Properties of Liquids, Solids, and Solutions: The Phase
Diagram of t-Butyl Alcohol
Vapor pressure, a manifestation of the tendency of a liquid or solid to become gaseous, is an important
property of pure substances and solutions, one which can give a variety of useful information about, for
example, the forces of attraction between molecules, molecular weights, and physical state. The vaporization
of a liquid is called evaporation; the vaporization of a solid is called sublimation. For any pure substance,
there are certain conditions of temperature and pressure at which the tendenc y of the substance to change
state in one direction (e.g., liquid to gas) is exactly balanced by its tendenc y to change state in the opposite
direction (e.g., gas to liquid). Under these special conditions, there is a state of equilibrium between the two
phases. For a pure substance, such as water, there is a set of pressure-temperature points at each of which
there is equilibrium between the gaseous and liquid phases. Since the liquid and gaseous phases coexist as
these points, they are called “liquid-gas coexistence curves”. Each of the points along a liquid-gas
coexistence curve is a boiling point; that is, the liquid will boil if the prevailing pressure is less than the
equilibrium vapor pressure at that temperature. The particular boiling point temperature that corresponds to
an equilibrium vapor pressure of exactly one atmosphere is called the normal boiling point; for water, this
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temperature is exactly 100 C (373.15K). At prevailing pressures less than one atmosphere, the substance
will boil at temperatures less than the normal boiling point.
Just as there are points at which equilibrium exists between the liquid state and the gaseous state, there are
pressure-temperature coexistence point at which there is equilibrium between the solid and liquid phases:
these are called freezing points or melting points. The normal freezing point of a substance occurs at a
prevailing pressure of exactly one atmosphere.
The unique point at which all three phases (solid, liquid, and gas) are at equilibrium is called the triple point.
This point is fascinating to observe in that, under pressure and temperature conditions at or very close to the
triple point, one should be able to see evidence of simultaneous boiling and freezing of the substance. For
water, the triple point occurs at 273.16K and 0.00603atm. By comparison, the freezing point of water at a
prevailing pressure of 1.000atm is 273.15K. In this experiment, you will not only find the freezing and melting
points of a substance, but also you will have an opportunity to observe its triple point. The substance under
observation is t-butyl alcohol, which has the following structure:
To get to the triple point, you must lower the pressure, which you will accomplish by attaching the test tube to
a water aspirator. This device provides suction by taking advantage of Bernoulli’s Principle, which says that
the pressure adjacent and perpendicular to a flowing fluid (liquid or gas) is less than the pressure in the
direction of the flow. The same phenomenon provides the basis for aerodynamics (why planes fly, etc.). A
water aspirator consists of a main tube which has a hole in it that leads into a side tube. The water flows
through the main tube; the pressure in the side tube is reduced, creating the suction. If the side tube from the
aspirator is connected with thick rubber tubing to the test tube, and the water is turned on, the pressure in the
test tube will be lowered, and eventually the liquid will begin to boil, because the pressure acting on the liquid
(the prevailing pressure) will be less than the vapor pressure at that temperature. Causing the liquid to boil in
this way lowers the temperature of the liquid, because vaporization is an endothermic process. Gradually the
temperature will continue to drop until it reaches a point at which the liquid will boil and freeze simultaneously.
You should ultimately observe a boiling slush of t-butyl alcohol.
PROCEDURE
TRIPLE POINT
1. Obtain a rubber stopper (#4 size, two holes, no split), thermometer, large test tube (25x200mm size), and
rubber tubing (approx. 50cm long) that has a short length of glass tubing in each end. Lubricate the bulb of
the thermometer and one hole in the stopper with a drop of glycerine, and, grasping the thermometer and
stopper with a towel, use a twisting motion to carefully slide the thermometer through the hole in the stopper.
Alternatively, you may use a stopper tubing tool to safely insert the thermometer into the stopper. The
Thermometer should be inserted far enough to reach nearly (but not entirely) to the bottom of the test tube.
Next, insert one end of the rubber tubing in the other hole of the stopper just far enough to go all the way
through the stopper. W ash off any glycerine with water when you are finished and dry off the thermometer
and glass insert.
2. Obtain another rubber stopper (#6 size, two holes, no split), 250mL filter flask, a pan or 1000mL beaker
half filled with ice water, and a short length of rubber tubing with glass insert in one end. Set up the trap and
connect it to both the water aspirator and the test tube as shown in the illustration:
There are several reasons for having such a trap, one being that it will prevent most of the t-butyl alcohol
vapors from being drawn into the water going down the drain. Another important reason is that the trap will
catch any water accidentally drawn back into the s ystem before it gets into the test tube containing the t-butyl
alcohol. Pure t-butyl alcohol freezes at a temperature near room temperature, so your instructor may have to
place the reagent bottle in warm water to keep it in a liquid state.
