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Glass
Scenario:
A body has been discovered by the side of the
road – it appears to have been struck by a
car. Upon closer examination, fragments of
glass are found embedded in the skin. It is
possible that these fragments may be from
the windshield of a vehicle, or could come
from another source, such as glass from
broken bottles thrown along the roadside.
Objective:
To determine if the glass recovered on the
victim is (uniquely) consistent with
windshield glass.
Problem: What methods can we use to distinguish
between types of glass?
Density (d=m/v)
Refractive index (RI)
Objective:
To measure the density of you glass sample
(A, B, C) and see which (if any) correspond
to windshield glass (or if we can tell).
Problem: What procedure(s) can we use to measure
the density of glass fragments?
Displacement (one or more pieces?)
Flotation
Objective:
To measure the density of you glass sample
(A, B, C) and see which (if any) correspond
to windshield glass (or if we can tell).
Problem: Which density method is best? How can
we tell?
Precision
Accuracy
Accuracy and Precision
• Accuracy can be
determined from
%error
% Error 
Experiment al Value - Accepted Value
Accepted Value
n
s

• Precision can be
determined from
standard deviation, s
( xi  x) 2
i 1
n 1
xi  individual data point
x  the mean of the data
n  the number of data points
X 100
Concepts covered/things to consider
•
•
•
•
•
•
•
Density
Precision
Mean (average) values
Standard deviation
Accuracy
Percent Accuracy
Significant figures
Writeup (Short written report)
• Objective
• Procedure
• Results
– table (samples A, B, C)
– sample calcs for density (one from each method
with correct sig figs!), stdev (one method, your
3 values), % error (one method, your 3 values)
Writeup (Short written report)
• Objective
• Procedure (describe steps foreeach method)
• Results
– table (samples A, B, C)
– sample calcs for density (one from each method
with correct sig figs!), stdev (one method, your
3 values), % error (one method, your 3 values)
[use 2.47 g/mL for windshield glass dens.]
• Discussion Conclusion – answer the following
questions:
– 1. Which method was most precise? How do you
know?
– 2. Which method was most accurate? How do you
know?
– 3. How does the number of sig. figs. determined for
your density values relate to the accuracy and precision
of the method? (see also question 4)
– 3. An analytical balance can measure mass in grams to
the fourth decimal place. For which method (if any)
might this have been useful? Why or why not?
– 4. What kind of glass was your sample (see table 2.3)?
Can you tell which sample is consistent with glass from
a car windshield? Why or why not?
– 5. Are there any other variables that you think are
important to the lab? How might it be improved?(if
something went wrong, what and why)