Physical Properties of a Pure Substance, Water
The chemical and physical properties of a substance characterize it as a unique substance, and
the determination of these properties can often allow one to determine the identity of an
unknown substance. Physical properties such as color, density, freezing point, normal boiling
point and refractive index are those properties which can be determined without altering the
chemical properties of the substance. That is, after the property has been determined, the
material can be recovered in its original state by physical methods. The determination of
chemical properties usually involves a chemical process, which changes the properties of the
material under study by transforming it into a new substance(s). Thus the identity of the
substance is altered in a positive test of a chemical property and the material cannot be
recovered in its original state by physical methods. Later you will learn more about chemical
properties, but the focus of this experiment is to determine some common physical properties
of a known substance, water. This will give you experience at making some routine laboratory
measurements such as measuring mass, volume, temperature and refractive index. This
experience will be practice for a future experiment in which you will attempt to identify an
unknown substance by measuring these four properties of the unknown, comparing your values
to those reported in a list of the possible unknown substances and determining the best match
for these four properties.
Property 1: Freezing Point/Melting Point
1A. Introduction:
The freezing point of a pure liquid is defined as the temperature at which the solid and liquid
states of the substance can coexist in contact with each other. (The melting point of the solid
is similarly defined.) This temperature is an intensive physical property, which is useful in
identifying the material. During the freezing process, many pure liquids will first supercool and
then crystallize very rapidly with a slight rise in temperature, making it very difficult to
determine the temperature at which crystals began to form. In this experiment, you will use a
graphical method to avoid this problem, making a “cooling curve” of time vs. temperature.
1B. Experimental Procedure for Determination of Freezing / Melting Point
Prepare a clean, 6-inch test tube by washing it with soap and water, and rinsing it with tap
water and then distilled water. Transfer enough water to fill the test tube ~1/3 full. Fill your
250 mL beaker with a mixture of ice and salt (~50 mL). Stir the mixture to make a salt ice
bath. Record the temperature of this mixture and check to ensure it drops to at least -5°C. If
this temperature is not reached, add more salt and stir until the bath is -5°C or lower.
Record the initial temperature of the water in the test tube (to the nearest 0.1°C) before you
place the test tube into the ice bath. You may need to warm this water to ~40-45°C to ensure
you have enough data points during the initial phase of the experiment. [This is the
temperature at time = 0.] Gently stir the water with the thermometer, and the ice bath with
the test tube. Read the thermometer temperature every 10 seconds. Keep a written record of
temperature vs. elapsed time. Do not remove the tube from the ice bath during this
procedure. Note if you can the temperature at which you first observe crystals forming in the
liquid. Continue recording the time and temperature until the temperature remains constant
(within 0.5°C) for five to six readings. If the thermometer reading will not remain constant for
this long you may have too much salt in the ice. Add some more ice to the mixture and try
again. In order to make a good graph, you need at least three (3) data points in the initial
range where temperature is changing rapidly and 8-10 points in the temperature range near
the freezing point where it is changing much more slowly or is giving constant readings. If you
do not have this number of data points, repeat the process until you have sufficient data points
to create two (2) appropriate cooling curve plots.
Sometimes a liquid will supercool and reach a temperature below its freezing point before any
crystals are observed. If the liquid reaches -3°C and no solid has been observed, scratch the
side of the tube gently with the thermometer to try to initiate crystal formation. If
crystallization does begin, the temperature should increase to the true freezing point and stay
at that temperature while the sample is freezing. If this does not happen, you may have
contaminated the sample and it will not be possible to get a definite freezing point temperature.
In this case, dispose of that sample, rinse your test tube again with fresh distilled water and
start over with a fresh water sample. If the water is frozen completely, the temperature will
begin to drop again, eventually reaching the temperature of the ice bath. It is not desirable to
cool the sample until it is frozen solid.
Once the thermometer reading is constant or is dropping very slowly for eight (8) or more
readings, you may remove the tube from the ice bath and warm again to about 40-45°C. Put
the test tube back into the ice bath and record a second cooling curve. Repeat the whole
process as necessary until you have two acceptable sets of data.
