Unit 1
How do we analyze
a system?
Chemistry XXI
Searching for Differences
Unit 1: Overall Goal
Chemistry XXI
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.
1. How do we identify an unknown substance?
2. How do we separate substances?
3. How do we use models to differentiate substances?
Chemistry XXI
Lab Session 2
How do we identify an
unknown substance?
Initial Ideas
In your own words:
 What are the central goals or purposes of
the experiment you will be doing today?
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 What knowledge and skills will help you
be successful in your work today?
 What do you expect to learn by doing this
experiment?
The Motives
Chemical thinking and practices are very useful in
determining the identity and analyzing the properties
of chemical substances.
Central Goals and Questions
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Detection:
Does the sample contain
substance X?
Identification:
What is the identity of the
substance in this sample?
Separation:
How can the substances
present in the sample be
separated?
Quantitation:
How much of substance X
is in the sample?
These are the types of questions we want to answer.
The Challenge
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.
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Given several samples of the
same unknown plastic:
What beginning questions
do you need to answer to
complete your job?
Beginning Questions
Examples:
What is the identity of the unknown plastics?
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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?
The Resources
These are the resources you have available
to complete your task:
 Pure samples of known
plastics.
 Balances (TC 4).
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 Vernier calipers.
 Glassware
 Samples of different
sizes of an unknown
plastic.
Initial Decisions
Given the resources available, discuss in your
groups how you propose to answer the central
question for this lab:
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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?
Let′s
explore!
Reference Values
As a class, decide which groups will be
determining the value of the property that
differentiates the different reference plastics.
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What should each group do?
How can you ensure reproducibility of
measurements?
How can you determine what technique is best?
Available resources:
 Pure samples of known plastics.
 Balances (TC 4).
 Vernier calipers.
 Glassware.
You have 30
minutes
Chemistry XXI
To Keep in Mind
 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.
Initial results
Report your results for the initial tests on the board.
Plastic
PETE
HDPE
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V
LDPE
PP
PS
Let′s think!
Your Reflections
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 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?
Did you use the right measuring tools?
Did you use them properly?
Did you report your results properly?
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Let′s think!
Some Reflections
The density of
commercial plastics
may vary from one
sample to another
depending on a variety
of factors
(synthesis methods,
processing, etc.)
Plastic
Density
(g/cm3)
PETE
1.29-1.4
HDPE
0.952-0.965
V
1.30-1.58
LDPE
0.917-0.940
PP
0.900-0.910
PS
1.04-1.05
Face Your Real Challenge
Based on what you have learned:
Chemistry XXI
Design and apply a procedure to
determine the identity of the of unknown
plastics provided.
Available resources:
 Pure samples of known plastics.
 Balances (TC 4).
You have 40
minutes
 Vernier calipers.
 Glassware.
 3 samples of different sizes of an unknown
plastic (you may have to share with other
groups)
Claims and Evidence
Report your claims about for the identity of each
plastic on the board.
Sample
1
2
3
Mass (g)
Volume (mL)
Density
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Identity
Present and discuss the evidence that you
have to support your claims.
Let′s think!
Final Reflections
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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 Ideas
In your own words:
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 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?
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?
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 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?
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 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 density to identify plastics?
Explain.
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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.
Chemistry XXI
Our Next Lab
How do we separate
substances?
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).
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What would you propose to do?
What questions do you need to
answer to complete your task?
Write your ideas in your notebook
before you come to lab.