Objective(s): The density of several unknown types of glass were measured using two
different methods and compared to known values for different types of glass to see if the
glass could be identified as being uniquely consistent with glass from a car windshield.
In addition, the accuracy and precision of the density methods used could be compared.
Procedure:
Displacement Method: The mass of either one or three pieces of glass of type A, B, or C
was determined. The glass was added to a graduated cylinder containing a previously
measured volume of water. After addition, the volume in the graduated cylinder was
recorded again. This process was repeated two more times using the same type of glass.
Flotation Method: A piece of glass of the same type as used for the displacement method
was placed in a vial along with a magnetic stir bar. Enough LST heavy liquid was added
so the glass was floating freely, well away from the stir bar. While stirring vigorously,
water was added dropwise from a Pasteur pipette. The stirrer was turned off periodically
to check if the piece of glass was no longer floating. If too much water was added and
the glass sank to the bottom, LST heavy liquid was added. This process was repeated
until the glass was seen to be evenly suspended in the liquid. At this point, a 2.000 mL
aliquot of the resulting solution was withdrawn using a volumetric pipette, and allowed to
drain into a previously tared Erlenmeyer flask on a balance. The mass of the solution
was recorded. The solution in the flask was poured back into the original vial, stirred,
and the mass of 2.000 mL was determined two additional times.
Results:
Sample calculation: Displacement
volume displaced: 5.3 mL final
- 5.0 mL initial
= 0.3 mL
Sample calculation: Flotation
d
d
m 0.603g

 2.01g / mL  2 g / mL
v
0.3mL
m
4.960 g

 2.480 g / mL
v 2.000 mL
Table 1: Densities of Glass Fragments A, B, and C, as Determined by
Displacement and Flotation methods.
Method
Density,
g/mL
Average
Standard
Deviation
known value
%Error
A Float
A 1Pc
A 3pc
B Float
B 1Pc
B 3Pc
C Float
C 1Pc
C 3Pc
2.196
2.189
2.193
2.246
2.245
2.244
2
3
2
2
2
2
2.567
2.583
2.584
2.478
2.378
2.523
2.540
2.544
2.492
2.535
2.555
2.436
2.481
2.456
2.482
2.487
2.476
2
3
2
2
2
3
0.8
2
2
2
2
2
2
2
2
2
2
2.470
2.385
2.385
2.418
2.441
2.401
2.502
2.511
2.528
2.440
2.439
2.436
2
2
2
2
1.1
3
2.6
2.6
2.6
3
3
3
2.219
2
2
2.506
2
2.446
2
3
0.02876
2.23
0.5007
0.6
2.23
5
0.0
2.23
1.E+01
0.05539
2.47
1.4456
0.5
2.47
2.E+01
0.04779
2.47
0.9582
0.6
2.47
2.E+01
0.2
2.47
1.E+01
Discussion/Conclusion
1. Which method was the most precise? How do you know?
In general, the flotation method appears to be more precise than the displacement method
because the value for the standard deviation is smaller. However, in one case, the
displacement method (A, 3pc) had a standard deviation of 0. This is apparently
coincidental. Also, since this method is good to only 1 significant figure, the standard
deviation may be smaller.
2. Which method was the most accurate? How do you know?
The flotation method was much more accurate than the displacement method - the
percent error values were much smaller.
3. How does the number of sig. figs. determined for your density values relate to the
accuracy and precision of the method?
Using more significant figures certainly lead to better accuracy. However, the use of
only 1 significant digit can sometimes give the appearance of the method having a high
degree of precision (if there is no difference to 1 sig fig) but this is an artifact of only
making the measurement to one significant figure.
4. 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?
Using an analytical balance would not have improved the accuracy or precision of the
method since the volume can only be measured to 2.000 mL, or 4 significant digits. The
use of a balance that could be used to measure the mass to 4 decimal places would give,
in our case, at least 5 significant digits since the mass was 5.XXXX grams. The balance
we used already gives 3 decimal places (4 sig. figs) which is sufficient.
5. 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?
