Measuring Melting Point and Freezing Point
Write everything (pre-lab, data etc) in your notebook, not on this sheet.
Background:
The melting point of a substance is defined as the temperature at which both the solid and liquid form of the
substance can co-exist. For example, under normal atmospheric pressure and temperatures below 0 oC, the stable
form of water is a solid (ice). At temperatures above 0 oC (and less than 100oC under atmospheric pressure), water is
stable as a liquid. At exactly 0oC, the addition or removal of heat can change the relative amounts of ice and liquid
water, but both are stable. Thus the melting point can also be characterized as a freezing point.
The melting point of a substance under prescribed conditions of pressure (usually, 1 atm) is a characteristic
property of that substance. Indeed, the Handbook of Chemistry and Physics indexes its extensive list of properties
of compounds by melting points. The melting point can also be used to determine the purity of a substance. When
one substance is dissolved in the liquid form of another substance, the melting/freezing point of the liquid substance
is lowered. For example, salt added to the surface of ice on slippery roads causes the ice to melt, even though the
temperature of the salt/water/ice mixture is below 0oC. This same approach can be used in ice cream makers to
freeze ice cream. This phenomenon is called freezing point depression. Based on this principle, the melting point
of an impure substance is expected to be lower than that of the pure substance. Thus the melting point can be used
to determine if a substance is pure or impure.
Purpose:
The purpose of this lab is to determine the freezing and melting point of an unknown substance, and to use this
information to identify it. Two different substances will be used (A and B) and you will be given ONE (not both).
Materials: To be written down from the set-up at your lab station along with a diagram.
Procedure:
1. You will receive a beaker with warm water, a hot plate and a thermometer with an attached cork.
2.
Calibrate the thermometer. This is done by putting the thermometer in an ice bath and recording the temperature
after the red indicator stops moving. Be sure to record your measurements with the correct number of significant
figures (remember: the rightmost digit is estimated). Warm the thermometer in your hand, and repeat this with
boiling water. You can assume that the thermometer behavior is linear and use this data to solve the linear equation:
Actual Temp = m (Measured Temp) + b (same as y = mx + b)
Repeat the measurements at 0oC and 100oC three times and then average them. You will then have data for two points
of a calibration curve: (measured freezing temp, 0oC) and (measured boiling temp, 100oC) (same as (x,y)). You then
solve for both “m” and “b” (use Excel!). You will use this formula to correct the measured temperature data.
3.
Once you have calibrated your thermometer, dry it and bring it to your teacher to receive your test tube with
Unknown (A) or (B). You will measure the initial temperature at the teacher workbench. Once you take your
unknown test tube, place it in your water beaker and immediately begin recording the temperature every 30
seconds. As your unknown material cools, in addition to taking temperature measurements, carefully observe it
visually to determine the point at which it makes the phase transition to a solid (freezes).
4.
Once your unknown material freezes, continue to take temperature measurements for about 5 minutes. You should
have a total of about 40 measurements. Record all your data in a table, and correct your temperature measurements
using your calibration curve. Then use Excel to graph the data as time (x) versus actual temperature (y) (Graph 1).
5.
Place the water beaker with your unknown test tube on the hot plate and SLOWLY raise the temperature so that
you can take measurements every 30 seconds. DO NOT get the sample too hot (not above 800C) by raising the
temperature of the hot plate. As your unknown material warms, in addition to taking temperature measurements,
carefully observe it visually to determine the point at which it makes the phase transition to a liquid (melts).
6.
Once your unknown material melts, continue to take temperature measurements for about 5 minutes. You should
have a total of about 40 measurements. Record all your data in a table, and correct your temperature measurements
using your calibration curve. Then use Excel to graph the data as time (x) versus actual temperature (y) (Graph 2).
7.
Allow the teacher to collect the unknown test tube and remove the water beaker from the hot plate.
8.
Determine the freezing and melting point temperatures from the graphs. Be sure that you have used your
calibration curve to correct these measurements.
Sample Data Table
Freezing Point (oC)
Measured Temp
Corrected Temp
Time (sec)
Melting Point (oC)
Measured Temp
Corrected Temp
0
30
60
90 etc.
Post Lab Questions:
1.
Which graph (1 or 2) is the “melting curve”? Which graph (1 or 2) is the “freezing curve”?
2.
How are the two curves similar?
3.
How are the two curves different?
4.
Why is there a flat region on each graph?
5.
Using a ruler, draw lines (sloped where the temperature is changing, flat where it is constant, as shown in
#11 below) through the data on both graphs 1 and 2. Note that not all data points fall on the lines. What
does this spread of data about the lines represent? How can you account for it?
6.
A list of materials with their melting point is listed on the front board. What is the material in your test tube?
7.
What evidence do you have to support your answer?
8.
Why are the melting points of stearic acid and palmitic acid different?
9.
Does the amount of the substance affect its melting point? Why or why not?
10. Does the amount of the substance affect its freezing point? Why or why not?
11. Examine the graph below:
B
C
Temp.
0
C
A
Tim e (m inutes)
a.
Does the graph show a melting or a freezing curve? How do you know?
b.
Which section represents only liquid?
c.
Which section represents only solid?
d.
Which section contains a mixture of both solid and liquid?
12. How does the size of the part of the thermometer in the material affect the measurement?
13. How does the rate at which we cool or heat the material affect the measurement?
14. Why did we need to calibrate the thermometer?
15. Calculate the percent error for your measured melting and freezing points. Why did your measurements
differ from the actual values?
16. What are other sources of errors in your measurements?
17. How would you redesign the experiment for a more accurate measurement?