Magnesium Chloride
De-Icing Agents:
How Do They Affect the Freezing Temperatures
on Our Roadways?

Experiments from Team Labs

Experiment Profile

Connections:		Chemistry (Supercooling, Heat of Crystallization, Freezing Point), Technology, Mathematics
Skills:		Measuring, Graphing, Analyzing, Inferring, Predicting
Duration:		1 Class Period with a pre- 2+ hour data collection trial
Team Size:		2-3 students per group, or whole class demonstration
Content Standards:		Science Standard A (grades 9-12) Science Standard B (grades 9-12) Science Standard E (grades 9-12) Math Standard 1, 4, 5, 11, 13 (grades 9-12)

Summary

In this experiment, you will need to obtain a sample of Magnesium Chloride. You can get this sample from your local department of transportation (if in an area that uses magnesium chloride as part of their road clearing management during snow and freezing rain storms) or you can purchase it through a industrial supplier, which is readily available online. This investigation will use an Extended Temperature Probe to analyze the liquid form of the de-icing agent Magnesium Chloride as it changes state to a solid. From this data you will determine its Freezing Point and note its associated Freezing Point Depression quality, Supercooling Effect, Heat of Crystallization and general physical properties.

Materials

• Computer
• ThinkStation SP16 Interface Kit
• ThinkStation Interface
• Power Supply
• Communications Cable
• Excelerator 2000 Software
• Extended Temperature Probe with API (Automatic Probe Identification)
• Freezer (a very low temperature non-frost-free scientific freezer works best. You may need to lower the temperature of the freezer you are using to -25° C or more to note the Supercooling Effect).
• 100 mL sample of liquid Magnesium Chloride de-icing product
(sometimes referred to by the trade name "FreezeGuard" as well as other self-titled products depending on the manufacturer)

Background

Magnesium Chloride has recently been introduced as part of a de-icing strategy and road management policy by many departments of transportation across the country and the world. It is a mostly clear liquid solution at room temperature that has a slightly viscous look and a pH of approximately 5.8, indicating it is a slightly acidic product.

Magnesium Chloride is usually just part of an overall mix that may include rust inhibitors, salts, and alcohols in the final product that is actually applied to the roads. It is spread on the roadways, usually preceding a storm if possible, in communities that experience freezing temperatures during rain and snowstorms. It is used to lower the temperature at which precipitation will freeze on the roadways, thus lowering the need to physically plow the road's surface in order to maintain a safer driving environment. It has also been employed for its ability to lower particulate pollution that occurs from the spreading of sand and gravel products that are kicked up into the atmosphere by the millions of vehicles on the roads. This particulate pollution that occurs with "traditional" methods of road management--plowing mechanically with blades against the surface and spreading salt/sand mixtures--is what often creates the haze in high population centers such as Denver, Colorado.

The use of Magnesium Chloride in the de-icing of roads has been shown in numerous studies (which you can readily find in an internet search) to significantly reduce the particulate matter pollution in communities that have piloted and introduced its use. It has also been shown to lower the cost of plowing services, as less plow runs along the road are necessary with its application. There are, however, concerns about its use that are still awaiting long-term study to ensure its effectiveness and safety. One concern is the bioaccumulation of the product in the waterways, animals and living fauna that lie along the roads in which it is applied. Another concern is the cost to vehicle owners to remove the product from their vehicles after storms to prevent the degradation of the paint and metal surfaces. As with most management practices, there are tradeoffs that must be weighed when creating an overall effective strategy, and the use of Magnesium Chloride is just one of the criteria that must be considered in the overall mix of a road management policy

As it is a product that is hygroscopic--attracts water--it has the ability to "grab" the precipitation, and combined with its lower freezing point quality, mixes with the precipitation to hold it in the liquid state at a significantly lower temperature than the natural freezing point of water. This allows vehicles to travel on roads that are less frequently frozen, leaving more traction on the surface for the tires to maintain higher and safer speeds than if they were snow packed and/or frozen.

This investigation will observe the characteristics of the Magnesium Chloride product that is used in our area by the Colorado Department of Transportation (CDOT) near and around Boulder, CO. We here at Team Labs were supplied with a sample of the product by our local CDOT department, and we would like to take this opportunity to thank them for their contribution and cooperation in this investigation.

