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Unit 1

How do we analyze a system?

Separating Substances

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Lab Session 3

How do we separate substances?

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Chemical thinking allows us to take advantage of the characteristic properties of

substances to separate the different components of materials.

Why would we want to separate substances?

Identification Characterization

Elimination Quantitation

Application Transformation

The Motives

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The Challenge

Imagine that you work for a clothing company interested in using natural pigments to dye fibers.

What would you propose to do to a) extract and

b) separate the pigments?

Your task is to devise the most efficient method to extract and separate the pigments in a colored natural product (chili see pods).

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Share Ideas

How can we extract the pigments from a natural product? Why?

Pigments are located in chloroplast walls inside cells. Solvent can’t get to them. Need to

break down cell walls.

Simple immersion in a solvent vs.

grinding?

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Many chemical separations are based on differences in the strength of the interactions

between particles of different substances.

A Useful ModelWhat solvent could work best to separate

components? Why?

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The solubility of one substance in another depends on the relative strength of the attractive forces between their particles

AA , BB >> AB Insoluble, Immiscible

AA, BB <= AB Soluble, Miscible

To Keep in Mind

In general,

LIKE DISSOLVES THE LIKE

How do you propose to separate the pigments in the extract?

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To separate the different components in your sample you will use Thin Layer Chromatography.

Separation based on unequal attractions between the different components of a mixture

an a stationary phase.

Mobile phaseStationary

Support

Separating Components

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With the proper selection of stationary and mobile phases you can separate materials based on

intermolecular forces.

Gas chromatography

Chromatography

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Thin Layer Chromatography (TLC)

TLC Strip

Support and Stationary

Phase

Developing jar

What is it?

Chromatography

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Spotting

Thin Layer Chromatography (TLC)

What’s the mobile phase?

What’s the stationary phase?

What makes the mobile phase move?

Chromatography

Running

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ChromatographyIn 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?

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Rf

Retention Factor

Chromatography

solvent by the traveledDistance

substance by the traveledDistancefR

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.

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Skill Development

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

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Claims, Evidence, Reflections

Share your results for the separation of the pigments in spinach leaves with your

classmates:

How successful were you in the separation?

What problems did you encounter?

What suggestions do you have to improve the separation?

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Discuss in your group what you propose to do.

Your task is to devise the most efficient method to extract and separate the pigments in a colored natural product (chili see pods).

Your Real Challenge

What questions do you need to answer in order to succeed in this task?

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Your Real Challenge

Design and implement an experimental procedure to extract and separate the

pigments in chili pods.

Available resources:

Solvents;

TLC strips;

Glassware.

You have 60 minutes

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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?

Are their claims clear?

Is the evidence reliable?

Is the evidence appropriate to support the claims?

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What did you learn from doing your experiment?

How would you improve what you did?

How have your ideas changed as a result of this lab?

What do you not completely understand?

What new questions do you have?

Final Reflections

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Beginning questions (2 p): What questions guided your explorations?

Safety Considerations (2 p):What did you do to stay safe in the lab?

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

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Claims (2 p): What can you claim to answer your questions?

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

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Post-Laboratory Questions:

Your Report

1. Is it practical to use TLC to separate pigments from natural products? Explain.

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?

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Our Next Lab

How do we use models to derive properties?

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The ChallengeThree 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