Freezing point depression
When the winter starts to fall, CaCl2 is good for all
Goeij, L.C. & Hagenaars, L.M.
’t Atrium, The Netherlands
13-04-2011
Summary
Road salt is used to prevent or remove ice on roads at winter time. Which type of road salt will give
the best result, is investigated in this article. The inquiry plan was simple: Five different types of
(alternative) road salt were compared to each other during a water-freezing experiment. A sensor
measured the temperature in the solutions while the heat was taken away. The results answered the
question. The hypothesis that was taken at the beginning of the experiment that calcium chloride has
the best lowering effect on the freezing point of water is thereby confirmed.
Introduction
When a new winter time has arrived, it
doesn’t stay unnoticed. The temperature is
reduced to far below zero and the roads get
all slippery. A solution is found to that last
problem: Road salt. But how does road salt do
this? That’s easy, salt lowers the freezing point
of water, so the rain that falls on the street
won’t turn into ice right away. A more
important question is: Which type of road salt
will give the best result? Is that the most used
salt sodium chloride, or is there another
substance that might have a better effect?
The research question thus is:
Which type of road salt has the best freezing
point-lowering effect on water?
We think that calcium chloride will turn out to
be the salt with the best lowering effect. This
expectation is based on the amount of
particles per molecule. A salt like sodium
chloride has got two types of ions when a
molecule dissolves in water: one sodium ion
and one chloride ion. When calcium chloride
dissolves, it dissolves into one calcium and
two chloride ions. The more ions get released
in the water, the more effect the salt has.
According to this theory, magnesium chloride
is also effective. We don’t think that both
molasses and urea will have a great working.
They do have many particles per molecule, but
not the whole molecule might split into loose
particles. Molasses and urea aren’t used as
road salt very often, so we think that the
reason for that is the low efficiency as a road
salt.
Experimental procedure
The inquiry question is investigated by taking
one independent variable, the substance,
measuring the temperature during the
experiment as the dependent variable and
keeping the rest even.
Materials needed for the experiment:
- 6 clean and dry plastic tubes
- 5 different types of road salt:
 NaCl (s)(Sodium chloride)
 CaCl2∙H2O(s)(Calcium chloride)
 CO(NH2)2 (s) (Urea)
 C12H22O11 (s)(Molasses)
 MgCl2 (s)(Magnesium chloride)
- Demi water
- 3 temperature sensors
- Computer
- Freezer (-25°C)
The salts were each put in a plastic tube
containing 25 ml of demi water. The salts
dissolved in the water to give separated ions.
Each solution had a molarity of 1,20M. this
amount is used because it is low enough to
dissolve quickly in the water and it is high
enough to notice clear differences in the
results.
The experimental set-up as shown on the
picture is used. The solutions were installed in
the set-up, three tubes at a time. There was a
temperature sensor in each of the tubes,
connected to a computer. This computer
visualized the measurements in a graph. The
whole installation was put in a freezer with
the temperature -25°C.
Results
The results obtained are shown in the graph
and table below:
Solutions
Molecular
Formula
Demi
water
Molasses
H2O (l)
Urea
C12H22O11
(s)
CO(NH2)2
(s)
NaCl (s)
Sodium
Chloride
Magnesium MgCl2 (s)
Chloride
CaCl2 (s)
Calcium
Chloride
Freezing
point
( ̊C)
-0,5
Freezing
point
compensated
0,0
-2,3
-1,8
-3,0
-2,5
-3,2
-2,7
-5,3
-4,8
-7,0
-6,5
Table 1: solutions with their measured freezingpoint
Graph 1 Temperature values
We observed the value of all the solutions.
These results are presented in graph 1. After
the solutions were put in the freezer, the
temperature of all the solutions descended
very fast. At the point when the temperature
is going to be almost constant, that’s the
freezing point. The freezing points of the
solutions are presented in table 1.
Conclusion and discussion
Based on the results, it can be concluded that
the salt which caused the lowest freezing
point of the water is calcium chloride with -7,0
°C. With this result, the hypothesis that was
thought of in the beginning is confirmed.
throughout the experiment, the same
variables
were
kept
constant:
the
environment temperature and the molarity in
each of the solutions. The same dependent
variable (the type of salt used for the solution)
was measured. In a way to check if the results
would be reliable, a tube with demi water only
was examined as well. With the knowledge
about the fact that pure water freezes at 0°C,
it was possible to see the difference caused by
wrong calibration. Because the found result is
that demi water freezes at -0,5°C, it is
necessary to change this difference in all the
other results as well. So every found freezing
point should be 0,5°C higher. This adaption
would bring them closer to reality.
Graph 1 shows that all the lines decline very
fast. The temperature falls fast in the first
twenty minutes. After this, the temperature of
the solutions remains approximately constant.
Magnesium chloride and urea remain less long
constant, they start to decline again. After a
while, all the graphs start to fall again, that’s
because at this point the whole solution is
freezing up. The order of the five salt solutions
are from high to low: Molasses, Urea, sodium
chloride, magnesium chloride and calcium
chloride.
With this as the result, calcium chloride
turned out as the best road salt to have a high
effect on the freezing point-lowering of water.
The theory mentioned in the beginning is true:
calcium chloride splits into more ions when it
dissolves in the water than when a salt like
sodium chloride does.
Calcium chloride is preferred over sodiumchloride releases energy upon forming a
solution with water, heating any ice or snow it
is in contact with. Calcium chloride
distinguishes itself from the other salts in that
way that it releases heat when the salt reacts
with ice (a reaction like that is called
exothermic). The released heat can be used
for the fight against the huge amounts of ice.
This heating-effect doesn’t work in a solution
form; the salt has to be applied in solid form.
The advantage of calcium chloride is that it
has a way better and longer lasting effect at
extreme temperatures than normal road salt
(-55°C compared to -22°C at normal road salt)
and that is has got the feature to take damp
out of the air very quickly. This causes a quick
brine formation.
A question arises: Why do we use sodiumchloride so often, then? A weakness of this
salt is namely that it tends to stimulate
corrosion much more than a salt like calciumchloride. However, calcium chloride is because
of its double amount of chlorine ions much
more toxic for plants and animals. Another
disadvantage is that it is much more expensive
to use as road salt than a salt like sodium
chloride.
Another prediction was that urea and
molasses wouldn’t have a high efficiency.
However, it is noticed that urea has got an
efficiency almost as high as sodium chloride.
The explanation why these two wouldn’t do
much was already given in the introduction.
They do have many particles per molecule, but
not the whole molecule might split into loose
particles.
Evaluation
On evaluating the experimental set-up, it is
believed that most of the control variables
used were managed well.
It might be better for the next time if the
solutions are stirred the whole time during the
freezing experiment. Without stirring,
differences in temperature occur. It hasn’t
been done this time because it would have
taken too long before the freezing would??
start. Now, it already took a couple of hours
per solution to freeze up completely.
Other questions have arised from this
experiment, such as: What causes the
difference between magnesium chloride and
calcium chloride? Both salts split into three
ions when they dissolve. Another experiment
that could be introduced is: What molarity
gives the best effect?
Bibliography
1. http://www.zout.be/winter/dossierstr
ooizout4.html
2. http://en.wikipedia.org/wiki/Freezingpoint_depression
3. http://nl.wikipedia.org/wiki/Vriespunt
sdaling
4. http://www.ar.cc.mn.us/chemistry/Ch
em1062/Labs/FreezingPoint/FpDepres
sionLab2.htm
5. http://www.chemprofessor.com/colli
gative.htm
6. ICY booklet