Metadata: Smälte, is, köldblandning, svensk, lärarversion
Why salt on slippery roads? (Teacher
version)?
Abstract:
This experiment shows how cold mixtures work and explains what
has happened chemically.
Theory
Pure ice (snow) melts when it becomes 0 ° C. At the melting point is the equilibrium
H2O (s)  H2O (l). This is because there are hydrogen bonds between
water molecules and these are strong.
When a solid is added to water energy is consumed from the environment to break the
existing bondings and form new bonds between water molecules and ions/molecules. This energy is taken from
material (ice) and therefore decreases the temperature. The new solution formed has a lower
melting point than pure water, because the new bondings are weaker.
The problem with slippery roads disappeared since the ice melted.
At the melting point we have the equilibrium:
H2O (s) + NaCl (s)  H2O (l) + Na+ + ClThe freezing point is an example of a colligative property. This means that
it is not important what chemical it is, but the decisive factor in this case, is the concentration of the dissolved
substance.
Melting of ice with roadsalt
If the salt is spread on the ice covering the streets and sidewalks, the ice melts
relatively easy. It is subject to the temperature outdoors, that can’t be
too low. This method of thawing is based on a well-known
phenomenon in thermodynamics, namely freezing point reduction. What
happens when you salt the roads, is that the salt lowers the melting point.
Normally snow and ice melts at 0 ° C. With the help of a little salt you can
melt snow on the roadway even when it is minus 12 ° C, depending
on the intensity of traffic. The salt "trick" the winter and we can
achieve a snow-and ice-free path even at zero temperatures. Road salt as
used on winter roads contains at least 97% pure sodium chloride (NaCl). The
remaining three percent is mostly damp and plaster;
(calcium sulfate, CaSO4). The salt also contains less than a hundredth of a
percent of sodium ferrocyanide (Na4Fe(CN)6 x 3H2O). The otherwise toxic cyanide ion is very stably bound
to iron. Therefore, the substance is harmless to the environment. Other salts can also be used
for the melting of ice tex CaCl2.
Corrosion
One problem that may occur when you salt the roads is that the cars rust
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easier. For the metal on the car to rust, an oxidation and a
reduction have to take place. The salt enhances
conductivity, and helps to retain moisture on the metal which
makes the corrosion is facilitated. Metals rust more quickly when
is hot, so we can say that the reaction is temperature dependent.
The salt on the road is made up of chloride ions, forming complexes
triggered by metals, so that rusting occurs more rapidly. To
prevent rusting cars, you can paint the metal and to protect the underframe, so you can put on a thick and tough
mass, underseal it. A new thought isto put sugar on the roads. Sugar does not contain
ions and hence do not contribute to corrosion.
Alternative cold mixtures
Ethylene Glycol)
In the winter when it's cold, the water in the water cooler in the car can freeze. What
happens, is that the radiator water expands when it becomes ice and tubes
freeze and break. To reduce the risk of coolant water in car engine to freeze
in winter, add some ethylene glycol (everyday term "glycol",
formula HO-CH2-CH2-OH). The substance dissolves readily in water because it can
form hydrogen bonds with water molecules. The addition of ethylene glycol
means that the freezing point of the radiator water drops. The more ethylene glycol
mixing in the water, the more it lowers the freezing point of radiator water. But if
it still becomes so cold that it freezes up, instead of ice one
viscous solution ensures no clots in the pipeline and therefore
no explosive force in the tubes.
Glycol and water mixture
Pure glycol has a lower freezing point than water, although there are hydrogen bonds
between glycol molecules. This is because the molecules are packed in a regular pattern as in water. Therefore, the
hydrogen bonds are not equally
strong as in pure water, where the molecules are arranged in a
regular pattern.
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Water and glycol have high solubility in each other because they form hydrogen bonds between
themselves, but the freezing point is lowered because they are so badly packed about each other. Water molecules
are prevented from organizing themselves in their regular, scattered pattern. In other words, the hydroxyl group of
glycol is acting as a lubricant for the water molecules in the mixture. The more glycerol, the worse the water
molecules are packed around each other and the lower the freezing point.
.
