Salts are cool? Brecht Berben and Victor Rieff ([email protected]

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Salts are cool?
Brecht Berben and Victor Rieff ([email protected] and [email protected])
Brecht Berben and Victor Rieff
K.S.G. De Breul, The Netherlands
K.S.G. De Breul, The Netherlands
Submission date: 15/03/2011
Submission date: 15-03-2011
Summary
To prevent icy roads, road salt is used in many countries to melt the ice. Mostly NaCl is used, but in countries
with very harsh weather conditions (temperatures below -20°C) CaCl2 is applied. In this report experiments
are done to prove that CaCl2 causes the largest freezing point depression compared to NaCl. The temperature
of melting ice to which salt was added was measured and compared to the theoretical freezing point
depression. It was found that NaCl resulted in the largest freezing point depression followed by CaCl2,
CH3COONa and Ca(NO3)2. On a relative basis the measured freezing point depressions corresponded to the
theoretical values. To improve the measurements it is suggested to do future experiments at lower
temperatures.
Introduction
All residents of The Netherlands have to deal
with snow and ice in the winter. These
weather conditions cause traffic-jams and
other problems in daily transportation and it is
preferred to remove ice and snow from the
roads. This is done by using salt. Salt on snow
(or ice) lowers the melting temperature. Ice
and snow (H2O(s)) melt at a temperature of
0°C. By adding salt, the melting temperature
will be lowered to, for example, -3°C, resulting
in the formation of liquid water that doesn’t
freeze. In The Netherlands, NaCl is the most
commonly used road salt (1). However, in
countries where temperatures are below 20°C, CaCl2 is used as road salt (2).
The question that has been examined in this
experiment is: Which salt gives the largest
freezing point depression, and is,
consequently, the best road salt?
Also, the freezing point depression of two
other salts are measured, namely a calciumbased salt and a sodium-based salt whose
anions contain no chloride.
We expect CaCl2 to be the best road salt (the
largest freezing point depression), because it
is used as road salt in areas where
temperatures are lower than -20°C. This may
be due to the fact that CaCl2 gives 3 moles of
ions while dissolving in water. NaCl gives 2
moles of ions when dissolving in water:
CaCl2 (s)  Ca2+ (aq) + 2 Cl- (aq)
NaCl (s)  Na+ (aq) + Cl- (aq)
The more moles formed, the larger the
freezing point depression.
The freezing point depression is a property
depending only on the number of dissolved
particles, not the identity of the particles (3).
The freezing point depression is given by the
following formula:

∆ = K ∙ M ∙ i
(4)
∆ is the freezing point depression in K. K is
the cryoscopic constant of the solvent in K ∙ kg
∙ mol-1, m is the total mass of solute per kg
solvent in kg, M is the molar mass of the
solute in gram ∙ mol-1 and i is the van ‘t Hofffactor which represents the number of formed
ions (4).
Experimental design
Take 50 grams of ice cubes. Crunch these ice
cubes in a cruncher. Let the temperature of
the ice rise to -10°C. When it reaches that
temperature, put the crunched ice cubes into
a funnel with a filter. Put 6 grams of salt over
the crunched ice. After waiting for 20 minutes,
measure the temperature of the melt water
from the ice with a thermometer. Repeat this
for every salt, three times.
In this experiment, the salt is the variable
factor. Different salts were used (NaCl,
Ca(NO3)2, CH3COONa and CaCl2). The quantity
of the ice and salt were kept constantly. The
experiments are done at ambient
temperature.
the different salts that are used in this
experiment.
Tabel 1. Freezing-point depression measured.
In the table the measured temperaturedepression (3 experiments) versus melting ice
containing no salts (0°C) is given in °C for
different salts.
ΔT
exp.
1
NaCl
6,0
Ca(NO3)2
1,9
CH3COONa 3,9
CaCl2
5,4
Salt
ΔT
exp.
2
6,1
2,2
4,4
5,8
ΔT
exp.
3
5,7
1,4
4,1
4,8
ΔT
average
5,9
1,8
4,1
5,3
These results show that when using the same
weight of salt the largest freezing point
depression is obtained with NaCl, followed by
CaCl2, CH3COONa and Ca(NO3)2.
The freezing point depression can be
measured with the following formula:
K is a constant. For water the value is 1,86 K ∙
kg ∙ mol-1. i is the number of formed moles of
ions from the salt. m is also kept constant,
namely 120 grams. This because 6 grams salt
were mixed with 50 grams of ice. This
corresponds with 120 grams on 1 kg. M is the
molar mass of the salt.
Figure 1. The used set-up.
Results
Shortly after adding the salt to the ice, the ice
begins to melt. The ice melts for about 10
minutes at a constant rate. After a longer
period of time, the ice forms less melt water.
Table 1 shows the measured temperatures for
The theoretic freezing point depression that
are calculated for the different salts are:
120
NaCl:
ΔT = 1,86 ∙ 58,44 ∙ 2 = 7,64°C
Ca(NO3)2:
ΔT = 1,86 ∙
120
164
∙ 3 = 4,08°C
CH3COONa: ΔT = 1,86 ∙
120
82
∙ 2 = 5,44°C
ΔT = 1,86 ∙
120
111
∙ 3 = 6,03°C
CaCl2:
Figure 2 shows the calculated and measured
temperature-depressions compared to each
other. The calculated values are larger than
the measured values but the relative order of
freezing point depression is the same. In
descending order of freezing point depression:
NaCl, CaCl2, CH3COONa and Ca(NO3)2.
10
8
6
4
2
0
The measured temperatures differ from the
calculated temperatures. This may be caused
by the fact that the experiment is done at
ambient temperatures. The temperature at
which the experiment has been done was
20°C. Because the melt water was exposed to
this temperature for a maximum of 20
minutes, the measured temperature drop is
too low. This effect was the same for every
experiment. The measured temperatures can
be compared on a relative basis.
measured
calculated
Evaluation
Figure 2. Freezing-point depression.
Discussion and conclusion
The average temperature-depressions were
compared. NaCl caused the largest freezing
point depression. Based on the temperature
effect, NaCl can be considered to be the best
road salt.
The hypothesis is therefore incorrect. This
experiment shows a larger freezing point
depression for NaCl compared to CaCl2. The
fact that CaCl2 is commonly used as road salt
in countries with very harsh weather
conditions, must be based on other properties
than only the freezing point depression. It is
known that mixing CaCl2(s) with water
releases heat (2). It is also reported that CaCl2
picks-up moisture from the air faster than
NaCl, which ensures a quicker formation of a
salt-solution (2).
The freezing point depression isn’t the only
characteristic of a salt when used as a road
salt to prevent icy conditions.
Looking back at the experiment, the ambient
temperature is an important point to be
improved. In case the experiment is done at a
relatively high temperature, the measured
freezing point depression is under estimated.
It would be better to perform the experiments
at a temperature just above 0°C. This ensures
that the melt water doesn’t warm up rapidly
and the results are closer to the theoretic
predictions.
Bibliography
1. http://www.rijksoverheid.nl/documenten
-en-publicaties/vragen-enantwoorden/hoe-wordt-gladheid-op-deweg-bestreden.html#anker-soortenstrooizout
2. http://www.zout.be/winter/dossierstrooi
zout4.html
3. Atkins, P.W. (1986). Physical Chemistry
third edition page 170-173, Oxford
University Press.
4. http://nl.wikipedia.org/wiki/Vriespuntsdal
ing
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