The effect of salty water on iron.
Noij, R. & Fölker, B.
June 1 2011
Summary
Road salt is used to de-ice roads in winter. The salt lowers the freezing point of water and
delays ice formation but does it has any disadvantages. This raised the question ‘What is the
effect of salt water on iron?’ The experiment shows that this salty ice combination increases
the rusting speed of iron objects. In the experiment four identical slices of iron were exposed
to certain condition. Only salt was used as a condition to see if it had any influences by itself.
Also a 10% salt solution was used that is commonly used as road salt. The salt solution was
maxed out to see if it increased the corrosion and water was used by itself to see if it had any
effect. The four iron slices were weighed before and after the experiment to see what the
differences in weight were.
Introduction
During winter time, snow falls down from
the sky. This structure of water can be very
beautiful, but it can also cause lots of
problems on the road. Scientists discovered
that salt lowers the freezing point of water
(1). This is why people spread salt on the
road to melt the snow and to prevent the
water from freezing again. It also makes it
a lot safer to drive in winter time, but does
is this road salt all good or does it bring
any disadvantages. Last January it was
noticed that bikes were more damaged then
previous years. Particularly the weeks with
snow caused a lot of corrosion on bikes.
That is why the question was raised about
the effect of salt on the rusting process of
iron. Because of how bike’s reacted to the
salty ice The hypothesis was made: ”iron
that is exposed to water with dissolved salt
will rust faster than iron that is exposed to
just water or just salt”. The oxidation of
metal is called corrosion, you can write
down this chemical reaction between
oxygen and iron as:
3 O2 (g) + 4 Fe (s) → 2 Fe2O3 (s) (2).
As you can see in the equation oxygen
molecules take the iron molecules out of
their lattice to form a bond. That’s why
less and less iron molecules stay in their
lattice and because of that the mass of the
original piece of iron lowers. This is the
base of the idea to compare four identical
iron slices exposed to different conditions
such as surrounded by water, salty water,
only salt and a saturated saline. These
conditions were chosen because of a
couple of reasons. Every car is troubled
with water each year, and with putting the
slice in only water we could see how much
effect this has. Solid salt was used to see if
it had any effect by itself. We used a 10%
salt solution because this is the ratio on the
roads (3) and we used a saturated solution
to see what more salt would do. The rest of
the conditions will be kept the same for
each beaker so only differences due to the
conditions we changed will be noticed. The
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temperature at which the experiments will
take place is 1 °C to imitate the conditions
of winter as much as we can without
freezing the water. The salt that is used in
the experiment is NaCl (s) because that is
the most common road salt. The iron slices
are weighed before and after the
experiment and compared these results
with each other. The prediction is that if
salty water causes iron to rust faster, then
the difference in weight, before and after
the experiment, of the iron slice that is
exposed to salty water will be bigger than
the difference in weight, before and after
the experiment, of the iron slice exposed to
just water.
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Materials
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Four beakers of 500 mL
Four slices of iron from 40 cm x 10
cm x 1,0 cm
About 1500 mL of demi-water
About 400 gram of NaCl (s)
Scales
Stirring tool
Marker
-
Method
-
-
-
so that each iron slice is completely
surrounded by water/salty
water/salt.
Mark each beaker with a number
from one till four.
Put four of the same iron slices in
each beaker and make sure that it’s
diagonal in it so the contact surface
is optimal.
Put the four beakers in the
refrigerator at a temperature of 1 °C
and first wait for two days. Take a
look at the four different beakers
and write down the differences you
can see. Take a picture off the
beakers. After this wait for a week
and a half.
After a week and a half get the four
beakers out of the refrigerator. First
write down the differences you can
spot between the four beakers and
take a picture of every beaker. Take
the four iron slices out of the
beakers and take a picture of them
as well.
Wash all of the rust of the iron
slices and dry them. Make sure you
still know which one belongs to
which beaker.
Weigh the iron slices again and
write your figures down in a table.
Take the four iron slices and weigh
them. Write the masses of each iron
slice down in a table.
Take the four beakers. Fill the first
one with demi-water, the second
one with NaCl (s), the third one
with a 10 volume % of NaCl (s) in
water and the fourth one with a
saturated saline of 360 g NaCl
(s)/L.
Make sure that each beaker is filled
enough with water/salty water/salt
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Results
After two days the experiment was
checked and observed (Fig. 1). The most
corrosion was detected on the iron slice in
beaker 3 with the 10 volume % of NaCl
(s). After that the iron slice in beaker 1
with demi-water had the most rust , no rust
had formed on the iron slice of beaker 2
with NaCl (s). In beaker 4 ( with the
saturated salt saline) no rust was detected
on the iron slice as well.
to get off (Fig. 3). Table 1 shows the
masses of the iron slices that we found
before the experiment and after the
experiment.
