The effect of molar mass on the melting of ice
Anouk Evers & Lisanne Jagt
’t Atrium, The Netherlands
Received April 2011
Summary
In the winter, a lot of ice lays on the roads because of the snowfall. Road salt will be used a
lot to make the ice on the road melt. Here in the Netherlands there is often too little road salt
at the end of the winter. Road salt lowers the freezing point of water and makes the ice melt.
Ice formation will be prevented and the roads will be available again for cars and bikes. The
most used road salt is NaCl (s). But is NaCl the best road salt to use? In this experiment we
will study the effect of molar mass on the working of salt on ice. For this inquiry we use NaCl
(s) and KCl (s). KCl has a higher molar mass than NaCl. We also wanted to use a salt with a
lower molar mass than NaCl, like NaF, but that is toxic for the environment, and thus not a
good road salt.
Introduction
Salt lowers the freezing point of water.
When NaCl (s) dissolves in water, it
divides into Na+-ions and Cl - -ions. Both
kinds of ions independently contribute to
the lowering of the freezing point. This is
shown in the following chemical equation:
NaCl (s) → Na+ (aq) + Cl- (aq)
One mole of NaCl produces two moles of
ions and causes a double lowering of the
freezing point. The freezing point
depression of 30 grams of water with 3.0
grams of NaCl is 6, 37 °C (1).
KCl does exactly the same thing, which is
shown in the following chemical equation:
KCl (s) → K+ (aq) + Cl- (aq)
The freezing point depression of 30 grams
of water with 3.0 grams of KCl is 4,99 °C
(1). The size of the lowering of the
freezing point depends on the amount of
ions. So NaCl has a bigger freezing point
depression. The molar mass of NaCl is
58,44 g/mol. The molar mass of KCl 74,55
g/mol. This raises the question: Will the
salt with a lower molar mass make the ice
melt faster?
Our hypothesis is that the salt with a lower
molar mass (NaCl) will make the ice melt
faster, because it will split into more ions,
which contribute to the melting of ice.
Experimental design
First off, we crushed the ice cubes with an
ice-crusher. We have weighed 30 grams of
crushed ice three times and put 30 grams
of ice in all three funnels with little holes.
We weighed 3.0 grams of the salt NaCl (s)
and spread it equally in one funnel. We
weighed 3.0 grams of the salt KCl (s) and
spread it equally in another funnel. We
wrote every 5 minutes down how much
water has melted in the measuring
cylinders. We did this until all the ice had
melted in all the funnels. For a reliable
result, we did the experiment three times.
A drawing of the set-up can be found on
the last page of this report.
We used three funnels with small holes,
three times 30 g of crushed ice, three
measuring cylinders (50 mL), 3.0 g of
NaCl, 3.0 g of KCl, a pair of scales, a
spoon and three stopwatches.
The independent variable is with or
without salt and what type of salt. In one
35
30
Melted water (mL) →
funnel we chose not to add salt, in another
funnel we chose to add NaCl and in the last
funnel we added KCl.
25
The dependent variable is the variable we
measured. That is the amount of melted ice
in the measuring cylinders.
The control variables during our
experiment were the air temperature, the
volume and weight of the ice at the
beginning of our experiment and the
weight of the salt.
20
ice without salt
15
ice +NaCl
10
ice + KCl
5
0
0 5 10152025303540455055
Time (minutes)→
For accurate measurements, we did the
experiments at the same time, so that the
environmental factors are the same. The
amounts of crushed ice and salt were
weighed with an accurate pair of scales (3
decimals). We spread the salt equally on
the ice and we made sure that the holes of
the funnels were not blocked. Furthermore,
we did the whole experiment three times.
The accuracy of the cylinders was one
stripe per 2 mL.
Figure 1: Amount of water (mL) in the three
cylinders, per 5 minutes.
Results
From the results shown in the graph and
table, we can conclude that the ice with
NaCl melts faster than the ice with KCl.
Both set-ups with salt melted faster than
ice without salt. The answer on our inquiry
question (Will the salt with a lower molar
mass make the ice melt faster?) is that ice
with a lower molar mass will make the ice
melt faster because it has more moles than
ice with a higher molar mass. Our
hypothesis (the salt with a lower molar
mass will make the ice melt faster) was
correct. The results that we saw, support
our hypothesis.
By keeping the same variables constant,
we were able to compare the results as
reliable as possible.
The average from each measuring moment
was calculated and put in a table, which
you can see below.
Time
(min.)
Ice
Ice +
Ice +
without NaCl
KCl
salt
(mL)
(mL)
(mL)
0
0,0
0,0
0,0
5
7,0
15
12
10
11
19
16
15
14
22
20
20
17
25
22
25
19
27
25
30
21
29
27
35
23
30
29
40
25
30
31
45
26
31
32
50
26
31
32
55
27
31
32
Table 1: Amount of water (mL) in the
measuring cups
The line that represents the melting of the
ice with NaCl is the steepest of all three
between 0 and 5 minutes. But between 10
and 35 minutes, the steepness of the three
lines is nearly the same. That means that
the melting speed of the three set-ups is
practically the same during that period.
Conclusion and discussion
Evaluation
It is defenitely possible that we were not
accurate enough somewhere during the
measuring process. That may have caused
some slight deviations in the results. For
example, there could have been some
crushed ice left behind in the cup after we
put the ice in the funnels. Therefore the
exact amount of ice would not be 30
grams, but a little less. This could also
have happened to the salt that we added. It
is also possible that the ice was not from
the same quality. Some of the icecubes
have stayed longer in the freezer than the
others. For the ice that has layed longer in
the freezer, it takes more time to melt.
This inquiry raises further questions, for
example: Is NaF (s) a better road salt, if it
was not so toxic for the environment,
because it has a lower molar mass than
NaCl (s)? Or: What is the exact right
amount of NaCl (s) that is required to melt
1 kilogram of ice as quickly as possible?
These are questions were we can only find
a answer to if we test them in experiments
and maybe it will help us further to keep
the roads ice free and the environment
clean.
Bibliography
1.http://nl.wikipedia.org/wiki/Vriespuntsda
ling
2.http://en.wikipedia.org/wiki/Sodium_chl
oride
3.http://en.wikipedia.org/wiki/Potassium_c
hloride
Set-up