
 An unknown substance may not have an Rf value
to compare it to.
qualitative analysis (Part 1)
Gas chromatography
In gas chromatography:
 the stationary phase is a liquid absorbed on an inert solid support; and
 the mobile phase is an unreactive gas (e.g. N2) called the carrier gas.
A gas chromatograph measures the time it takes for each substance in the
mixture to pass through the column. This is called its retention time and it
can be used to identify the substance.
 The more soluble a substance in the liquid phase, the longer it takes to
pass through the column  longer retention time.
 The higher the boiling point of a substance, the longer it takes to pass
through the column  longer retention time.
The areas under the peaks are proportional to the amount of each
substance in the mixture.
 The greater the area under the peak, the higher the proportion of that
substance in the mixture.
 The further to the right the peak, the longer the retention time.
Calibration curves in gas chromatography
To calibrate the chromatograph,
we measure the areas under the
peak for various known
concentrations of the substance
and use these to plot an external
calibration curve. This enables
us to work out the concentration
of that substance in the mixture.
The retention time can be affected by:
 the length and packing of the column;
 the nature and flow rate of the carrier gas; and
 the temperature of the column.
The limitations of gas chromatography include:
 Some substances have similar retention times.
 An unknown substance may not have a retention time to compare it to.
 Substances with high boiling points cannot be separated.
Mrs Sudbery
6.3.1 Chromatography and
Tests for organic functional groups
 Alkenes
qualitative analysis (Part 2)
Test: Reaction with bromine water
Positive result: Bromine is decolourised (orange  colourless)
 Aldehydes
Test: Reaction with Tollens’ reagent (ammoniacal silver nitrate)
 Haloalkanes
Test: Reaction with aqueous silver nitrate in ethanol
Positive results:
- Chloride (Cl–): white precipitate (AgCl)
- Bromide (Br–): Cream precipitate (AgBr)
- Iodide (I–): Yellow precipitate (AgI)
 Phenols
Test: Reaction with sodium hydroxide, followed by reaction
with sodium carbonate
Positive result: Sodium hydroxide is neutralised due to the weak
acidity of phenol, but there is no reaction with
sodium carbonate and therefore no fizzing
 Carbonyl compounds
Test: Reaction with 2,4-DNP (2,4-dinitrophenylhydrazine)
Positive result: Yellow precipitate
Positive result: Silver mirror formed due to Ag+ ions being
reduced to Ag metal as the aldehyde is oxidised
to a carboxylic acid
CH3CHO + [O]  CH3COOH
 Primary and secondary alcohols
Test: Reaction with acidified sodium or potassium dichromate
(Na2Cr2O7 or K2Cr2O7)
Positive result: Oxidation of the alcohol takes place, leading to
an orange to green colour changes chromium
is reduced from the +6 to the +3 oxidation state
CH3CH(OH)CH3 + [O]  CH3COCH3 + H2O
 Carboxylic acids
Test: Reaction with sodium carbonate
Positive result: Bubbles of carbon dioxide gas
2CH3COOH + Na2CO3  2CH3COONa + H2O + CO2
Mrs Sudbery