Different freezing-points with different road salts
Broeke, J. van den & Pouwels, J.
Kandinsky College Nijmegen, the Netherlands
Received April 2011
Summary
The spreading of road salts in the winter lowers the freezing-point of water on the roads. Therefore ice
formation is delayed. Different road salts are used to cause this effect. This raises the question which road
salt has the most effect on the freezing-point of water with an equal amount of weight. Comparison of the
melting-graphs from different solvents (1gr of road salt resolved in 15 ml water) shows that NaCl (aq) lowers
the melting-point of water the most. However it also raises questions, such as which road salt is the best for
the environment, and are there any road-salts that we didn’t test that lower the freezing-point of water
even more than NaCl with the same amount of weight?
substance dissolved in water than the other
salts give (table 1).
Introduction
In the winter the municipalities use road-salt
to prevent the streets from being slippery.
This means that the salt which is used to
sprinkle on the streets, NaCl(s), has an effect
on the freezing-point of water.
Apart from NaCl, four other road-salts are
commonly used. These are CaCl2, C12H22O11,
CO(NH2)2 and MgCl2.
It was experimentally determined that one
mole of resolved substance per litre water will
lower the freezing-point of water with 1,86°C
(1). This value is called the freezing-point
depression constant (kf). This value can be
different in other liquids than water and
depends on the liquid in which the salt is
solved and not on the salt itself (2).
The five road-salts have a different molarity
and dissolved in water there are differences in
the number of ions. Because of this one gram
of each road salt dissolved in water gives a
different number of moles, thus the freezingpoint of each dissolved salt will be different
(table 1).
This raises the question: Which of the five
different road salts lower the melting point of
water most with an equal amount of weight?
The following road salts are used: CaCl2(aq),
C12H22O11(aq), CO(NH2)2(aq), MgCl2(aq) and
NaCl(aq).
Our hypothesis is that NaCl will lower the
freezing point of water most, because each
mole of resolved substance per litre water will
lower the freezing-point of water with 1,86°C
and one gram of NaCl gives more mole of
Road salt
CaCl2
MgCl2
C12H22O11
NaCl
CO(NH2)2
Mole of substance
when one gram
road salt is
dissolved in water
27,0x10-3
31,5x10-3
2,92x10-3
34,2x10-3
16,7x10-3
Expected
drop in
freezingpoint in oC
3,3
3,9
0,35
4,2
2,0
Table 1. This table shows how much moles are solved
when the road salt is dissolved in water and the
expected drop in freezing point for each road salt.
Experimental procedure and
approach
We took 6 identical test-tubes each filled with
15 ml of water. In each test-tube we added
one gram of one of the five road salts and to
the sixth test tube we added nothing. Then we
have frozen all of the test-tubes by a constant
temperature with a temperature sensor in it.
The temperature during the melting-range of
each solvent by the same environmental
temperature was measured by the 0135isensors with an accuracy of 1⁄10 degree and
recorded by the computer program Coach6
(figure 1).
Figure 2. Melting graph of MgCl2(aq)
Figure 1. The experimental set-up.
Data gathering and analysis
During the melting process the temperature of
each solvent was measured with the sensors
for four hours and recorded by the computer
program Coach6. The program Coach6 has
made a graph from the melting process of
each solvent. The program has also calculated
the differentiation of the melting graphs. The
first period the graph of the differentiation
incline to be zero, is the time the meltingrange takes place. The temperature of the
solvent during this period is the temperature
of the melting-range.
The calculated drop in freezing-point and the
measured melting-range can not be compared
with each other, because the one is only one
figure and the other is a period. To compare
the calculated drop in freezing-point with our
measuring-results, we take the average of the
starting temperature of the melting-range and
the ending temperature of the melting-range.
Figure 3. Melting graph of C12H22O11(aq)
Figure 4. Melting graph of NaCl (aq)
Results
We observed that there was a melting-range
visible in each graph, and that there was a
visible difference between the melting-ranges
of the different solvents.
Figure 2 to 7 show the melting graphs of the
different solvents (blue line) and the
differentiation of the melting graphs (red line).
