Chemistry PAG 3: Separation techniques
Combined Science PAG C3: Separation techniques
Suggested Activity 3: Kitchen chromatography
Instructions and answers for teachers
and technicians
These instructions cover the learner activity section which can be found on page 10. This Practical
activity supports OCR GCSE Chemistry and Combined Science.
When distributing the activity section to the learners either as a printed copy or as a Word
file you will need to remove the teacher instructions section.
This is a suggested practical activity that can be used as part of teaching the GCSE (9-1)
Gateway Science (A) and Twenty First Century Science (B) specifications.
These are not controlled assessment tasks, and there is no
requirement to use these particular activities.
You may modify these activities to suit your learners and centre. Alternative activities are
available from, for example, Royal Society of Biology, Royal Society of Chemistry, Institute of
Physics, CLEAPSS, SSERC and publishing companies, or of your own devising.
Further details are available in the specifications (Practical Skills Topics), and in these videos.
OCR recommendations:
Before carrying out any experiment or demonstration based on this guidance, it is the
responsibility of teachers to ensure that they have undertaken a risk assessment in
accordance with their employer’s requirements, making use of up-to-date information and
taking account of their own particular circumstances. Any local rules or restrictions issued
by the employer must always be followed.
CLEAPSS resources are useful for carrying out risk-assessments:
(http://science.cleapss.org.uk).
Centres should trial experiments in advance of giving them to learners. Centres may
choose to make adaptations to this practical activity, but should be aware that this may
affect the Apparatus and Techniques covered by the learner.
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Introduction
This is an activity that can be used to refresh learners’ knowledge of chromatography which is likely
to have been studied at Key Stage 3 and prior. It could provide a hook to formally introduce
concepts of stationary and mobile phase, calculation of Rf values and differential separation of
substances with different mobile phases, e.g. by use of the OCR Suggested Activity 2 for the
Practical Activity Group ‘Separation Techniques’ : http://www.ocr.org.uk/Images/323640-pagactivity-chemistry-separation-techniques-suggestion-2.doc.
As a ‘kitchen chemistry’ practical, this could also be set as homework for learners to use the food
dyes they have at home, and then ask your learners to bring in their chromatograms. Ensure you
provide clear instructions to your learners to seek permission from their parents/guardians
to use household solvents before they carry out any work.
Aims
To investigate the different separation of food colouring components with different solvents
Intended class time
30-45 minutes
DfE Apparatus and Techniques covered
The codes used below match the OCR Practical Activity Learner Record Sheet (Chemistry /
Combined Science) and Trackers (Chemistry / Combined Science) available online. There is no
requirement to use these resources.
By doing this experiment, learners have an opportunity to develop the following skills:
4 [9]: Safe use of a range of equipment to purify and/or separate chemical mixtures including: iv)
chromatography
6 [11]: Safe use and careful handling of gases, liquids and solids, including: i) careful mixing of
reagents under controlled conditions
Links to Specifications:
Gateway Science (Suite A) – including Working Scientifically (WS)
C2.1g describe the techniques of paper and thin layer chromatography
C2.1h recall that chromatography involves a stationary and a mobile phase and that separation
depends on the distribution between phases [to include: identification of the mobile and stationary
phase]
C2.1i interpret chromatograms, including measuring Rf values [to include: the recall and the use of
the formula]
C2.1j suggest suitable purification techniques given information about the substances involved
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C2.1k suggest chromatographic methods for distinguishing pure from impure substances [to include
paper, thin layer (TLC) and gas chromatography]
WS1.2e evaluate methods and suggest possible improvements and further investigations
WS1.3a presenting observations and other data using appropriate methods
WS1.3b translating data from one form to another
WS1.3c carrying out and representing mathematical and statistical analysis
WS1.3e interpreting observations and other data
WS1.3f presenting reasoned explanations relating data to hypotheses
WS1.3g evaluating data in terms of accuracy, precision, repeatability and reproducibility
WS1.3h identifying potential sources of random and systematic error
WS1.4a use scientific vocabulary, terminology and definitions
WS2a carry out experiments
WS2b make and record observations and measurements using a range of apparatus and methods
WS2c presenting observations using appropriate methods to include descriptive, tabular
diagrammatic and graphically
WS2d communicating the scientific rationale for investigations, methods used, findings and
reasoned conclusions
Twenty First Century Science (Suite B) – including Ideas about Science
(IaS)
C5.1.4 recall that chromatography involves a stationary and a mobile phase and that separation
depends on the distribution between the phases
C5.1.5 interpret chromatograms, including calculating Rf values
C5.1.6 suggest chromatographic methods for distinguishing pure from impure substances [including
the use of a) paper chromatography, b) aqueous and non-aqueous solvents, c) locating agents]
C5.1.8 suggest suitable purification techniques given information about the substances involved
IaS1.8 use appropriate scientific vocabulary, terminology and definitions to communicate the
rationale for an investigation and the methods used using diagrammatic, graphical, numerical and
symbolic forms
IaS2.1 present observations and other data using appropriate formats
IaS2.4 be able to translate data from one form to another
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IaS2.8 when analysing data identify patterns/trends, use statistics (range and mean) and obtain
values from a line on a graph (including gradient, interpolation and extrapolation),
IaS2.9 in a given context evaluate data in terms of accuracy, precision, repeatability and
reproducibility, identify potential sources of random and systematic error, and discuss the decision to
discard or retain an outlier
IaS2.10 evaluate an experimental strategy, suggest improvements and explain why they would
increase the quality (accuracy, precision, repeatability and reproducibility) of the data collected, and
suggest further investigations
IaS2.11 in a given context interpret observations and other data (presented in diagrammatic,
graphical, symbolic or numerical form) to make inferences and to draw reasoned conclusions, using
appropriate scientific vocabulary and terminology to communicate the scientific rationale for findings
and conclusions
Mathematical Skills covered
No specific mathematical skills are covered in this activity.
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Technical Requirements – PER GROUP
Chemicals
Identity
Approximate
quantity
required or
produced
PER GROUP
Hazard information
food
colouring
< 1 cm3
isopropanol
solvent
c. 10 cm3
WARNING: Flammable, irritant
(skin, eye)
c. 10 cm3
DANGER: Highly flammable.
Harmful if swallowed. Irritant (eye).
May cause damage to organs
surgical
spirit
(ethanol)
acetone
based nail
varnish
remover
c. 10 cm
Not currently classified as hazardous
3
Risk
information
Concentrated
pastes can
temporarily
stain the skin
Ensure there
are no naked
flames present
– handle
solvents with
care and
dispose of
responsibly.
DANGER: Highly flammable.
Irritant (eye). May cause
drowsiness or dizziness.
Equipment