Water freezes at 0.0 ° C and pure glycol at -13 ° C. The mixture between them
bring about a freezing point reduction according to Raoult's law. This law says
that a solution’s freezing point is lower than the solvent’s. The reverse applies to
boiling point elevation.
Glycerol
Glycerol
Insects that live in the arctic zones produce large amounts of glycerol (CH2OHCHOH-CH2OH), which is a substance similar to ethylene glycol. This includes our common housefly. The effect is that
the freezing point of blood drops and the insects can survive at very low temperatures
Technical information
This activity uses two or more temperature sensors. Is there more than two inputs, one can measure the temperature
change at additives of different substances or different amounts of an additive, otherwise you can share the results
between groups.
Equipment
Equipment
• Data logger with 2 temperature sensors
• Beaker
• Different salts, eg NaCl, CaCl2, sugar, ethanol, glycol, glycerol
• Ice and likely an Ice-slush, in order to disperse the ice.
Arrangement and performance
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This experiment is appropriate for implementation of student-oriented activities with two
degrees of freedom, ie the students themselves must to come to the topics to be explored and
how the investigation should be carried out. (With a reference test). The composition requires only two beakers with
ice and the temperature sensors and data logger.
Questions for discussions
• Which substance lowers the temperature at the slowest rate?
• What substance gives a minimum of temperature drop?
• Why is the temperature of the pure ice increasing during the experiment?
• Why does the temperature increas after a while, eg after 40s with NaCl?
• Give arguments fore and against for sugar or salt should be used for deicing.
• How do you think animals can survive in arctic conditions, the blood is indeed a water solution?
• How to remove ice from aircraft wings? Advantages/disadvantages.
Results
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1. NaCl – Ice
2. CaCl2 – Ice
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3. Ethanol – Ice
4. Sugar – Ice
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Variations
In addition to the above tests, students can themselves propose topics, but also glycol and glycerol can be
investigated.
• We can explore what happens when you add different quantities of a single substance eg glycols of different
concentration: pure water, pure glycol, 50% of each, 30% glycol-70% water and 10% glycol-90% water . Then you need
for a freezer that is at least -25 ° C.
• Ice heaving-Add a cotton thread on an ice cube, and try to lift the ice cubes with the thread. (Tip: Pour salt on the
thread) Explanation: When the salt sprinkled on the ice dissolves some salt in the water layer on top. This solution
process, a spontaneous endothermic process, stealing the heat from the environment, ie it allows for the heat to
dissolve the salt in the water. The heat is taken including the water layer on top of ice cubes, which then freezes into
ice. In this way the thread freezes solid and you can lift up ice cubes with the thread.
• How did they make ice cream before the freezer? (Ice was available) Pour a little juice in a plastic bag for ice cubes
and immerse it in a cold mixture.
Comments to the graphs: The activities were started when the contaminant was added. Temperature increases at
the beginning of the experiments due to the agitation of the solid ice-mass, due to that the temperature sensor was
placed in air pockets under some/a few seconds, and this can be easily seen on the graphs. At the end of the activities
the temperature increases in all the graphs. At that time the new solution will no longer take energy from their
surroundings, and simultaneously begins to adopt to the ambient temperature.
Risk analysis
Glycols (ethylene glycol) is toxic! Glycol should certainly not be ingested because it is highly poisonous! Lifethreatening poisoning can occur. It is very harmful to the nervous system and kidneys. Avoid even the inhalation of
glycol fumes in closed spaces.
Unused glycol is collected in plastic bottles or cans and is submitted to environmental deposit sites.
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Environment
Propylene glycol is an environmentally friendly alternative to ethylene glycol with at least as good anti-freeze
properties. It may be included in solvents that are tagged with Good Environmental Choice. Propylene glycol is used
even as a sweetener in food. Pure propylene glycol has low toxicity, but the propylene glycol you buy at the gas
station contains corrosion inhibitors that are toxic.
In the early 80's 1000 tonnes per year of ethylene glycol was used for de-icing of aircraft. Due to recycling of wastage
we only use just over 90 tons per year. Impact on land and water is no longer a major environmental hazard. The
decrease also depends on using the environmentally friendly propylene glycol.
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