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Unit 1

How do we analyze a system?

Searching for Differences

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Unit 1 of your General Chemistry course, both in lecture and lab, will be focused on

helping you understand how to go about separating and identifying

the chemical substances present in systems of interest.

Unit 1: Overall Goal

1. How do we identify an unknown substance?

2. How do we separate substances?

3. How do we use models to differentiate substances?

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Lab Session 2

How do we identify an unknown substance?

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Initial Ideas

In your own words:

What are the central goals or purposes of the experiment you will be doing today?

What knowledge and skills will help you be successful in your work today?

What do you expect to learn by doing this experiment?

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Chemical thinking and practices are very useful in determining the identity and analyzing the

properties of chemical substances.

Detection: Does the sample contain

substance X?

Identification: What is the identity of the substance in this sample?

Central Goals and Questions

Separation: How can the substances present in the sample be

separated?

Quantitation: How much of substance X

is in the sample?

The Motives

These are the types of questions we want to answer.

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Imagine that you are employed by a recycling firm.

You have been assigned to devise a quick and cheap method for identifying

certain type of plastic.

The Challenge

What beginning questions do you need to answer to

complete your job?

Given several samples of the same unknown plastic:

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Beginning Questions

Examples:

What is the identity of the unknown plastics?

What properties can be measured to distinguish one type of plastic from another?

How do we best measure these properties (accuracy and precision)?

What are the values of the properties for known and unknown plastic samples?

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The Resources

Pure samples of known plastics.

Balances (TC 4).

Vernier calipers.

Glassware

Samples of different sizes of an unknown plastic.

These are the resources you have available to complete your task:

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Initial DecisionsGiven the resources available, discuss in your groups how you propose to answer the central

question for this lab:

What is the identity of the unknown plastic?

What differentiating characteristics would you use?What will you measure?

How will you measure it?What would you do with these measurements?

How will you stay safe?

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Reference ValuesLet′s explore!

As a class, decide which groups will be determining the value of the property that

differentiates the different reference plastics.

What should each group do?How can you ensure reproducibility of

measurements?How can you determine what technique is best?

You have 30 minutes

Available resources: Pure samples of known plastics. Balances (TC 4). Vernier calipers. Glassware.

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The tools you use needs to measure the property that you want (length, volume, etc.) The tools need to measure in the range you need (accuracy). The tool needs to meet the precision you need (tenths, hundreths, etc.) The measurements need to be repeated. The measurements need to be reported taking into account the precision of your tools.

To Keep in Mind

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Initial resultsReport your results for the initial tests on the board.

Plastic

PETE

HDPE

V

LDPE

PP

PS

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Your Reflections

Discuss the variation in the data and the way the data are reported. How do you explain the

differences? Why did you chose to report the numerical data as you did?

Let′s think!

Did you use the right measuring tools? Did you use them properly?

Did you report your results properly?

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Plastic Density

(g/cm3)

1.29-1.4

0.952-0.965

1.30-1.58

0.917-0.940

0.900-0.910

1.04-1.05

PETE

HDPE

V

LDPE

PP

PS

Some ReflectionsLet′s think!

The density of commercial plastics may vary from one sample to another

depending on a variety of factors

(synthesis methods, processing, etc.)

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Design and apply a procedure to determine the identity of the of unknown

plastics provided.

Based on what you have learned:

Face Your Real Challenge

You have 40 minutes

Available resources: Pure samples of known plastics. Balances (TC 4). Vernier calipers. Glassware. 3 samples of different sizes of an unknown plastic (you may have to share with other groups)

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Claims and Evidence

Sample 1 2 3

Mass (g)

Volume (mL)

Density

Identity

Report your claims about for the identity of each plastic on the board.

Present and discuss the evidence that you have to support your claims.

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Discuss how the results obtained by the different groups compare with each other? Which are most credible? Which are least? What is your evidence?

What were the strengths and weaknesses of the methods you used to identify a plastic?

How could you avoid or reduce the difficulties that you encountered?

Final Reflections

Let′s think!

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Final Ideas

In your own words:

What were the central goals or purposes of the experiment you did today?

What knowledge and skills would have helped you be more successful in your work today?

What did you learn by doing the experiment today?

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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:

1. Is it practical to use density to identify plastics? Explain.

2. Propose another way to identify plastics making use of a property different from density.

3. Of the measurements you made, which were more precise; the mass measurements or the volume measurements? Why?

4. In your own words, what was the main purpose of this lab experiment.

Your Report

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Our Next Lab

How do we separate substances?

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The Challenge

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

Your task is to devise the most efficient method to separate the pigments in a colored natural product

(such as green leaves, a vegetable, or a fruit).

Write your ideas in your notebook before you come to lab.

What would you propose to do?

What questions do you need to answer to complete your task?