Activity—Unknown Substance
Name ____Class Set________________
Physical Science/MTHS
The physical properties of substances belong to that substance and that substance alone. So far we have
investigated density, melting and boiling points. Now you will put your skills into identifying a mystery substance.
Each team will conduct several lab investigations designed to collect evidence to support a claim. Each team will
share data with the other teams in place of multiple trials.
Unknown Substance Part I: Calculating Density
Safety Precautions:
 Wear googles for measuring out liquid
 Keep long hair and loose clothing pulled back.
 Wash hands after handling substance
 Return used substance to container marked Waste.
Materials
 Unknown substance
 Graduated cylinder
 pipette
 Scale
 Waste container
Procedure:
(1) Obtain a clean, dry 10-ml graduated cylinder and determine its mass to the nearest 0.1 grams
(2) Record in Part 1 data table.
(3) Fill the graduated cylinder so that the bottom of the meniscus is even with the 10-ml mark in the graduated
cylinder. Note: if you are above or below the line add drops of the unknown liquid with a pipette until the
liquid level is even with the 10-ml line.
(4) Note any observations concerning the unknown liquid’s color, odor and appearance.
(5) Re-weigh the graduated cylinder with the unknown liquid inside and record the mass of the cylinder and
unknown liquid in your density data table.
(6) Calculate the density (in g/ml) of the unknown liquid and add your data to the class data table.
(7) Copy the class data onto your data table & calculate average density of the unknown.
(8) Compare your calculated value with the known values in the known density data table.
(9) Complete Part 1 questions/task.
p. 1
Unknown Substance Part II: Measuring Boiling Point
Safety Precautions:
 Never use direct heat an unknown substance. Always
place sample in a hot water bath.
 Use a boiling chip to prevent dangerous boil over.
 Wear googles while heating and/or handling liquid.
 Keep long hair and loose clothing pulled back.
 Closed shoes only!
 Wash hands after handling substance.
Materials
 Test tube with unknown substance





Test tube rack
Test tube holder
Boiling chip
Thermometer
Hot plate
Procedure:
(1)
(2)
(3)
(4)
(5)
Fill a 250 ml beaker with about 150 ml of water.
Place the beaker on a hot plate and turn the hot plate on to highest setting.
Obtain test tube with unknown substance.
Place one boiling chip into the test tube. The chip will keep liquid from flash boiling.
Wait until the water begins to boil and then turn the dial reading to a low setting and place the test tube (with
thermometer) into the hot water bath.
(6) Carefully observe the unknown substance until it boils vigorously and record the boiling point of the unknown
liquid in a data table. (CAUTION: the chemical will produce vapor that is very hot and unhealthy to breath.
Do not let the chemical totally evaporate for this reason).
(7) Using a test tube holder pick up the test tube and discard any extra liquid (with the boiling chip) into the
designated waste container.
(8) Copy your data onto the class data table.
(9) Record the class data onto your data table & calculate the average boiling point.
(10) Compare your calculated value with the known values in the known boiling point data table.
(11) Complete Part II questions/ tasks.
p. 2
Unknown Substance Part III: Freezing Point
At this point you are probably feeling more confident about the identity of the unknown substance, but you still
get to do one more test to give you more certainty about the identity of the substance. We’ll test the melting
point/freezing point of the unknown liquid.
Safety Precautions:
 Wear googles for handling liquid.
 Keep long hair and loose clothing pulled back.
 Wash hands after handling substance.
Materials List
 250 mL beaker
 Ice
 Thermometer
 Test tube with unknown substance
Procedure:
(1) Prepare an ice bath in your 250 ml beaker by filling the beaker about ½ full with ice and then filling the beaker
with water up to the 150 ml line.
(2) Place about (does not need to be precise) 5 ml of your unknown liquid in the large test tube and place the clean
and dry thermometer in the substance.