Figure 2: Beakers after 12 days
Figure 1: Beakers after 2 days
12 days after we started the experiment we
finally checked it to gather the results (Fig.
2 and 3). Beakers one till three showed not
much of a difference with our first check.
Beaker two still didn’t show any corrosion
Beaker one and three showed more
corrosion and the third beaker still showed
the most. In both beaker 1 and 3 rust had
deposited on the bottom of the beaker. In
beaker four the situation had changed
drastically. Some kind of salty crust of
about 1 cm had formed on the edges of the
beaker. The iron slice in the beaker now
was corroded and there was also rust on
and around the bottom of the beaker. There
was not just rust on the iron slice but a
crust of rust and solid salt had formed on
the surface of the iron slice that was hard
Figure 3: Iron slices after 12 days
Number
of
beaker
slice of
iron was
in.
1
2
3
4
Mass
Mass after
before
Difference.
experiment.
experiment.
329,00 g
328,93 g
0,07 g
328,50 g
328,47 g
0,03 g
328,60 g
328,36 g
0,24 g
329,70 g
329,60 g
0,10 g
Table 1: Mass (g) of the different slices of
iron before and after the experiment.
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Conclusion
Based on the results of the experiment and
the observations we did regarding these
experiments, we can conclude that
dissolved NaCl speeds up the process of
rusting. The hypothesis that iron exposed
to water with dissolved salt will rust faster
than iron exposed to just water is hereby
confirmed, because the difference in
weight, before and after the experiment, of
the iron slice that is exposed to salty water
will be bigger than the difference in
weight, before and after the experiment, of
the iron slice exposed to just water.
Experiment two shows that only dissolved
NaCl speeds up the process of corrosion
and NaCl (s) doesn’t, because we didn’t
see any corrosion. No sensible conclusion
can be drawn from experiment four.
The results of the experiments didn’t differ
that much. The masses of each of the iron
slices were in the beginning almost
identical to the masses at the end of the
experiment. This is what makes the inquiry
less reliable. The conclusion is though still
right because the observations were clear.
To make this inquiry more reliable thinner
slices of iron could be used in a following
experiment. The surface of the slices will
pretty much remain the same whereas the
mass of the iron slice will be much less.
This causes the percentage of mass of the
iron slice that is used for corrosion will be
higher and by that the experiment will be
more reliable. Experiment four was done to
check if more dissolved NaCl would speed
up the process of corrosion. It turned out
afterwards that O2 (g), which is required
for corrosion, couldn’t dissolve anymore
because of saturated saline. That could
explain what was observed during the first
check. The slice of iron didn’t corrode and
because the O2 (g) couldn’t reach it. In the
time that the beakers were in the
refrigerator the water could still evaporate
a little bit at 1 °C and therefore
crystallization could take place on the
edges of the beaker forming a crust. This
means that there is room again for O2 (g)
because part of the dissolved NaCl is out
of the saline. Because of this corrosion can
take place again. That might be the reason
why there was rust in the beaker when we
checked for the second time. It was also
weird that a crust of NaCl (s) and rust had
formed on the iron slice of experiment
four. The crust prevented the iron slice
from corroding any further and that’s not
fair compared to the other experiments. To
prevent all that happened in the fourth
experiment a less saturated saline could be
used in a following experiment. To draw a
sensible conclusion in a following
experiment a for example 50 volume %
NaCl saline could be used. The conclusion
drawn from experiment 2 might be invalid
because no control experiment was done
with regard to this experiment. You should
have compared the iron slice in beaker 2 to
an iron slice exposed to the air. Our
conclusion drawn from experiment 2 is
however though valid because half of the
iron slice was exposed to air and half of the
iron slice was exposed to NaCl (s). No
differences were found between these two
halves and that’s why NaCl (s) has no
effect. During this inquiry a couple
questions remained unanswered. We don’t
know if only dissolved NaCl speeds up the
process of corrosion. Other salts might too
and they might even work better. We also
don’t know if only the oxidation of iron is
catalysed by salt. Other metals might
corrode faster or might form a layer that
oxygen can’t fit through as we thought we
saw in experiment four. Whatever the case
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may be, using salt on roads not only brings
advantages but it also carries disadvantages
such as corrosion but there might be other
negative effects on for example the
growing of plants. That’s why we need to
come up with more ideas and test different
types of salts to solve these problems and
make the roads saver and happier again.
References
(1), (3) Iru, P et al. (2011). Journal of
Anorgano chemistry. Vrije Universiteit
Amsterdam.
(2) Wikipedia: the free encyclopedia,
Redox. http://en.wikipedia.org/wiki/Redox
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