Freezing or Boiling Point of Two Alcohols
Introduction
In this experiment the freezing point of octadecanol will be determined by utilizing a cooling
curve. The cooling curve will be determined by plotting measurements of temperature
versus time for octadecanol. Also, the boiling point of methanol will be determined by
raising the liquid to a constant temperature while continuously heating the sample.
Theory
All substances can exist as a solid, a liquid or a gas. The transition from one state to
another is dependent upon the temperature of the substance. The temperature at which a
substance changes from a solid to a liquid is referred to as the melting point. Conversely,
the temperature at which a substance changes from a liquid to a solid is the freezing point.
The freezing point is the same temperature as the melting point for a particular substance.
The temperature at which a substance changes from a liquid to a gas is the boiling point.
The temperature at which a gas condenses to form a liquid is the condensation point. Again
the boiling point is the same temperature as the condensation point.
The boiling point is greatly affected by the atmospheric pressure. The lower the atmospheric
pressure, the lower the boiling point. For example, the boiling point of water at sea level is
100 °C. At higher altitudes (e.g. Lethbridge) the boiling point of water is about 98 °C.
The melting point, freezing point, boiling point and condensation point for a particular
substance are characteristic physical properties of that substance.
During the transition from one state to another, the temperature of the substance remains
constant until the change of state is complete. The temperature at which state changes
occur can be determined graphically by measuring and plotting time and temperature for a
particular substance. The change of state is seen as a horizontal line or “plateau” on the
graph.
Purpose
A. To determine the freezing (melting) point of octadecanol.
B. To determine the boiling point of methanol.
Procedure
Part A – Freezing Point Determination for Octadecanol
1. Mass out about 15 g of octadecanol into a 25x200 mm test tube fitted with a 2-hole
stopper, a stirring wire and a thermometer as shown in the diagram below. These
typically will be provided to you ready to go. Use the top loading electronic scales
and a creased piece of paper for the weighing. Place about half of the octadecanol
into the test tube before inserting the stirring wire. Assemble the apparatus as in the
diagram, placing the test tube in a 600 mL beaker about ¾ full of cool tap water to
act as a water bath. A second thermometer must be placed in the water bath to
monitor its temperature but the readings are taken from the other thermometer.
2. Heat the water and octadecanol until the water temperature reaches about 70 C,
then shut off the heat. Stir and poke at the octadecanol until it is completely melted.
There should be no flakes of octadecanol at this point.
3. When the octadecanol is completely melted, no flakes left, remove the apparatus
from the water bath. Set a zero time and record the temperature of the octadecanol
as it cools every 2 minutes. Stir vigorously and consistently as the octadecanol cools.
Record the temperature for about 20 minutes after the octadecanol has frozen. All
temperatures should be measured to the nearest degree Celsius only.
Part B – Boiling Point Determination for Methanol, (CH3OH)
1. Place a hot plate on a retort stand in a fume hood. Fill your largest beaker about ¾
full with tap water and place this on the hot plate. Put about 15 mL of methanol in a
15x150 mm test tube and add two boiling chips. Place this test tube into the beaker
of tap water on the hot plate and clamp it to the retort stand so that the level of the
methanol is below the level of the water in the beaker.
2. Clamp a thermometer to the retort stand above the test tube so that the top of the
bulb of the thermometer is held about 1 cm below the surface of the methanol in the
test tube.
3. Heat the water and, indirectly, the methanol using the medium heats setting on the
hot plate. Continue heating the methanol until the temperature no longer increases –
indicating that the methanol is “boiling.” Record the temperature to the nearest 0.1
°C. Continue heating the boiling methanol taking a temperature reading every
minute for 5 minutes. Average these readings and record the average as the boiling
point of methanol.
Data
1. From the temperature/time data collected for the freezing of the octadecanol,
construct a graph of temperature versus time. The procedures for graphing are found
in the Lab Report Guidelines. Locate the horizontal (“plateau”) portion of the graph.
Draw a horizontal line from the plateau to the temperature scale (vertical axis). This
is the freezing point (melting point) of octadecanol.
2. Record the boiling point of methanol
3. Refer to a chemical reference text or manual to obtain the normal boiling point of
methanol. Compare this value to the experimental obtained value.
Conclusion
Summarize your results for the freezing and boiling points. Discuss, in terms of
experimental error, any factors or reasons why your value for the boiling point of methanol
differs from the reference value. Refer to the Lab Report Guidelines for helping determine
experimental error.