(3) Allow the thermometer to adjust to the temperature of the liquid and then place the test tube in the ice water
bath so that you can carefully watch the temperature.
(4) Note the temperature on the thermometer when the temperature remains fairly constant. This is the
freezing/melting point of the liquid. Record this temperature in a data table and share your data with at least
two other groups and calculate an average freezing/melting point.
(5) Remove the thermometer from your sample and return test tube where instructed.
(6) Copy your data onto the class data table.
(7) Record the class data onto your data table & calculate the average boiling point.
(8) Compare your calculated value with the known values in the known boiling point data table.
p. 3
UNKNOWN SUBSTANCE RESEARCH
Date ________ Test tube # _____
Research Team Members: _______________________________________________________________________
Initial Observations:
Appearance: _____________________________________________________________
Odor: __________________________________________________________________
PART I: Density
Density Measurements:
Class data
Team
Density
(g/mL)
1
Our data
Mass of 10 mL graduate cylinder
empty (g)
Mass of 10 mL graduate cylinder
with substance (g)
Mass of substance (g)
2
3
Volume of substance (mL)
4
Density of substance (g/mL)
5
Average Density of Trials 1-7
6
(include units and sig figs.)
7
Average
Analysis:
Our Density Calculation: Include the equation, the values plugged into the equation and proper units with the correct
number of significant figures.
Average Density of Class data Calculation: Include the equation, the values plugged into the equation and proper
units with the correct number of significant figures.
Which 5 substances could match the density you calculated for the unknown liquid? Why did you choose these?
If you were a chemist whose job depended on identifying this chemical, how confident are you that you correctly
identified the unknown liquid? On a scale of 1-10, 10 being 100% sure, rank your confidence and, with several
sentences, explain why you chose the number (on the scale of 1-10) that you did.
p. 4
PART II: Boiling Point
Data Table—Boiling Point:
Our Boiling Point (oC)
Team
1
2
3
4
5
6
7
Class data
Boiling Point (oC)
Average
Analysis:
Identify at least 5 substances that COULD match the boiling point you measured for your unknown liquid and, with at
least one sentence, explain why you chose these substances as a possible match for the unknown.
Look back at the chemicals that you identified as possible matches for the density test. Were any of these chemicals
on your list for boiling point? If so, list them. If not, develop a list of 5 chemicals (either from the density test or the
boiling point test) and explain why you developed this list instead.
If you were a chemist whose job depended on identifying this chemical, how confident are you that you correctly
identified the unknown liquid? On a scale of 1-10, 10 being 100% sure, rank your confidence and, with several
sentences, explain why you chose the number (on the scale of 1-10) that you did.
p. 5
PART III: Freezing Point
Data Table—Freezing Point:
Our Freezing Point (oC)
Team
1
2
3
4
5
6
7
Class data
Freezing Point (oC)
Average
Conclusion: Answer the following question on a separate sheet of paper.
