Pre-Lab 10B: Determining
Freezing/Melting Point
Purpose
• A cooling/heating curve is a plot of temperature vs.
time/energy graph.
• It illustrates the effect on a substance as the
temperature decreases/increases through a phase
change.
• In this experiment, you will study the effects of cooling
and heating cetyl alcohol through a phase change.
• The data will be recorded using a data collector and
then plotted on graphs.
• Based on your observations, measurements and
graphs, you will be able to determine the
freezing/melting point of cetyl alcohol.
Key Question
• How do you determine the
freezing/melting point of cetyl alcohol?
Background
• Define:
Phase change
Freezing point
Melting point
Safety:
wear goggles with heating and chemical
use
Drawing
• Draw this:
note probe is in
the test tube
IMPORTANT PROBE DATA COLLECTOR USE:
• probe must be in chemical before you press “Go”
• when “Go” is pressed, data collection begins, even
though you can not see it
• press “graph” to view graphing data
• there is no pause button
• mark last data point you want by pressing screen at
that point on the data line. This will put a line on the
graph marking the end of your data, BUT the temp
will keep going: do not use this extra data
• DO NOT REMOVE TEMP PROBE FROM
SOLUTION UNTIL THE GRAPH HAS BEEN
DRAWN
Procedure
Read directions very slowly and accurately
there are two parts to this experiment:
part one, melting point: in a 250ml hot water
bath, measure/graph the temperature of the test
tube with solid turning to liquid
You will get the 250ml hot water from me at the
side of the class, carefully hold the lip of the
beaker and return to your lab station
part two, freezing point: remove the test tube
with melted solid from the water bath and
measure/graph the temperature the liquid turns
to solid
Section 2:
The first part of the lab:
you do not need a 250ml beaker for cold water, just put
the temperature probe under cold running water
before use
Step 4 has a typo-error:
Measure the temperature of the hot water to make
sure it is within the range in step “1”; this number
should be step 2, please change this in our lab books
• Place the temperature probe into the test tube and put
the test tube containing the solid into the hot water
bath and press START on the Data Collector.
• View the experiment in graph mode.
Section 3:
• Describe and carefully sketch the shape of
the graph from Part 2.
• remember this graph is a sketch: do not
use graph paper
• use straight edge for x and y axis
• write title
• label x and y axis: variable and unit
• record numbers on each axis
• Include data line
Section 4:
The second part of the lab:
• Observe the cetyl alcohol in the test tube to
make sure it is still all liquid.
• If some solid has formed, pour out the water in
the warm beaker and add more hot water to the
beaker.
• You must start the experiment w/ all the
chemical in the test tube in the liquid state
• Remove the test tube from the warm beaker,
stir (?) and start a new experiment on the data
collector. DO NOT HOLD THE T.T.?
Section 5:
• Describe and carefully sketch the graph
from Part 4.
• Wash your temperature probe so that
no chemical remains on it , why?
Post Lab 10B: Determining
freezing/melting point
Key Question:
How do you determine the
freezing/melting point of cetyl alcohol?
Background
• Define:
Phase change
Freezing point
Melting point
Purpose
• A cooling/heating curve is a plot of temperature vs.
time/energy graph.
• It illustrates the effect on a substance as the
temperature decreases/increases through a phase
change of cetyl alcohol
• you will be able to determine the freezing/melting
point of cetyl alcohol.
Questions:
1) what is the frz/mlt point temperature
range for a pure substance?
2) If the ceytl alcohol becomes
contaminated, how would this effect the
fz/mlt pt?
3a) What is the effect of sample size on the
melting/freezing point?
3b) Describe and carefully sketch the shape
of the graph from heating the sample.
5a) Describe and carefully sketch the graph from cooling
the substance.
5b) Referring to both of your graphs, determine the
melting and freezing point of cetyl alcohol. Are they
the same? Should they be?
5c) Based on the shapes of your curves, which data do
you think is more reliable - the heating or cooling
data? Why do you think this is so?
5d) What is happening to the molecules of cetyl alcohol
during the diagonal portions of the heating curve?
What about the plateau?
5e) Your graph from Part 2 shows that during a change
from solid to liquid, the temperature stays the same.
Explain why the temperature does not increase, even
though energy is being added.