three food colourings solutions/gels [look for products that contain some of the same
colouring agents – e.g. E133 – brilliant blue, E110 sunset yellow, E102 tartrazine – e.g.
Colour Splash Concentrated food colouring] – avoid powder colourings
three small containers, e.g. egg cups
coffee filters – preferably white
small drinking glass
large measuring spoon
fine tipped paint brush
pencil
ruler
bull-dog clip
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Notes
Most homes contain common solvents in a variety of forms. For example, isopropyl alcohol
(isopropanol) can be found in rubbing alcohol or electronics cleaning solvent (e.g. Maplin RE79L), or
cleaning liquids in nail varnish kits. Ethanol is the major component of surgical spirit, used for
cleaning and disinfecting skin. Acetone is the major component of some nail varnish removers
(check the label carefully).
Health and Safety
Avoid using powdered food colourings.
If selecting other solvents, ensure they are properly risk assessed by a competent person.
Many non-aqueous solvents are highly flammable – ensure there are no naked flames or other
sources of ignition present when carrying out this activity.
Wear eye protection during practical work.
Ensure any kitchen apparatus used in the experiment is thoroughly cleaned after use
Method
This method makes use of apparatus and reagents likely to be available in the home / alternative
provision establishments. Despite this, the separation of mixtures by chromatography can be readily
demonstrated, and provides a useful stepping stone to higher quality chromatography and
measurement of Rf values.
Learner should be aware of the equipment that would be used in a standard laboratory
chromatography practical – see questions.
Images from trials
Figure 1: Images from the trial
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Analysis of results – trial results
You can draw your own table, or use the one below:
Figure 2: Chromatograms of ‘Colour Splash – Concentrated food colouring – Sunflower, Peacock
and Dark brown’ in i) tap water, ii) surgical spirit, iii) rubbing alcohol, iv) nail varnish remover
1.
Describe qualitatively the difference using different solvents had on the separation
Answers will depend on the food colourings used.
In water, all food colours produced smeared chromatograms. There was some separation
of a yellow and orange colouring in Sunflower. In Dark brown, the purple colour did not
move significantly, while the yellow moved to the solvent front.
In surgical spirit, there was very little separation from any of the food colouring, with large
smears produced for each colour.
In rubbing alcohol, yellow and orange colour moved about the same distance, with the
yellow moving slightly further. There was some separation in Dark brown between the
yellow/orange and purple. There was much less smearing compared with water and
surgical spirit.
In acetone there was not real separation of the coloured components.
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Extension opportunities
1.
Describe which equipment you would use to carry out this practical in a standard
chemistry laboratory.
In place of a glass, a beaker could be used.
In place of the coffee filter, a (Whatman No. 1) filter paper could be used.
In place of the egg cuts, a multi-well plate could be used.
In place of the paint-brush a fine tipped capillary tube could be used.
2.
Explain why using your chromatograms would make calculation of accurate Rf
values difficult.
The spot on the chromatogram are in general very smeared, so it would not be possible
to accurately identify the spot centre.
There is also significant overlap of the spots in some of the chromatograms.
3.
Describe how you could use a different chromatographic technique to produce
improved chromatograms.
In general, chromatograms from thin layer chromatography have better resolved spots
(less smearing).
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Document updates
v1.1
February 2017
Published on qualification page
v1.2
June 2017
Updated hazard and risk information
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Chemistry PAG 3: Separation techniques
Combined Science PAG C3: Separation techniques
Suggested Activity 3: Kitchen chromatography
Learner Activity
Introduction
In this activity you will refresh your knowledge of chromatography which you are likely to have
studied in Key Stage 3 and prior.
You will separate a range of food colouring dyes by paper chromatography, making use of solvents
usually available in the home. By varying the proportion of water to organic solvent, you can observe
different separation of the coloured components of the food colouring.
Aims
To separate the coloured components of food colouring using commonly available solvents.
Intended class time
30 minutes
Chemicals and equipment (per group)