1C. Calculations for Freezing Point of water
Make a cooling curve from the data in each of your freezing point trials by graphing with Excel®
the observed temperature on the y-axis and the elapsed time on the x-axis. (You can not plot
two trials on the same graph unless you have exactly the same number of xy data points in
each experiment and all of the x-data points are the same for both graphs. Therefore, it is
necessary to plot separate graphs). On each graph, using a straight edge, draw the “best
straight line” through the initial linear portion of the data and then extend (extrapolate) this line
until it extends below the temperature of the “constant” temperature plateau section of the
graph. With the straight edge, draw another straight line through the data points on the
plateau (horizontal) portion of the graph. The temperature at which these straight lines
intersect is the observed freezing point and should be recorded as the Tfp(1) and T(fp,2). These
two values should be averaged.
Look up the freezing point of water in the handbook and compare it with the value obtained
from this experiment.
Data
Thermometer readings for the freezing point of Water (Tfp (avg))
(Record to nearest 0.1°C)
Trial 1
Trial 2
Time
Temp (°C)
Time
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Trial 3
Temp (°C)
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Observed freezing point temperatures of water (From the Graphs)
Trial 1_________°C
Tfp,avg = __________ °C
Handbook Value __________ °C
Trial 2_________°C
Temp(°C)
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Property 2: Boiling Point/Condensation Point
2A. Introduction
When a liquid boils, the vapor pressure of the liquid is equal to the pressure of the atmosphere
over it. Therefore, a liquid can boil at various temperatures at different atmospheric pressures.
If the surrounding atmosphere is at 760 mm/Hg pressure (1 atm), then the temperature is
called the normal boiling point. When the surrounding pressure is greater than one
atmosphere, the liquid will boil at a temperature above its normal boiling point. At a lower
pressure, such as occurs in the typical laboratory, it will boil below the normal temperature.
Boiling temperatures measured at pressures other than one atmosphere should be reported
with the pressure noted. The most useful temperature is the normal boiling point, since this
value will be reported in the literature and can thus be used in the identification of a substance.
Although it is not possible for us to measure the normal boiling point directly, we can apply a
calculation to convert the experimental boiling point to the temperature at which boiling would
have occurred at one atmosphere if we know the pressure at which the experimental value was
measured.
2B. Experimental Procedure for Determination of Boiling Point
In order to determine the boiling temperature you must use a distillation flask. Clean the flask
with soap and water and rinse thoroughly with tap water and then with distilled water from the
white squeeze bottles. Fill the bulb of the flask ~2/3 full with distilled water from the carboy
and add one or two boiling chips. The latter do not change the boiling point, but makes the
boiling go more smoothly. Obtain a cork that will fit your flask and insert your thermometer
into the cork. Hold the thermometer near the end to be inserted in the cork and carefully push
and twist the thermometer through the cork. Do not force the thermometer, as this could be
dangerous. Either make the hole larger or apply a small amount of stopcock grease. If
stopcock grease is used, be sure to wipe the thermometer clean with a paper towel before
continuing the experiment.
Now, set up the distillation apparatus on the hot plate. Your instructor will provide a sample
apparatus for you to use as a model. Place the cork on your thermometer and insert it into the
neck of the flask. Position the tip of the thermometer in the flask near the point where the
flask begins to flare into the bulb portion of the flask. Do not place the thermometer directly
into the liquid. You will need to collect the distilled water so it is necessary to put a receiving
test tube over the side arm of the distilling flask. Then, insert this test tube into a beaker of
cold water (water with a small amount of ice in it) to condense the distilling vapor. Before you
begin to heat the flask for the first time, have your instructor inspect your apparatus for safety.
Carefully heat the water on the hot plate until it begins to boil and adjust the heat to maintain a
gentle boil. Begin to record the temperature to the nearest 0.1°C every 15 seconds
beginning when the temperature nears 90°C. By then, water should be condensing
on the thermometer and dripping back into the flask. Continue boiling the liquid until
you have recorded eight to ten (8-10) consecutive temperatures that do not vary by more than
0.5°C. Record this thermometer reading as the observed boiling point of water. Replace about
½ of the water in the flask, add two more boiling chips and repeat the determination of the
boiling point. Repeat as necessary until two trials agree to within ±0.5°C. Average the values
for the two trials and record this average as the observed boiling point of water.
