Unit 1
How do we analyze
a system?
Chemistry XXI
Separating Substances
Chemistry XXI
Lab Session 3
How do we separate
substances?
The Motives
Chemical thinking allows us to take
advantage of the characteristic properties of
substances to separate the different
components of materials.
Chemistry XXI
Why would we want to separate substances?
Identification
Characterization
Elimination
Quantitation
Application
Transformation
The Challenge
Imagine that you work for a clothing company
interested in using natural pigments to dye fibers.
Chemistry XXI
Your task is to devise the most efficient
method to extract and separate the pigments
in a colored natural product (chili see pods).
What would you propose
to do to a) extract and
b) separate the pigments?
Share Ideas
How can we extract the pigments from a
natural product? Why?
Chemistry XXI
Simple immersion in
a solvent vs.
grinding?
Pigments are located in chloroplast walls
inside cells. Solvent can’t get to them. Need to
break down cell walls.
A Useful Model
What solvent could work best to separate
components? Why?
Chemistry XXI
Many chemical separations are based on
differences in the strength of the interactions
between particles of different substances.
To Keep in Mind
The solubility of one substance in another
depends on the relative strength of the
attractive forces between their particles
AA , BB >> AB
Chemistry XXI
AA, BB <= AB
Insoluble, Immiscible
Soluble, Miscible
In general,
LIKE DISSOLVES THE LIKE
How do you propose to separate the
pigments in the extract?
Separating Components
To separate the different components in your sample
you will use Thin Layer Chromatography.
Chemistry XXI
Separation based on unequal attractions
between the different components of a mixture
an a stationary phase.
Mobile phase
Stationary
Support
Chromatography
Chemistry XXI
With the proper selection of stationary and mobile
phases you can separate materials based on
intermolecular forces.
Gas chromatography
Chromatography
Thin Layer Chromatography (TLC)
Chemistry XXI
What is it?
TLC Strip
Support and
Stationary
Phase
Developing jar
Chromatography
Thin Layer Chromatography (TLC)
 What’s the mobile phase?
 What’s the stationary phase?
Chemistry XXI
 What makes the mobile phase
move?
Running
Spotting
Chromatography
Chemistry XXI
In TLC, the selection of the solvent used as the
“mobile” phase should be made very carefully to
induce the best separation possible.
The substances most
attracted to the
solvent will be the
ones moving faster
through the plate
(carried by the
solvent).
Different combinations of
solvents induce different
degrees of separation.
Which one is best?
Chromatography
Rf
Retention
Factor
Chemistry XXI
Distance traveled by the substance
Rf 
Distance traveled by the solvent
Different substances are expected to have different
Rf values. Thus, the Rf can provide corroborative
evidence as to the presence of different
components in a sample.
Skill Development
Chemistry XXI
Extract and separate the pigments from
spinach leaves.
Available resources:
 Solvents: Acetone, hexane.
 Pigment sources: Spinach leaves;
 TLC strips
 Others: Sand, mortar and pestle, glassware.
You have
60 minutes
Claims, Evidence, Reflections
Share your results for the separation of the
pigments in spinach leaves with your
classmates:
Chemistry XXI
 How successful were you in the
separation?
 What problems did you encounter?
 What suggestions do you have to improve
the separation?
Your Real Challenge
Your task is to devise the most efficient
method to extract and separate the pigments
in a colored natural product (chili see pods).
Chemistry XXI
Discuss in your group
what you propose to do.
What questions do you need to answer in
order to succeed in this task?
Your Real Challenge
Design and implement an experimental
procedure to extract and separate the
pigments in chili pods.
Chemistry XXI
Available resources:
 Solvents;
 TLC strips;
 Glassware.
You have
60 minutes
Claims and Evidence
Based on the results of your experiments,
present your major claims and the evidence that
you have to support the answer to your
beginning questions.
 Did the group answer their
beginning questions?
Chemistry XXI
 Are their claims clear?
 Is the evidence reliable?
 Is the evidence appropriate to
support the claims?
Final Reflections
 What did you learn from doing your
experiment?
 How would you improve what you did?
Chemistry XXI
 How have your ideas changed as a result
of this lab?
 What do you not completely understand?
 What new questions do you have?
Your Report
 Beginning questions (2 p):
What questions guided your explorations?
 Safety Considerations (2 p):
What did you do to stay safe in the lab?
Chemistry XXI
 Procedures and Tests (2 p):
What experiments did you do to answer your
questions?
 Data, calculations, and representations (6 p):
What observations did you make?
What data did you collect?
What calculations and representations helped
you make sense of the data?
Your Report
 Claims (2 p):
What can you claim to answer your questions?
Chemistry XXI
 Evidence and Analysis (6 p):
How did you interpret your results to support
your claims?
 Reflections and additional questions (10 p):
What did you learn?
What do you not completely understand?
How have your ideas changed as a result
of this lab?
What new questions do you have?
How would you improve what you did?
Your Report
 Post-Laboratory Questions:
1. Is it practical to use TLC to separate pigments
from natural products? Explain.
Chemistry XXI
2. What other separation techniques may you use to
face a similar challenge to the one posed in this
experiment?
3. How would you use the particulate model of
matter to explain a major result from your
experiment?
Chemistry XXI
Our Next Lab
How do we use models
to derive properties?
The Challenge
Chemistry XXI
Three volatile solvents have been found in
different containers in an illegal drugs lab.
Based on prior experiences, you infer the
potential identity of these substances:
Butanone
C4H8O
Ethanol
C2H6O
Hexane
C6H14
How can you determine the identity of the
solvent in each container?
BRING A LAPTOP WITH EXCEL