Metadata: Smlte, is, kldblandning, svensk, lrarversion

Why salt on slippery roads? (Teacher version)?

Abstract:

This experiment shows how cold mixtures work and explains whathas happened chemically.TheoryPure ice (snow) melts when it becomes 0 C.At the melting point is the equilibriumH2O (s) ( H2O (l).This is because there are hydrogen bonds betweenwater molecules and these are strong.When a solid is added to water energy is consumed from the environment to break theexisting bondings and form new bonds betweenwater molecules and ions/molecules.This energy is taken from material(ice) and therefore decreases the temperature.The new solution formed has a lowermelting 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++ Cl-

The freezing point is an example of a colligative property.This means thatit 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 roadsaltIf the salt is spread on the ice covering the streets and sidewalks, the ice meltsrelatively easy.It is subject to the temperature outdoors, that cant betoo low.This method of thawing is based on a well-knownphenomenon in thermodynamics, namely freezing point reduction.Whathappens 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 canmelt snow on the roadway even when it is minus 12 C, dependingon the intensity of traffic.The salt "trick" the winter and we canachieve a snow-and ice-free path even at zero temperatures.Road salt asused on winter roads contains at least 97% pure sodium chloride (NaCl).Theremaining three percent is mostly damp and plaster;(calcium sulfate, CaSO4).The salt also contains less than a hundredth of apercent of sodium ferrocyanide (Na4Fe(CN)6 x 3H2O).The otherwise toxic cyanide ion is very stably boundto iron.Therefore, the substance is harmless to the environment.Other salts can also be usedfor the melting of ice tex CaCl2.

CorrosionOne problem that may occur when you salt the roads is that the cars rusteasier.For the metal on the car to rust, an oxidation and areduction have to take place.The salt enhancesconductivity, and helps to retain moisture on the metal whichmakes the corrosion is facilitated.Metals rust more quickly whenis hot, so we can say that the reaction is temperature dependent.The salt on the road is made up of chloride ions, forming complexestriggered by metals, so that rusting occurs more rapidly.Toprevent rusting cars, you can paint the metal and to protect the underframe, so you can put on a thick and toughmass, underseal it.A new thought isto put sugar on the roads.Sugar does notcontainions 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.Whathappens, is that the radiator water expands when it becomes ice and tubesfreeze and break.To reduce the risk of coolant water in car engine to freezein winter, add some ethylene glycol (everyday term "glycol",formula HO-CH2-CH2-OH).The substance dissolves readily in water because it canform hydrogen bonds with water molecules.The addition of ethylene glycolmeans that the freezing point of the radiator water drops.The more ethylene glycolmixing in the water, the more it lowers the freezing point of radiator water.But ifit still becomes so cold that it freezes up, instead of ice oneviscous solution ensures no clots in the pipeline and thereforeno explosive force in the tubes.

Glycol and water mixture

Pure glycol has a lower freezing point than water, although there are hydrogen bondsbetween glycol molecules.This is because the molecules are packed in a regular pattern as in water. Therefore, the hydrogen bonds are not equallystrong as in pure water, where the molecules are arranged in aregular pattern.

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 solutions freezing point is lower than the solvents.The reverse applies to boiling point elevation.

Glycerol

GlycerolInsects that live in the arctic zones produce large amounts of glycerol (CH2OH-CHOH-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.

EquipmentEquipment 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 performanceThis 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.Results1. NaCl Ice

2. CaCl2 Ice

3. Ethanol Ice

4. Sugar Ice

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!Life-threatening 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.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.Detta verk r licensierat under en Creative Commons Erknnande 2.5 Sverige Licens. Les mere om projektet p www.dlis.eu. Sida 1 av 8