Kerry Cornelisse
AP Chemistry Lab
Hour 4
Identification of Substances by Physical Properties
Introduction
Identification is important in many areas of life. While deer hunting one must be
able to determine what kind of deer it is before harvesting the deer. It has to be decided if
it is a buck or a doe, a fawn or adult deer, a buck with 2 points or 4. It also had to be
decided if the deer is really worth the time cleaning and processing it. To do this, traits
are used. It can be determined that a deer is a fawn if it has spots, it is small, or it is with
a doe. It can be decided that it is a buck if it has antlers, if it is a doe it will not have
antlers. It can be determined whether or not the deer is worth the time by the size and if
it has enough antlers. All these are traits used to determine and an unknown. Using traits
to determine an unknown is also used in chemistry. The unknown is the substance that is
not known and is being tested. The unknown can be compared to a known or something
that is already determined and qualities are already set. Traits used to identify unknown
substances are physical properties, or traits that can be determined without changing the
chemical composition of the sample. These physical properties include solubility, does a
sample dissolve in the solvent. Melting point is at what temperature the sample changes
to a liquid. Boiling point is at what temperature the sample changes to a gas. And finally
density mass per volume. Physical properties like these do not change and can be
compared to that of an unknown. If physical properties are accurately determined, then
an unknown substance may be identified by comparison of properties to a standard
reference
Materials and Methods
The first step done in the lab was that test tubes were found and some of them
were cleaned. Water, ethanol, and cyclohexane were obtained and poured into the test
tubes. Next, naphthalene was obtained and added to the 3 different solvents in the test
tubes. The results were recorded. The test tubes with the 3 solutions were cleaned out
and replaced with more water, ethanol, and cyclohexane. Samples of toluene were then
placed in the test tubes with the 3 solvents. Observations were taken and recorded. The
test tubes were then cleaned and dried. More samples of water, ethanol, and cyclohexane
were obtained and placed in the clean test tubes. The unknown liquid was also obtained
and added to the solvents. Observations were taken and recorded. The test tubes were
cleaned and dried. The next step was to obtain more water, ethanol, and cyclohexane.
The unknown solid was obtained and added to the samples. Observations were taken and
recorded. The next step was a sample of the unknown solid was taken and the mass was
measured. A sample of cyclohexane was taken and placed in a graduated cylinder. The
volume of the sample of cyclohexane was measured and then the unknown solid sample
was added to the cyclohexane. The volume of the cyclohexane and unknown solid was
recorded. Then, the graduated cylinder was cleaned out and the mass of the empty
graduated cylinder was measured and recorded. The unknown liquid was then obtained
and added to the graduated cylinder. The mass of the liquid and the graduated cylinder
was recorded. The volume of the unknown liquid was recorded. Next, the unknown
liquid was placed in a test tube and the mass of the empty graduated cylinder was
measured again. The liquid was placed back into the cylinder and the mass of the liquid
and graduated cylinder were measured again. The volume of the liquid was also
measured again. All 3 of these measurements were recorded. The graduated cylinder
was cleaned out and then a Bunsen burner was obtained. A capillary tube was also
obtained and was rotated in the fire until the end was closed. The sample of the unknown
solid was crushed and a small part of it was placed in the capillary tube. Next the
apparatus for determining melting point was set up. A ring stand was placed with wire
gauze on it. The Bunsen burner was placed underneath. Then water was put in a beaker
and placed on the wire gauze. Water was also placed in a test tube and the capillary tube
and the thermometer were placed in a stopper and the stopper placed on the test tube.
The test tube was then connected to the ring stand so that the bottom of it was in the
water in the beaker. The Bunsen burner was lit and the temperature was recorded when
the solid sample melted. The test tube was taken apart and cleaned out after the
apparatus cooled. Next a sample of liquid unknown was obtained and placed in a test
tube. A thermometer and glass tube were placed in the stopper and a rubber tube
attached to the glass tube and pointed into the sink. The stopper was then placed on the
test tube with the unknown liquid in it. The test tube was then connected to the ring stand
and placed so that the bottom of the test tube was in the water of the beaker. The Bunsen
burner was again lit and the temperature was recorded when the unknown boiled. The
apparatus was then partially taken apart and put away.
Results
Table 1: Solubility of Samples
Water
Ethanol
Cyclohexane
Naphthalene
Insoluble
Sparingly Soluble
Soluble
Toluene
Insoluble
Soluble
Soluble
Unknown Liquid
Sparingly Soluble
Soluble
Soluble
Unknown Solid
Soluble
Insoluble
Insoluble
Table 2: Other measured physical properties
Density
Boiling Point
Melting Point
Unknown Solid
1.6 g/mL
Not Determined
83.20 ºC
Unknown Liquid
0.78 g/mL
79.63ºC
Not Determined
Table 3: Sample calculation for density
(42.66 g-40.47 g)/2.8 mL = 0.78 g/mL
Conclusion
The hypothesis, if physical properties are accurately determined, then an unknown
substance may be identified by comparison of properties to a standard reference, was
partially supported by the experiment. It was determined what the unknown solid was
and the unknown liquid could be one of two liquids. If further testing had been done the
liquid unknown could have been determined. The unknown solid in the experiment was
magnesium nitrate. This is believed, because the actual density of magnesium nitrate is
1.63 g/mL and the result found was 1.61 g/mL, which is close. The melting point
determined of the unknown solid was 83.20 ºC which is also close to that of magnesium
nitrate which is 89.00ºC. The magnesium nitrate is soluble in water and insoluble in
cyclohexane and ethanol. These are close to the results found in the lab except for in
cyclohexane. The problems and discrepancies could have been caused by human error.
There could have been some water spilt while measuring density and the test tube could
have been read for solubility inaccurately or the solid might not have been given enough
time to dissolve. The unknown liquid cannot be fully determined but can be one of two
liquids, isopropyl alcohol or t-butyl alcohol. The physical properties known in these two
liquids are the same except for the boiling point which was not determined for the
unknown. Therefore, because the physical properties of these two substances are very
close to the properties determined in the unknown, it can be said that the unknown liquid
is one of these two liquids. Because further testing was not done on the unknown to
determine the boiling point it cannot be determined which of these two liquids it actually
was. The results for the unknown liquid make sense and the properties determined for
the unknown are very close to that of these two liquids. The boiling point for isopropyl
alcohol and t-butyl alcohol is 83.00ºC and the unknown was determined to be 79.63ºC.
There could have been error if the temperature was not fully up to boiling and it was not
given enough time. The densities of these two known liquids are 0.79 g/mL and the
density of the unknown was 0.78 g/mL. These results are also very close and the
difference could have been caused by reading the top of the meniscus instead of the
bottom. Overall the results make sense and other classmates determined the same
substances to be the unknowns, there were no discrepancies found. It is also important to
test known items along with unknown substances so that the results can be compared.
This is also important so that students can learn how to read solubility of substances
before the unknown substances are tested. Determining unknowns by physical properties
is vital to every day living. People use physical properties of another person to determine
who they are. Some examples of these properties would be how the other person dresses,
what he sounds like, what he looks like, how old he is and many more. People use these
characteristics to determine a familiar face or when meeting someone that they have only
heard about. They use these traits or physical properties to remember the other persons
name or character.