Article:
Effects
of NaCl
on
water
Team 17:
Merve Orta V5B
Martine Wijnhorst V5B
1st of June 2011
Summary
The inquiry is about the freezing of
water in comparison with the freezing
of the mixture water and salt. We
wanted to know which one freezes
faster and how this works. To find out
which one freezes faster, we made up
an inquiry. We used two small boxes
and we put in one only 15 ml of pure
water and in the other one also 15 ml
of water but in this one we solved
some salt by stirring. We put both
boxes in the fridge and we looked
every 10 minutes to slowly see ice
forming. We wrote down our
observations. This experiment was
repeated 4 times to be able to
compare the results and take an
average. The observations were
different than we expected. The box
with the mixture water and salt did not
freeze very fast in the beginning, but
after a certain point it was completely
frozen. After some research, we
learned that salt lowers the freezing
point of water and that salt has a great
effect on the melting and freezing of
water.
Introduction
In The Netherlands, it does not snow
very often but when it does snow,
measures are taken quickly by the
government. You will see salt trucks on
the streets. Salt is used to melt the ice
to prevent slipping and accidents. The
melting point of water is 0˚Celsius and
the melting point of sodium chloride is
801˚Celsius. When you solve a
substance in water the freezing point
gets lower. It gets harder for water to
form a solid. The melting point of a
water-NaCl mixture is around 6
degrees lower than pure water when
the percentage salt is 10%. The
melting point drops 16 degrees if the
percentage salt is 20%.
In class, we have seen how water in
the form of ice melts and how a
mixture of water and salt in the form
of ice melts, and surprisingly water
melted faster than the mixture of
water and salt. We wanted to know
which one freezes faster, and so we
made up an experiment. With all the
information above, we have thought of
an expectation what would happen.
Our inquiry question was: “Does salt
have any influence on the freezing of
water?” Our hypothesis was: “Water
with salt will freeze slower than pure
water.” This expectation is mostly
based on the fact that salt is used to
melt ice on winters days. Salt is thrown
on snow and mixes with the H2Omolecules, to lower the freezing point
of the water. So we expected to see
ice earlier in the pure water box than
in the salty water box.
Experimental design
The experiment was not too hard to
carry out. We just used two boxes, one
with water and one with salt water, we
put them in a fridge and looked every
10 minutes what happened. We chose
to take small amounts, because small
amounts freeze faster and in this way
it would be easier for us to see the
water freezing and to do the
experiments at home. The first time
we did the experiment, we used a very
small amount of salt. In this way, it is
very hard to see if the salt has any
influence on the freezing of water,
because there is so less salt. So the
other times we did the experiment, we
used more salt to make it easier to see
any difference in the freezing process.
We thought about what kind of water
to use, and we chose to use distilled
water. We did this, because we wanted
to make sure that no other materials
were in the water, so no other
reactions would take place. We used
one kind of salt, which was sodium
chloride, to make sure the results of all
the experiments would be almost the
same. By doing all these things, we
tried to make sure that there was only
one variable different. The variable
was that one of the boxes consisted
pure water, and the other one pure
water mixed with some salt. This made
clear what effect salt has on the
freezing process of water.
Results
We did the experiment 4 times. The
first time, we looked only after an hour
but after an hour, both boxes
consisted of ice and so we did not see
which one froze faster. Beneath the
results of the other 3 experiments are
showed.
1 Time
After 10 min.
After 20 min.
After 30 min.
After 40 min.
After 50 min.
After 60 min.
After 70 min.
After 80 min.
Water
liquid
some ice formed
more ice formed
same, most ice is on top
mostly frozen but it still consists some liquid
seems to have a little bit more liquid than before
almost completely frozen
completely frozen
Mixture water and salt
liquid
liquid
still liquid
still liquid
completely frozen
completely frozen
completely frozen
completely frozen
2 Time
After 10 min.
After 20 min.
After 30 min.
After 40 min.
After 50 min.
After 60 min.
After 70 min.
After 80 min.
Water
liquid
liquid
ice formed on top
more ice formed
more ice formed
still contains some liquid
almost frozen
completely frozen
Mixture water and salt
liquid
liquid
liquid
liquid
liquid
most is frozen
completely frozen
completely frozen
3 Time
After 10 min.
After 20 min.
After 30 min.
After 40 min.
After 50 min.
After 60 min.
After 70 min.
After 80 min.
After 90 min.
After 100 min.
Water
liquid
liquid
some ice formed
more ice formed on top
more ice formed
almost frozen
still some liquid
still some liquid
still some liquid
completely frozen
Discussion and Conclusion
It is known that the freezing point of
water is 0˚Celsius, however when salt
is added to it, its freezing point
decreases. From our observations we
can conclude that sodium chloride has
Mixture water and salt
liquid
liquid
liquid
liquid with some small ice particles
liquid with some small ice particles
some ice formed
most is frozen
completely frozen
Completely frozen
Completely frozen
an influence on the forming of ice. The
results of our experiment make clear
that salt water has a lower freezing
point, because ice was already formed
in the box with only distilled water
while salt water was still liquid. At that
point, so after approximately 20
minutes, the boxes must have reached
a temperature of 0˚ Celsius, because
otherwise distilled water would not
have started freezing. The box with a
mixture of salt and distilled water was
still liquid, proving that it needs a
lower temperature (below 0˚ Celsius)
to form ice.
The theory behind the fact that a
mixture of salt and distilled water
freezes later than just distilled water is
because of the different substance
properties.
0˚Celsius is the freezing point of pure
water. When the temperature declines,
water molecules lose heat and so their
kinetic energy and start to freeze at
0˚Celsius. Approximately after 80
minutes, the temperature is too low for
enough movement to be liquid and all
the water molecules turn into solid:
ice.
The freezing process of salt water was
totally different. The mixture was still a
liquid after 20 minutes when it
probably had reached a temperature of
0˚ Celsius. The NaCl-molecules disrupt
the freezing process of water. This is
because salt water is denser than pure
water and by this is it is heavier and
will freeze more slowly.
As we told in the introduction, salt
counters the freezing of water. Below
0 degrees the water will not freeze due
to the salt. But when finally the new
freezing point is reached, the water
Bibliography
can finally freeze because the NaClmolecules are hydrated. Because the
new freezing point is way lower than
the actual freezing point of water, the
water will freeze immediately. When
The formed ice consists of separate
salt crystals (NaCl·2H2O) and ice
crystals.
Evaluation
The first time we did the experiment
the amount of sodium chloride we
used was too small compared to the
amount of pure water. The effect of
this mistake was that we could not see
any differences in the results. There
was not much influence of the salt at
the freezing process of water. We also
overestimated the time we had to
check on the boxes to make good
observations. We thought that it would
take longer to freeze both boxes but it
took less than an hour. We improved
on these mistakes by using more
sodium chloride and we decided to
look every 10 minutes what the
changes were, so these problems are
already solves. What we could have
done, but didn’t do, is repeat the
experiment more often. Because the
first time failed, we only had 3 times
the experimental results, and so there
was not a lot to compare. Over all, we
carried out the experiment very good,
and we were very precise. The inquiry
question was clear and the experiment
to investigate this was very good.
For the experiment and all the theory, we used:
- The booklet “ Icy – road salt”
- http://www.delftintegraal.tudelft.nl/info/index7483.html?hoofdstuk=Artikel&Ar
tID=2046
- http://science.howstuffworks.com/nature/climate-weather/atmospheric/roadsalt.htm
- http://www.worsleyschool.net/science/files/saltandfreezing/ofwater.html