Figure 6. Melting graph of CO(NH2)2(aq)
Figure 7. Melting graph of H2O
Table 2 shows the start of the melting-ranges,
the end of the melting ranges and the average
drop in freezing point.
Figure 1. Melting graph of CaCl2(aq)
Road salt
Start
meltingrange
End
meltingrange
CaCl2
MgCl2
C12H22O11
NaCl
CO(NH2)2
H2O
-5,2oC
-6,0oC
-3,0oC
-7,8oC
-3,5oC
0,0oC
-2,3oC
-3,0oC
1,0oC
-5,7 oC
-2,8oC
3,0oC
Average
drop in
freezingpoint in oC
3,8
4,5
1,0
6,8
3,2
-1,5
Table 2. This table shows the results of the
measurements of our experiment and the average drop
in the freezing point of the different solvents.
Conclusion and discussion
8
7
6
5
4
3
2
1
0
-1
-2
expected
drop in
freezing-point
measured
drop in
freezing point
difference
between
expected and
measured
Figure 8. This graphic shows the difference between the
expected and the measured drop in freezing point.
For all solvents the calculated drop in
freezing-point doesn’t match with the
measured drop in freezing-point (figure 8).
However, the biggest difference is the
difference between the expected and the
measured freezing-point of H2O. there are
several possible explanations for this
difference.
First it is a possibility that the temperaturesensors were not good gauged, because the
measured freezing-point of H2O is 1,5°C and it
is clear that the freezing-point of H2O is in
reality 0°C. Figure 7 shows that melting graph
of H2O starts at 0oC, as expected, but than the
temperature suddenly goes up and becomes
very irregular. Because of this, we think that
something else went wrong with the
measuring of the freezing point of H2O.
This isn’t the case with the other melting
graphs, because they are all quite regular.
It is possible that the sensor somehow moved
in the melting ice towards the lowest point of
the test-tube where the temperature could be
higher, due to the fact that it is far away from
the melting ice that floats in the top of the
test tube.
It is also possible that the measuring of the
amount of water and the amount of road salt
weren’t accurate enough. That would explain
the difference between the expected drop in
freezing-point and the measured drop in
freezing-point of the solvents.
Despite the problems mentioned above the
proportions between the expected drop in
freezing-point of the different solvents are the
same as the proportions between the
measured drop in freezing-point of the
different solvents. Therefore we think that the
proportions of the experiment are reliable,
and we can draw the conclusion from the
experiment that NaCl lowers the freezingpoint of water most with the same amount of
weight.
Evaluation
During our different experiments we have
tried to keep the control variables constant:
the environmental temperature, the amount
of water, the amount of salt, the size of the
test-tubes, the type of the temperaturesensor and the temperature of the frozen
solutions. We measured the same dependent
variable (the melting-point) in our
experiments with six different salts. We have
also calculated the expected drop in freezingpoint so we can check whether our theory is in
agreement with the measured results (figure
8).
As seen in ‘Conclusion and discussion’ there
were problems with keeping the control
variables constant. We thought we kept the
control variables constant, but according to
the results they differed too much.
To improve this experiment the temperaturesensors have to be gauged before the
experiment. To make the measuring-results
more reliable there are more measurements
per solvent needed. Also the amount of water
and road salt should be measured more
accurately and the temperature-sensor should
be positioned in the melting ice.
The conclusion of our experiment means that
NaCl is probably best of the five road-salts to
use to prevent the streets from being slippery.
However, literature shows that the use of
road-salts can contribute to environmental
pollution (3). This raises further questions for
inquiry: are there other road salts that are
environmental friendlier than NaCl? We can
also ask ourselves if there are other road salts
that we didn’t test that lower the freezingpoint of water even more than NaCl with the
same amount of weight.
Bibliography
1. Zumdah, S.S. (2009). Chemical
principles, pp. 865. Houghton Mifflin
Company.
2. Iru, P, Luib, A. & Nelem, D. (2010).
The effect of NaCl(s) on ice, pp. 135.
Journal of Anorganic Chemistry.
3. Kelting, D.L. & Laxon, C.L. (2010).
Review of Effect and Costs of Road
De-icing with Recommendations for
Winter Road Management in the
Adirondack Park, Saranac Lake:
AdkAction.