Will plants beside the roads survive?
Bay, B., Bruin, R., & Veerkamp, L.
_______________________________________________Received
April 2011_____________________________________________
Summary
In areas which have cold winters, road de-icing is necessary to maintain the safest possible
traffic situation. De-icing is done by spreading salt or a salty solution on roads. Salt affects
H2O(s) by speeding up the melting process, caused by the lowering of H2O(l)’s freezing point.
In the Netherlands, NaCl (s) is the most common used road salt. The question is though: is
NaCl (s) the best road salt available, concerning the environment? We tested the effect on the
growth of plants and the duration of the melting process using different concentrations or
amounts of salt. By comparing the effect of NaCl (s) on watercress to the effect of
Mg(CH3COO)2 (s) on the same plant, it was found that there was a better reduction of
material affected by Mg(CH3COO)2 (s). Because of the over measure we used in our first
experiment, both plants died. However, the plants affected by Mg(CH3COO)2 (s) had mould
on them, whereas the NaCl (s) affected plants didn’t. The experiment showed that there was
life possible on the watercress affected by Mg(CH3COO)2 (s). This raised more questions
which will result in further experiments.
___________________________________________________________________________
Introduction
In highway de-icing, salt is used to lower the
freezing point of H2O (l). What often isn’t
considered, is the negative effect salt can have
on plant growth. Regarding to 1 CMA is an
eco-friendly alternative to NaCl (s) because it
is a natural acid with the same chemical
properties as vinegar. Therefore, CMA would
be a better de-icing salt considering the
environment.
Another question which comes to mind
though, is whether this alternative road salt is
equally efficient as the one already used. For
this matter, the following chemical formula with which the freezing-point depression can
be calculated - is used:
ΔTF = KF · m · i
 ΔTF, the freezing point depression
(lowering of freezing point).
 KF, the cryoscopic constant, which is
dependent on the properties of the solvent,
not the solute (for water, KF = 1.853
K·kg/mol).
 m, the molality (mol solute per kg of
solvent).
 i, the van 't Hoff factor.
1
http://www.sustainabilityconsulting.com/blog/2010
/3/1/views-eco-friendly-de-icing-alternatives-tosalt.html
It is found that the freezing point depression of
Mg(CH3COO)2 (s) is a little more effective
than NaCl(s):
NaCl (s): ΔTF = 1,853 • 1,711 • 2 = 6,342
Mg(CH3COO)2 (s): ΔTF = 1,853 • 1,120 • 3 =
6,669
In theory CMA is better to be used on both the
environmental as the effective side. We now
try to prove this in practice: How does
Mg(CH3COO)2 (s) affect H2O (l)’s freezing
point and the growth of plants, compared to
NaCl (s)?
Our hypothesis is that Mg(CH3COO)2 (s)
melts ice faster than NaCl (s), considering the
freezing point depression. Moreover, we think
Mg(CH3COO)2 (s) is more environmentalfriendly than NaCl (s), based on their
properties.
Approach of the experiments
In our investigation, we focused on two
aspects of the replacement of NaCl (s) by
Mg(COOH3)2 (s).
First, the lowering of the melting point of
water was inquired, using the two kinds of
salts. We added 10 mL of 0,17 M NaCl
and 0,17 M Mg(COOH3)2 (s) to two
identical measuring cylinders and funnels,
already filled with 10 grams of ice. We
took track of the time and the melted
water. These measurements were
compared by putting them in a table and
transferring them into a graph.
Secondly, the effects of the different salts
on the environment were tested by adding
salted water to watercress. We let the
watercress seeds germinate in damp and
warm surroundings.
Then we put the plants in ten Petri dishes,
ten germinated seeds in each:
two control groups, without added salt
two lower concentration groups, with 0,17
M salt, one NaCl, one Mg(COOH3)2
two higher concentration groups, with 0,51
M salt, one NaCl, one Mg(COOH3)2
two groups with 0,025 grams of salt, one
NaCl, one Mg(COOH3)2
two groups with 0,05 grams of salt, one
NaCl, one Mg(COOH3)2
The concentrations and amounts of mass
were determined by a literal source2.