By comparision of the results to table 2.3, it appears that sample A was borosilicate, and
samples B and C were soda glass. Given that the car windshield was actually sample C,
density is not sufficient to make an absolute determination. However, if the density of
the sample were not the same as the soda glass, one could determine if the sample were
not from a windshield. It might be possible to further narrow down which of the two
samples was actually from a windshield by looking at how the glass fractures. (see also
#6 below)
6. Are there any other variables that you think are important to the lab? How might it be
improved?
Temperature could be a factor in determination of the density, although the density of a
solid does not change much with changes in temperature, so comparison to the table is
probably valid. Using larger volumes to measure (5.000 mL) instead of 2.000 mL might
improve the accuracy of the method. Another possibility would be to measure the
refractive index of the glass and use this as additional conformation to determine the type
of glass. Examination of the morphology (i.e. shape of the pieces and how they
fragment) could be useful as well.
Notes for Glass Density Lab
1. Equipment
Graduated cylinders (10 mL or 25 mL)
Volumetric pipettes (2 mL) with green or red bulbs.
Large vials with caps and stir bars (1 per student/group)
Stir Plate
Balance
LST Heavy Liquid
Forceps – straight for taking out stir bars.
Broken Glass Pieces
2. Procedure
Discuss with students possible ways of identifying type of glass
Refractive Index (we won't be doing this, but it's a good way)
Density
getting density - ways to get most precise and accurate data
Show students how to use balances.
-Displacement
Students may mention that using more pieces of glass may improve the situation
Size/shape of graduated cylinder may be an issue as well
Take three measurements - dry off glass in between, clean cylinder (don't have to
though if tare with a few drops in it.)
-Flotation
Prepare solution so glass is neutral buoyancy using H2O or LST as needed with
stirring. Take three measurements using volumetric pipette. Can pour liquid
back into vial on balance to get weight (or use beaker).
Go over use of lab notebook
-Report class data on excel data for A, B, C (displacement & flotation), use to calculate
mean (average), standard deviation, and % error - get class data table.
Go over operation of excel.
-Discuss class table in class
-Students take home and write up lab:
Objective
Procedure
Results
Table
Sample Calcs
Discussion/Conclusion
Answer specified questions
Correction For Glass Density Lab
I. Objective (2 ponts)
A. Measure density of several unknown types of glass to se if could identify [1]
B. Learn about accuracy/precision/sig figs. [1]
II. Procedure (2.5 points)
A. Displacement Method [1]
i. Massed 1/multiple pieces, measured volume
ii. Repeat 2 more times
B. Flotation Method [1.5]
i. Put glass in LST heavy liquid in vial, stir, add H2O until buoyancy
neutral.
ii. Measure density of resulting liquid by sucking 2.000 mL out with
volumetric pipette, taring vial, letting back in.
iii. Repeart part ii 2 more times
III. Results (6 points)
A. Attached excel file of data correctly labeled/calculated (s.f. if desired, not
essential here this time - I can explain about how to truncate data with cells) [.5]
B. Attached excel file of your data correctly labeled/calculated [.5]
B. Sample calculations
i. Density by displacement calc [0.5] with correct sig. figs [.5] and read to
nearest 0.1 ml or 0.05 mL [.5]
ii. Density by flotation calc [.5] with correct sig. figs [.5] and read to
nearest 0.01 ml [.5]
iii. Standard deviation (must get average here first) [1]
iv. Percent error [1] -.25 if wrong sf here
IV. Discussion/Conclusion (9 points)
A. Method that was most precise was determined based on standard deviation, discuss for
your data and class data [1.5]
B. Method that was most accurate was determined based on % error, discuss for your
data and class data [1.5]
C. More sig figs. may improve the accuracy, but sometimes you can get good
precision if only one sig fig b/c appears to be good reproducibility [1.5]
D. Using a balance to four decimal places would not have improved either method
because the best measuring device used (the volumetric pipette) was good to only
3 sig figs. [1.5]
E. Idenity of sample was (by comparing to table). Can tell definitively if not from a
windshield, but not beyond that because soda glass is used for other things. [1.5]
F. Other variables, possible sources of error, etc. [1.5]
V. Grammar/Spelling (0.5 points)
A. Used past passive or active, no imperative