3. Place three boiling stones and enough t-butyl alcohol in the test tube to cover the bulb of the thermometer.
4. Clamp the test tube to the ring stand, and ensure a tight seal when replacing the stopper in the test tube.
5. To properly operate the apparatus constructed, you should turn on the water aspirator BEFORE you close
off the tubing connected to the trap, or stopcock. Touch your finger to the end of the stopcock tubing to
ensure there is good suction.
6. Close the stopcock with parafilm, a pinchcock clamp, or your finger. The temperature should begin to drop
almost immediately. After a short period of time, the liquid should start to boil, and eventually you should
observe simultaneous boiling and freezing. Note the temperature at that point, which is the triple point
temperature. W atch carefully or you may miss this very interesting phenomenon! (If you don’t see boiling
after a couple of minutes, your apparatus may have air leaks which are lowering the vacuum quality. Consult
your instructor for possible solutions.)
7. After you observe the triple point, first open the stopcock on the trap to relieve the vacuum, and then you
may turn off the water aspirator. (If you do this in the reverse order, water may be sucked into the trap, so
make sure you have the correct order down.)
8. Remove the thermometer and tubing from the rubber stopper in the test tube. Keep the test tube with tbutyl alcohol in it; you will need it for your other two tests. Break down and return the equipment used for the
trap, as well as the stopper that was in the test tube.
The point at which all three phases are present simultaneously, the triple point of t-butyl alcohol, occurs at a
pressure of 42.4mmHg (0.0558atm). Record the triple point temperature and pressure on your worksheet.
BOILING POINT AT LABORATORY PRESSURE
1. Prepare a hot water bath by filling a 600mL beaker about two-thirds full of tap water, adding three boiling
stones and heating it on a hot plate until it just starts to boil. T-butyl alcohol is flammable and must be kept
away from open flames.
2. Get a rubber stopper (#4 size, two holes, with a split) and insert your thermometer into the split hole. The
stopper has been split to facilitate reading the thermometer. Adjust the thermometer so that the bulb sits
about 1cm from the surface of the alcohol in the test tube (it is better to measure the temperature of the vapor
rather than the liquid, even though, in principle, they should be equal).
3. Clamp the test tube to the ring stand. W hen the hot water bath is boiling, turn off the heat and lower the
test tube into the beaker.
4. Record the boiling temperature on your worksheet (which should be very close to the normal boiling point).
Your instructor will report the current barometric pressure in the laboratory, in which you should also record
this value on your worksheet.
FREEZING POINT AT LABORATORY PRESSURE
1. Using the same test tube setup, insert a long wire stirrer into the other hole of the rubber stopper. Push
the thermometer down far enough so that it will be submerged in the liquid. Make sure the thermometer scale
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is visible in the range from 15 C to 40 C. Then fill a 400mL beaker half full of cold tap water. You may adjust
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the temperature of the water, monitoring it with a second thermometer, with crushed ice to about 18 C.
2. Immerse the test tube into the cold water. W hile constantly stirring (to minimize supercooling), take
thermometer readings every 15 seconds until the liquid freezes. Once it reaches this point, take an additional
four readings, continuing at 15 second intervals. Check the thermometer in the cold bath periodically,
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ensuring that it remains at approximately 18 C. Add ice as needed.
3. Record the temperature and time data for the t-butyl alcohol in your notebook. The constant temperature
attained during freezing is taken as the freezing (and melting) temperature. Record this value on your
worksheet.
DATA ANALYSIS
Using graphing paper, apply all the information you’ve gathered about t-butyl alcohol on your worksheet to
construct a phase diagram for t-butyl alcohol. You should have the following details:
1.
2.
3.
4.
5.
Title
X and Y axes labeled with proper units
Identify the solid, liquid, and gaseous regions
Boiling, freezing points at laboratory pressure, labeled
Triple point, labeled
Name: ______________________________
Class/Section: ________________________
DATA—Phase Diagram of t-Butyl Alcohol
TEMPERATURE
PRESSURE
TRIPLE POINT
__________
__________
BOILING POINT
__________
__________
FREEZING POINT
__________
__________
Name: ____________________________
Class/ Section: _____________________