KHCO2 effecting water’s freezing point
Berend Smeiting, Koen Wesselink en Rik Helwegen
KSG De Breul, Zeist, The Netherlands, 15 March 2011
Summary
Salt is a good solution to icy roads, but is it the best solution? In this research article we will talk about our findings in on the
road to our conclusion and how we were inspired to test with potassium formate. When the time came that we had to hand in
the scenario for our research Berend made an inquiry about something he had been told about from his father, a salt used on
airports to de-ice the runways. This substance was a good de-icer, but will it be a good replacement for salt? We felt that this
was a good subject for our review. With our main question: 'What is the effect of KHCO2 (aq) on the freezing point of water?' we
hoped to begin a review that was relevant for others too. The next step was putting it into practice. After collecting reliable
results we were able to pull off a conclusion, there was proof to conclude that there is a linear connection between the mass of
potassium formate and the freezing point of water. It turned out that the freezing point of a water solution will be around -0.6
times the mass percentage of the potassium formate.
Introduction
In icy winter days roads are sprinkled with NaCl(s, this
causes corrosive damage to metals and pollution due the
salt.
We wanted to look for a new salt that is not corrosive and it
witch is biodegradable. We found out that potassium
formate meets these requirement. To find out if the new
salt helps to keep the roads ice free, we are going to do
research about the freezing-point depression.
The phenomenon that causes a lower freezing point of a
solvent (liquid) by adding a other substance to this pure
solvent is called Freezing-point depression. The Freezingpoint depression is caused by a lowering of the vapor
pressure of a solution. When a substance with 2 different
phase states, like liquid water and solid ice, are being put
together they will exchange Molecules. This exchanging
causes the vapor pressure. The molecules will leave the
phase state with the highest pressure and join the one with
the lowest pressure. When the 2 substances have the same
pressure, then you have reached the freezing point of the
solution. Potassium formate will dissolve better in a liquid
phase state then in a solid state. So more Potassium
formate Molecules will go the liquid phase state. Raoult’s
law says that this cause a lowering of the vapor pressure in
the liquid phase state.
And as described earlier, the molecules will go from the
solid phase state (highest pressure) to the liquid phase
state (lowest pressure). This will stop when there is a
balance.
With an ideal solution, the freezing-point depression is only
influenced by the solute concentration. To calculate the
freezing-point depression in such situation, you can use this
formula:
ΔTF = KF · m · i
ΔT = the freezing point depression
KF = the freezing-point depression constant (for water this
is: 1.853 k · kg/mol)
m = molality of solvent
i = the van’t Hoff factor, for potassium formate this is 2
But this linear relationship with the freezing-point
depression constant is only relevant when there is a ideal
solution. But our solute is most likely not ideal, because our
Molecules in the solution are not chemical identical. So this
raises the question: What is the effect of KHCO2 (aq) on the
freezing point of water?
Our hypothesis is that the potassium formate will lower the
freezing point of water, because the described theory
above tells you this. But how much it lowers and what kind
of relationship there is between the concentration KHCO2
and the freezing point of water we don’t know. We do think
that it is not exactly linear, because we have no ideal
solution
Experimental design
The main dedication of this experiment is finding out the
temperature during melting processes. A melting process
takes time and energy, mostly in the form of heat, this
causes he rising temperature of the substance to hold stable
for a while during the process. We made several solutions
adding potassium formate to pure water.
We got our potassium formate as a solution with water
already. So in a pre-experiment we evaporated the
substance and determined the mass percentage in a reliable
way. By measuring the volume of any liquid with a graduated
cylinder we always looked at the lowest point of the
meniscus, this is a recognized method to prevent differences
in measures. The solutions we made for our experiment had
after being watered a mass percentage of thirty, twenty, ten,
five and zero percent. The last one serves as a control of the
validity of our results. Every solution was made twice to
make sure no major errors were caused and not noted from
the very beginning. We putted the solutions in a freezer for 2
days. The freezer could do minus 18 degrees Celsius, which
wasn’t enough to freeze the 30 % solution at all. They other
solutions were frozen, we could start the measurin
Analysing
When a solution was put out in room temperature, it was
provided with a thermometer. We observed the melting
processes looking for a stable temperature that could
refer the freezing point of the solution in case. Every five
minutes we notated the current temperatures of the
solutions. To exclude measure differences by different
thermometers we swop them a couple of times after first
having them cleaned and dried.
Picture 1: Shows the used experimental set-up
Table 1 presents the average data (out of 2 measurements)
of the defrosting from the 4 solutions with different
amounts of potassium formate. The room temperature was
18 degrees (°C). Solution 4 was not completely melted after
115 minutes; the 1 degree is probably a measurement
inaccuracy.
Time (in
minutes)
0
5
10
15
20
25
30
35
40
45
50
55
60
65
70
75
80
85
90
95
100
105
110
115
°C
Solution
1
-18,0
-17,0
-16,0
-14,5
-13,0
-12,5
-10,5
-10,1
-10,0
-8,0
-3,0
0,0
°C
Solution
2
-19,0
-17,0
-16,0
-14,0
-12,0
-11,0
-10,0
-9,5
-9,0
-8,0
-7,0
-7,0
-6,5
-6,8
-5,0
-4,0
-3,0
-2,6
-1,0
0,0
°C
Solution
3
-19,0
-18,0
-15,0
-11,0
-9,0
-8,0
-7,5
-7,0
-6,8
-6,3
-6,0
-5,5
-4,8
-4,3
-4,0
-3,5
-3,3
-3,0
-3,0
-2,8
-2,5
-2,0
-1,0
0,0
°C
Solution
4
-19,0
-10,0
-5,0
0,0
1,0
1,0
1,0
1,0
1,0
1,0
1,0
1,0
1,0
1,0
1,0
1,0
1,0
1,0
1,0
1,0
1,0
1,0
1,0
1,0
Picture 2: Shows the made solutions for the inquiry
Table 1: shows the temperature of 4 different solutions
over a amount of time.