Procedure

Collecting Data with the Extended Temperature Probe

1. Prepare your experiment by finding a Magnesium Chloride-type product, either commercially or via a local source. (You will need about 200-500 mL of the solution; make sure you note its ingredients).
2. Attach an API Extended Temperature Probe to the ThinkStation interface.
	3. Position the temperature probe in a small Nalgene-type (plastic) bottle of approximately 150 mL with 100 mL of the Magnesium Chloride solution. Use a cork or rubber stopper with a hole in the middle that will accommodate the temperature probe and hold it in place towards the middle of the liquid sample. (You can adjust the size and sample depending on your resources and time constraints).
Show me	4. Next, launch Excelerator 2000 and click the Connect&GOTM icon. Excelerator will automatically identify the temperature probe and create a graph of Temperature vs. Time. The software also sets a default sample rate and duration for the experiment, which you will change in the next step.
	5. To change the sample rate and duration of the experiment, click the Edit Clock icon located on the Excelerator toolbar.
Show me	Set the sample rate to 1 sample per second and the duration to 4 hours (14,400 seconds). Note: A longer duration may be required depending on the efficiency and temperature of the freezer you use; therefore, adjust accordingly.
	6. Starting from Room Temperature, place the 100 mL sample with the Extended Temperature Probe held in place via a cork or rubber stopper in the freezer. You are ready to record some data. Click the green GO button on the left side of the Excelerator toolbar.
View screen	7. Excelerator will record the Temperature emitted from the probe for exactly 4 hours at a sample rate of once every second, and will display a graph similar to the one below. Different models and configurations of freezers will vary in the time and temperature drop; however, the data should be similar in trend. It may be necessary to click on Rescale in the Tools menu to see all of the data. Save the trial after completing.

Analysis of the Data

1. Use your Fast Graph "Analysis" functions to "Select" and note the lowest temperature reached, its time interval and the point at which the solution changed state to a liquid. If your freezer has a low enough temperature, you will be able to note the Supercooling Effect and The Heat of Crystallization of the Magnesium Chloride solution. You can create graphs that look like the following using Excelerator's new Analysis and Right Click functions:
View screen	View screen	View screen
2. The graphs above show the precise capability of the Exclerator/ThinkStation system to analyze and confirm the exact characteristics of our sample of Magnesium Chloride.

Conclusions

A. Magnesium Chloride solutions, when used as a de-icing agent, will lower the freezing point of the precipitation on roadways, as this substance is characteristically shown to only freeze (when pure) at very low temperature (just below -20 °C). As the liquid solvent Magnesium Chloride is mixed with precipitation (the solute) on the road, it lowers the overall solution's freezing point. The lowering of the freezing point caused by dissolved substances is called the Freezing Point Depression.

B. Magnesium Chloride readily shows the Heat of Crystallization as it changes state from liquid to solid. (This investigation observed a 25.29 °C temperature change as the Magnesium Chloride changed from a liquid state to its solid frozen state).

C. The Supercooling Effect is evident in this example. The Magnesium Chloride solution was able to temporarily attain temperatures (-45.79 °C) well below its characteristic freezing point (-20.50 °C) before releasing the energy necessary to change state and crystallize into its solid and frozen form.

D. Magnesium Chloride solutions have, by their very nature as a liquid product, less particulate matter than solids-based road treatments such as sand/salt/gravel.

Extensions

Use probeware to investigate the following:

• Mix solutions of your Magnesium Chloride product with varying ratios of water (perhaps distilled, tap and even collected rain/snow water), and then note the observed cooling curves to imitate what may be naturally occurring in your area (i.e. linked to the type and amount of precipitation you receive in your area and how Magnesium Chloride may or may not affect freezing point).
• Calculate the costs associated with using Magnesium Chloride versus other "traditional" methods of road maintenance.
• Compare various salt solutions in similar ratios to water for their freezing point characteristics. What is the most cost efficient strategy? What is the least effective mixture in lowering the freezing point of water?

About the author... Marc Mueller is the Secondary Curriculum Specialist at Team Labs. His background includes packaging and mechanical engineering, secondary science, technology and vocational instruction. His real-world experience is mirrored in his curricula as he has been focused on engineering, creating applied technology laboratories, and the creation of pre-engineering, computer technology and vocational coursework and activities throughout his career.

If you have a great experiment idea, please send mail to the webmaster.

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