We evaluated the condition of the cress by
judging the plants’ number, colour and
structure. These conditions will differ
from each other mainly due to
concentration variables, since we tried to
keep other variables, like temperature or
light intensity, constant.
The results of time measured are as follows
(see table 1 and figure 1+2).
Time
(in
min.)
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
0,17
mole/L
NaCl
0
0
0,6
1,2
1,8
2
2,1
2,5
2,8
3
3,1
3,1
3,2
3,4
3,9
4
0,17
mol/L
CMA
0
0
0,7
1,1
1,3
1,8
2
2,5
2,8
3
4,3
4,8
5
5,3
5,8
6
Table 1
Results
Before we could start measuring, there first
was some math to be done.
Molar masses:
NaCl: 58,44 g/mole
Mg(COOCH3)2: 142,398 g/mole
We used two different concentration values in
the solution: 0,17 mole/L and 0,51 mole/L. In a
solution of 10 mL, this came down to:
NaCl
Mg(COOCH3)2
0,17 mole/L
0,1 grams
0,242 grams
0,51 mole/L
0,3 grams
0,769 grams
These quantities of salt were added to 10 mL,
which was spread on 10 grams of ice.
2
Figure 1 - 0,17 mole/L NaCl/CMA
http://onlinepubs.trb.org/onlinepubs/sr/sr235/126138.pdf
Figure 2 - 0,51 mole/L NaCl/CMA
0,51
mol/L
NaCl
0
1,9
2,2
3
3,8
4
4,5
4,9
5,1
5,4
6,1
6,2
6,9
7
7
7,1
0,51
mol/L
CMA
0
1,8
2,1
3
3,8
4
4,2
4,9
5
5,2
6
6,1
7
7,8
8,1
8,2
Conclusion and Discussion
As shown in figure 1+2 the ice with
Mg(COOH3)2 (s) melted faster. This became
even more clear in the more dense solution. It
proves Mg(COOH3)2 actually is a more
effective salt to melt down ice with, as was
predicted by the theory.
After the melting process we planted
watercress. When they had grown large
enough we put 10 cress plants in a Petri dish.
At first we used the same (high) molarity for
each salt, as to see a clear difference. As you
can see in the appendix, the NaCl (s) was more
lethal for the plants than Mg(COOH3)2 (s).
We also did an experiment with the same
amount of grams, thereby trying to imitate true
conditions in which vegetation will have to
grow when salt is spread on the roads. The
amount used here is therefore approximately
the same as used when de-icing. In this
experiment we saw hardly any effect: the
amount of salt must have been insufficient to
damage the cress.
Because both tests didn’t show a significant
effect, we can conclude that both salts used in
the given concentrations aren’t directly
destructive for the environment.
However, after a few days, mould showed in
the Mg(COOH3)2 Petri dish. This proofs
Mg(COOH3)2 is after all better for the
environment. Plants might accidentally take in
more salt than they can bear. When this
happens, there isn’t forming any mould on the
cress with NaCL(s), there is forming some on
Mg(OOCCH3)2 (s).This means that, when
added in a lethal quantity, Mg(OOCCH3)2 (s)
gives better results for letting the plant be
biodegradable. The vegetation is decomposing,
recycling, and gives nature the opportunity to
start new life. If there isn’t mould after plants
die, the decomposing will be slower and the
plants would stink during the process. This
won’t be good for the environment
Evaluation
First, the watercress was really simple to get.
In our school we made the whole experimental
set-up, we didn’t have to buy anything.
The co-operation went well too. Although we
were with three persons instead of two, we
didn’t think this was a disadvantage.
The only thing we would do better next time
was using the same amount of gram and not
the same molarity as we did now. In real life
they don’t look at the molarity of the salt with
spreading salt, they look to the amounts of
gram. In overall though, the experiment and
writing the report went very well.
Bibliography


http://www.sustainabilityconsulting.co
m/blog/2010/3/1/views-eco-friendlyde-icing-alternatives-to-salt.html
http://onlinepubs.trb.org/onlinepubs/sr/
sr235/126-138.pdf
Appendix



Photo of our group
Drawing of experimental set-up
Pictures of experimental set-up,
information added about condition
plants