Writing as if you were a professional chemist, with one FULL page (minimum) identify the unknown chemical
from the last three labs. Explain, using as much evidence as possible, why you are certain that you correctly
identified the unknown chemical. Lastly, discuss any possible sources of error (at least two required) & how you
believe those errors affected your results. You should point to specific pieces of data to explain exactly how those
errors affected your data. For example, the known boiling point of water is 100 degrees Celsius and we
determined that the boiling point of the unknown was only 99.5 degrees Celsius. This means that an error must
have been present. That error was… (10 pts)
p. 6
Known Density Data Table:
Substance
Density (g/mL)
Substance
Acetic Acid
Acetone
t-butanol
s-butanol
Ethanol
Ethyl acetate
4-methyl-2-pentanone
Glycerol
1.049
0.792
0.789
0.807
0.789
0.901
0.80
1.261
Methanol
2-propanol
Toluene
Water
Pentane
Hexane
Methyl Salicylate
1,1-dimethoxyethane
Density
(g/mL)
0.796
0.785
0.867
1.00
0.626
0.66
0.87
0.847
Substance
Ethyl propyl ether
Benzaldyhyde
Methyl acetate
Nitrobenzene
2-pentyne
2-propanethiol
1-propanol
Trichloro- trifluoroethane
Known Boiling Points Data Table:
Substance
Boiling
Point (°C)
118.1
Substance
Substance
Acetone
Boiling
Point (°C)
56.5
2-propanol
Boiling
Point (°C)
82.4
t-butanol
82.8
Toluene
s-butanol
99.5
Ethanol
Acetic Acid
Substance
Methanol
Boiling
Point (°C)
64.6
Substance
Ethyl propyl
ether
Substance
Boiling Point
(°C)
64.0
Benzaldyhyde
Boiling Point
(°C)
178
110.6
Methyl acetate
57.1
Water
100
Nitrobenzene
210.8
78.5
Pentane
36
2-pentyne
56.0
Ethyl acetate
77.2
Hexane
69
2-propanethiol
60.0
4-methyl-2pentanone
Glycerol
115
Methyl
Salicylate
1,1dimethoxyethane
125
1-propanol
97.2
64.5
Trichlorotrifluoroethan
47.6
Substance
Freezing Point
(°C)
-79.0
182
Known Freezing Points Data Table B:
Substance
Substance
Acetic Acid
Freezing
Point (°C)
16.6
Methanol
Freezing
Point (°C)
-97.8
Acetone
-95.0
2-propanol
-98.1
Ethyl propyl
ether
Benzaldyhyde
t-butanol
25.5
Toluene
-95.0
Methyl acetate
-98.1
Substance
Substance
Water
Freezing
Point (°C)
0
Substance
s-butanol
Freezing
Point (°C)
-115
Nitrobenzene
Freezing Point
(°C)
5.7
Ethanol
-117.0
Pentane
-130
2-pentyne
-109.3
Ethyl acetate
-83.6
Hexane
-94
2-propanethiol
-130.7
4-methyl-2pentanone
Glycerol
-80
Methyl
Salicylate
1,1dimethoxyethane
-8
1-propanol
-127.0
-113.0
Trichlorotrifluoroethan
e
-35.0
20
p. 7
-26
Density
(g/mL)
0.747
1.0415
0.933
1.198
0.711
0.806
0.804
1.510
Conclusion Rubric
Conclusive statement correctly answers the experimental question or correctly states whether the
hypothesis/prediction was correct
A vague conclusive statement cannot be credited
A response with an incorrect conclusive statement or no conclusive statement may not be
credited any points
A response with both a correct and an incorrect conclusive statement cannot be credited
Supporting data: Should at minimum be over the entire range of the conditions investigated.
Density
Boiling Point
Melting Point
Explanatory language: Separate from the conclusive statement, is used to connect or compare the
supporting data to the conclusive statement. How does the data support the conclusion
Can only be credited when at least one numeric value or the text from a descriptive data table
for the manipulated or responding variable is included in the response.
A copy of the conclusive statement cannot be credited for explanatory language. However, a rephrased credited conclusive statement can be credited.
Source of Error: Identifies at least 2 plausible sources of error using specific evidence from the
data.
Human error must not include reading the measurement tool wrong or making calculation
mistakes.
A
18-20
B
16-17
C
14-15
D
12-13
F
5-11
Team Working Rubric
Active participant in class discussions.
A positive model for others.
Takes leadership role in getting work done and helping others learn.
Often participates in class discussions.
Positive role model for others
Often takes initiative to get work done, and works well with teammates.
Usually answers questions if called on in class discussions.
Usually works well with teammates, but is willing to let others do the work/ has difficulty
sharing the workload.
Often unable/unwilling to participate in class discussions.
Often talking at inappropriate times.
Tends to socialize or be off task instead of working on assignments.
Frequently disrupts learning with comments or behavior.
Often talking at inappropriate times.
Usually socializes or goofs off instead of working on assignments.
p. 8