food colourings solutions/gels [look for products that contain some of the same colouring
agents – e.g. E133 – brilliant blue, E110 sunset yellow, E102 tartrazine – e.g. Colour Splash
Concentrated food colouring] – see note below
household care / cleaning solvents, e.g. surgical spirit, rubbing alcohol, nail varnish remover
(DANGER: flammable, harmful, irritant) – see note below
non-plastic containers, e.g. ceramic egg-cups
coffee filters – preferably white
small drinking glass
large measuring spoon
fine tipped paint brush
pencil
ruler
bull-dog clip
Health and Safety

Avoid using powdered food colourings.

If selecting other solvents, ensure they are properly risk assessed by a competent person.
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
Many non-aqueous solvents are highly flammable – ensure there are no naked flames or
other sources of ignition present when carrying out this activity.

Wear eye protection

Ensure any kitchen apparatus used in the experiment is thoroughly cleaned after use.
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Method
STAGE 1: Setting up the chromatogram
1. Cut enough chromatogram papers from the coffee filter for each of the solvents you are
using (plus one for water) - about 5 cm wide and about as tall as your glass
2. Draw pencil lines about 2 cm from one end of your pieces of paper
3. Mix a small amount of the food colouring in about the same volume of water in a small
container (e.g. egg cup)
4. Using a fine paint brush, paint small spots of the food colouring mixture onto the pencil lines
and gently blow to dry the spots. Repeat once more.
5. Repeat Steps 3 and 4 with the remaining food colourings.
STAGE 2: Running the chromatogram
6. Add about 1 cm of water into the glass
7. Using the bulldog clip, attach the top of your paper to the pencil, and lower the paper into the
flask – ENSURE the top of the water is BELOW the pencil base line.
8. Allow the water to soak up the paper until about 1 cm from the pencil.
9. Remove the paper from the glass, and mark the highest point of the solvent with a pencil
line.
10. Place the chromatogram on a piece of absorbent towel until dry.
11. Repeat stage 2 with the remaining solvents.
Figure 1: Experimental set-up for running the paper chromatography
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Analysis of results
Take a picture of the three chromatograms you ran and stick them in below:
Your ability to analyse your observations may depend on how much of the GCSE
Chemistry/Combined Science course you have studied. Your teacher will let you know which
questions you should focus on:
1.
Describe qualitatively the difference using different solvents had on the separation
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Extension opportunities
1.
Describe which equipment you would use to carry out this practical in a standard
chemistry laboratory.
2.
Explain why using your chromatograms would make calculation of accurate Rf
values difficult.
3.
Describe how you could use a different chromatographic technique to produce
improved chromatograms.
DfE Apparatus and Techniques covered
If you are using the OCR Practical Activity Learner Record Sheet (Chemistry / Combined Science)
you may be able to tick off the following skills:
Chemistry
4-iv
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Combined Science
9-iv
11-i
6-i
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