Experiment #3– Using Physical Properties
To
Determine the Identity of an Unknown
Laboratory Overview
CHEM 1361
August 2012
Gary S. Buckley, Ph.D.
Department of Physical Sciences
Cameron University
Learning Objectives
•Distinguish between physical and chemical properties
•Experimentally determine the boiling point of a compound
•Determine the index of refraction of a material, time and
instrumentation permitting
•Experimentally determine the miscibility of two liquids
•Use physical property data to identify an unknown from a list of
possible compounds
Table of Contents
(you may click on any of the topics below to go directly to that topic)
•Density
•Boiling Point
•Index of Refraction
•Solubility
Density
Density is defined to be the mass of a sample of a substance
divided by its volume. In mathematical terms,
density 
mass
volume
In this experiment, you will determine the density of your
unknown as a means of helping determine its identity. The
measurement is a simple matter of determining the mass of a
known volume and carrying out the division above.
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Density’s Role in this Experiment
Density will be used in this experiment as one of the
physical properties to help identify your unknown
from the list at the right. There are a couple of
important things to notice:
•If only one significant figure is measured
experimentally, 5 of the 9 compounds would have the
same density (0.8 g/mL)
•The instructions for measuring the density will lead to
masses and volumes to at least three significant
figures. Your density values should all have three
significant figures as well as your average. Don’t be
lazy with significant figures – remember that if you
divide , for example, 2.94 g by 1.47 mL, the density is
2.00 g/mL – not 2 g/mL.
Possible Unknowns
Compound
water
ethyl acetate
Density
(g/mL)
1.00
0.902
ethanol (95%)
0.816
1-propanol
0.804
methanol
Acetone
2-propanol
0.791
0.791
0.785
cyclohexane
0.779
hexane
0.659
Boiling Point
The liquid state of matter is comprised of atoms or molecules that are virtually in
constant contact with each other moving relatively slowly past each other. The
vapor state on the other hand contains atoms or molecules that are spaced far
away from each other on the atomic scale and are moving rapidly with frequent
collisions.
In any liquid sample atoms or molecules are constantly escaping into the vapor
phase and condensing into the liquid phase. As the temperature of the liquid
increases, the fraction of molecules in the vapor phase increases.
The pressure of the vapor above the sample is called the vapor pressure of the
liquid. The temperature at which the vapor pressure of the sample equals the
external pressure on the sample is called the boiling point. The temperature at
which the vapor pressure of the liquid is 1 atmosphere is called the normal boiling
point.
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Boiling Point
The animation on the right is
intended to give some visual
sense of the effect of
temperature on the vapor
pressure of a liquid. Watch the
temperature and pressure
gauges and the distribution of
atoms/molecules between the
liquid and vapor phases.
Pressure Med.
Pressure
Pressure
High
Low
High
Med
Low
Click anywhere on this slide to
start the animation.
Temperature
Low Temperature
Medium Temperature
High Temperature
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Boiling Point - Experimental
The boiling point of your unknown will be determined by
heating a test tube containing the unknown in a
temperature bath. A stainless steel temperature probe
connected to a handheld LabQuest 2 data acquistion
device will be placed just above the surface of your
unknown. As the unknown is heated the temperature will
rise but it will level off once the unknown begins to boil.
Once the temperature levels off, you will find the average
temperature in the leveled-off region and record that as
your boiling point.
Instructions for using the LabQuest 2 may be found under another item in
Blackboard in the same location as this slide show. It is a 20-minute video
showing you how to work the LabQuest 2.
Index of Refraction
You are probably aware that light travels incredibly fast – 3 ×
108 m/s or 186000 miles/s. These speeds refer to light
traveling in a vacuum – it travels more slowly in other
materials.
The index of refraction, η, is the ratio of the speed of light in a
vacuum to the speed of light in the material of interest.

speed of light in vacuum
speed of light in substance of interest
Since light travels fastest in a vacuum, η will always be
greater than one. We can use the value of η to help
distinguish between the possible unknowns.
Possible Unknowns
Compound
nD
Refractive
index
Cyclohexane
1.426
1-propanol
1.384
2-propanol
1.377
Hexane
1.375
ethyl acetate
1.372
ethanol (95%)
1.363
Acetone
1.359
Water
1.333
Methanol
1.329
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Index of Refraction
Ocular Piece
You may or may not have an opportunity to
carry out index of refraction measurements – it
depends on instrumentation availability the
day you do the lab.
The instrument used to measure the index of
refraction is called a refractometer.
A liquid sample is placed on the stage and the
cover is gently closed. If the switch on the lefthand side of the instrument is moved up, a
light can be seen through the ocular part of
the refractometer.
Switch
Stage
(Description continued on next slide)
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Index of Refraction
The goal is to turn the knob on the right-hand
side of the instrument until the light-dark
horizontal separation in the ocular is aligned at
the crosshairs.
Once this positioning is accomplished, the
switch on the left-hand side is moved to the
downward position and the index of refraction
reading is made on the visible scale. The
reading in the window to the right would be
1.3596. The lower scale is not used here.
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Solubility
Solubility is the fourth of the physical properties you will investigate. Two liquids
that mix with each other are said to be soluble, or miscible. If the liquids do not
mix with each other they are said to be insoluble, or immiscible.
Possible Unknowns
As the table to the right indicates, the solubility
behavior of the unknowns is not a great
differentiator between the compounds. But, when
coupled with the other information, these data may
be helpful in confirming your unknown identification.
You will check the solubility of your unknown in two
solvents – water and cyclohexane. Pay careful
attention to the instructions in the lab manual
regarding looking for wavy lines and the behavior
following stirring.
Compound
solubility solubility in
in water cyclohexane
ethyl acetate
sl. s
s
water
1-propanol
s
s
i
s
2-propanol
s
s
ethanol (95%)
s
sl. s
methanol
acetone
cyclohexane
s
s
i
sl. s
s
s
hexane
i
s
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End of Slide Show