Thermometer readings for the boiling point of water (Tbp observed)
Time
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Trial 1_________°C
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Trial 2_________°C
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Trial 3_________ °C
Average ________°C
Barometer Reading in mm Hg
__________mm Hg
Boiling Point of Water at this Barometric Pressure
__________ ° C (See handbook)
2C. Calculations for Boiling Point of Water
Average the observed boiling points from your two trials to give Tbp,avg. Boiling points are
usually listed as “normal Boiling Points”, i.e., the boiling point at 760 mmHg pressure.
Therefore, you will need to convert your observed boiling point to its corresponding value at
760 mmHg by using the following empirical equation.
T
normal,bp
= Tbp
observed
+ 0.050°C/mm Hg x (760 - Barometric Pressure)
Tnormal,bp = ______°C + 0.050°C/mm Hg x (760 mm Hg - ______mm Hg) =
______°C
Handbook value of boiling point (normal) of water __________°C
Property 3: Density
3A. Introduction
Density, mass per unit volume, is very useful for identification of a substance. Comparison of
an observed value with a literature value should show excellent agreement.
3B. Experimental Procedure for Determination of the Density of Water
Weigh a clean, dry, 10 mL graduated cylinder to the nearest 0.001 g on the balance in your
work area. Add about 2.5 mL distilled water to your graduated cylinder. Measure the volume
of the water as closely as possible and re-weigh the cylinder plus the liquid. Repeat the volume
measurement and weighing for ~5 mL, ~7.5 mL, and ~9.5 mL of the liquid. Do a second trial.
Data for the Density of Water
Mass
Trial 1
Volume
Empty ________g
Mass
Trial 2
Volume
Empty ________g
¼ full ________g
________mL
¼ full ________g
________ mL
½ full ________g
________ mL
½ full ________g
________ mL
¾ full ________g
________ mL
¾ full ________g
________ mL
“full” ________g
________ mL
“full” ________g
________ mL
Temperature of the water __________°C
Handbook Density for Water at this Temperature __________g/mL
3C. Calculations for Determination of the Density of Water
An approximate value of the density of water could be obtained by subtracting the mass of the
empty graduated cylinder from the mass of the cylinder plus water and then dividing by the
volume of the water. However, a more accurate way of obtaining the density is by
determining the slope of the mass vs. the volume of the liquid for different amounts
of sample. For a graph with the mass of the cylinder and water as the y-axis and the volume
of the water as the x-axis, the slope of the trend line will be the density of the water.
Make an Excel graph for each trial. Plot the mass of the cylinder plus water vs. the volume of
water in the cylinder. Plot all of your mass-volume data points. Don’t forget to include the
mass of the empty cylinder and zero volume as a point. Add a trendline to determine the slope
of the line produced. Plot each trial as a separate graph and determine a separate slope.
Average the two slopes to obtain an average value for the density of water.
Density, trial 1 __________g/mL
Density trial 2 __________ g/mL
Average Density __________ g/mL
Property 4. Refractive Index
4A. Introduction
When light passes from one medium into another, its speed changes, and this results in a
change in the direction of the light ray. This “bending” of the light ray is termed refraction.
The angle between the initial light ray and a line perpendicular to the refracting surface is called
the angle of incidence Θi. The angle between the refracted ray and this perpendicular line is
called the angle of refraction Θo. The refractive index, η, is defined as follows:
η=sin Θi /sin Θo
The refractive index is an intensive property of a pure compound and is useful in identifying
materials and evaluating their purity.
4B. Experimental Procedure for Determination of the Refractive Index of Water
See handout on student refractometers.
Refractive Index Data
Trial 1 __________
Trial 2 __________
Trial 3 __________
Average Refractive Index for water __________
Handbook Value of Refractive Index for Water __________
Summary of Results for Water
Freezing Point
Density
Boiling Point
Refractive Index
Lab Values
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Accepted Values
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