Results
Figure 1 shows the data of table 1 plotted in a graph. The
lines with their corresponding colour and name refers to
the names of table 1 (solution 1, 2, 3 and 4).
Melting process solutions
5
Temperature (°C)
We observed that the solutions with potassium formate
were a little bit yellow, and the solution with the most
potassium formate was the yellowiest. We also noticed that
the solution with the least amount of potassium formate
melted the slowest and the one with biggest amount the
fastest.
With our gained results we can make a figure in which we
follow the melting process of the several solutions. Based
on the theory explained in the introduction, we are able to
find the new freezing-points of the solutions. The freezing
point can be found at the value of the y-axis when the
melting process is staying horizontal for a while. That’s
because the melting process takes energy and this is pulled
out of the rising temperature energy. So the stabilisation of
the rising temperature line indicates that the freezing point
is reached.
Time (in minutes)
0
-30
-5
20
70
-10
-15
-20
Figure 1: Data of table 1 plotted.
120
solution 3
solution 1
solution 2
solution 4
Solutions
:
0
1
2
3
4
Mass
percenta
ge
KHCO2
Freezing
point
(°C)
30%
20 %
10 %
5%
0%
<18,0
-10,1
-7,0
-3,0
1,0
Table 2: shows the mass percentage potassium formate in
the solutions with their freezing points in degrees celcius.
freezing point solution at different mass percentages
of potassium formate
Freezing point solution
( °C )
0
5
10
20
30
5
0
-5
-10
-15
-20
mass percentage of potassium formate (%)
Figure 2: The data of table 2 plot in a graphic connected
with a solid line. For the solution with 30 mass percent
potassium formate there is given an indication line, we only
know the ending point of this line must be below minus
eighteen degrees.
Conclusion and Discussion
After an interesting inquiry we’re able to say we have
collected reliable results and can now draw a valid
conclusion. Our inquiry question was: What is the effect of
KHCO2 (aq) on the freezing point of water? When doing
research we’ve strictly bounded us to the rules of becoming
reliable measurements. We have reason to believe that the
deviation of most of our controlled variables is in most
measurements negligible, because we’ve checked
everything a second time and did not run into alarmingly
differences. Between the solutions with the difference
mass percentage of potassium formate can be made a good
comparison of the freezing-point.
There are a lot of different connections that could be found
between our dependent and independent variable but it
turned out to be for the lower mass percentages the easiest
of all, a linear connection, easy to recognise in figure 2.
When hitting the origin, which is necessary because pure
water has a freezing point of zero, the slope of a linear fit
would be about -0,6. That means that the freezing point will
be -0.6 times the mass percentage of potassium. With our
experiment we found out this is a wide approach of the
truth, the measurements we did are not very accurate
approached this way. New questions rise immediately, like
is this linear fit a tangent of the twenty mass percent
solution? And how will the connection continue at higher
mass percentage rates?
Obvious is that some measured points approach our find
connection much closer than others, this can be explained
as the result of a false connection. But we think the
accuracy of the test results could have been improved if we
had done a few things different, we believe it’s not possible
to reach the optimal accuracy. When a project is based on
temperature measurements, there is always a possibility
that there is an error in either the observation of the
temperature gauge or there has been a bad timing in
reading off the numbers. There might also have been a
small incorrect measure when the potassium formate was
added to the water. This however will not lead to significant
differences in our results because the percentage
difference between the solutions are doubled every time.
Another interesting point, there were several cases where
our temperature gauge couldn't be placed in the middle of
the Solution, or the frozen solution was impenetrable and
the temperature gauge couldn't pierce the ice. To
overcome this we had to drill a hole in the ice, the friction
between the steel spiral drill and the ice might have
warmed it up a little bit and this could have let to
temperature differences in the first, and maybe the second
measurement. After a while the surrounding ice will have
cooled off again, so the influence can’t have been very
large. The most influential error in our inquiry is according
to us the used thermometers. We’ve used a lot of different
thermometers for our measurements and were not sure if
they were all as valid as the others. We have tried to
minimize this problem by switching the thermometers
around every now and then. When looking at the results of
solution 4, we see that the result is incorrect by one degree.
Evaluation
All by all we think we’ve made a good experimental set-up.
Points of improvements lay in the used tools, next time we
would start with a pre-experiment in which we do some
tests to get the most reliable thermometers. But also we
would arrange to get disposal of a colder freezer, so we can
broaden our inquiry and make sure every solution we make
gets in solid phase. This would also give us the opportunity
to do further inquiry. After finding a linear connection
between the freezing point of water solutions and their
mass percentage KHCO2 we’re languishing to know how
this relationship continues at higher mass percentages.
Bibliography
Title
edition
publication
Writer
publisher
Binas
5e
2004
NVONcomisie
WoltersNoordhof
http://en.wikipedia.org/wiki/Freezing-point_depression