Deicing the Town
Teacher’s Instructions
Brief Description of Activity
Students are casted as the town snow and ice removal crew. Students will experiment with three common types of road salts to see which is the most effective ice melter. Salts, however, do not decompose. Rather, they seep into water and soil, where they have many harmful effects upon accumulation. The amount of salt in water can be measured through the conductivity of water, due to salt being an electrolyte. Dirty and clean snow samples will be compared for conductivity. A demonstration will then be done to illustrate how salt is removed from soil through leaching. Finally, after considering several factors provided (cost of corrosion, toxicity to plants and animals, drinking water pollution, and cost of the salt itself), students will have to justify their choice as to which salt they will choose for their town. Further research into their individual town’s road practices will be encouraged.
Learning Goals
- Learn to evaluate the effectiveness of salts and compare them.
- Learn about watersheds and the consequences of salt application in them.
- Learn how to remove salt from soil.
- Incorporate multiple factors into their decision-making.
- Come up with solutions and alternatives to a real life problem in the hope of applying it to their lives.
Recommended Grades: Grades 4-10
Estimated Time Required: 60 minutes
Key Concepts and Explanation of Terms:
Electrolyte:Substances that give ions when dissolved in water.
Conductivity: The ability of water to pass a charge, measured in Siemens/centimeter.
Watershed:Land area that drains into a specific waterbody.
Colligative properties: Properties of a solution that depend on the amount of solute in it.
Turbidity:Cloudiness of water.
Ions: Atoms or small groups of atoms that have an electrical charge.
What Happens and Why:
Salt is an electrolyte that dissociates in water. By doing so, it destroys ice bonds, allowing ice to melt. Calcium chloride works best, followed by sodium chloride, and finally potassium chloride. Following the winter season, the snow and ice melt, allowing these deicers to runoff into adjacent waters. Because of each salt’s ability to dissociate, chloride ions are released, which are toxic to a variety of life forms. In rock salt, sodium, too, is released, potentially raising the sodium levels in water, which can cause health issues. Calcium and potassium ions are not as harmful to the environment. Chloride ions from the salts can also increase corrosion of metals, such as those found in cars and road structures, another externality of salt use. In order to measure the amount of salt that makes its way into waters, a conductivity test can be used to see how easily a current passes through water. Current passing through more easily indicates higher salt levels.
Soil can also be contaminated by salt. Salt cannot be neutralized and is bad for plants. The only way to remove salt is by flushing soil through excess water in order to leach the salt down to lower horizons or to drain it out. Without this, the soil is compromised. When deciding on what type of salt to use, a number of factors need to be considered, such as amount spent on corrosion control, toxicity to living things, price of the salt, and alternatives.
Materials Needed(per group):
Sodium chloride, 1 g
Calcium chloride, 1 g
Potassium chloride, 1g
Calcium magnesium acetate, 1 g
(Really, any salt can be used and tested, but I recommend having at least one chloride salt)
Wood ashes, 2 tbsp
Sand, 2 tbsp
Beakers
Deionized water
Potting soil, 1 cup
1 cup, clean snow
1 cup, dirty snow
4 samples of snow in small paper cups for snow melt race
Two zip shut bags (gallon size)
Samples of local waters (optional)
Conductivity meter
Balance
Coffee filter
1 tablespoon of salt
Safety Information
Use caution not to get any of the salt or salt solutions into eyes.
General Outline of Procedures:
Students will investigate the effect of three different salts. Conductivity should be illustrated by a teacher demonstration to start, because it is essential for students to understand that electricity can travel through an ion solution, especially for younger learners. A set up with a lightbulb circuitcan be obtained or made. Simply leave a gap in the circuit that can be inserted into water. Students will see no light when the missing circuit is bridged by plain water, but salt water will light the bulb, due to the electrolyte being able to conduct electricity.
Explain ions, and where they come from. Colligative properties can also be introduced here, start by adding a little bit of salt (a teaspoon or so), and the light bulb will light a little bit. Add more, and the light bulb will be brighter. This will help explain colligative properties. For older students that already know about the conductivity of electrolyte solutions, this part can be omitted or replaced with an explanation of a conductivity meter.
To run the conductivity meter, first ensure that the meter is calibrated. Stick the probe into the waterand allow the meter to stabilize. After taking the reading, rinse the probe with deionized water to ensure that no cross contamination is happening. Note that the probe needs to be immersed in deionized water whenever not in use so the probe doesn’t ruin.
The first student activity is a race to see which salt will melt snow the fastest. Four small cups should be filled and packed down with snow, or can be frozen with water to produce ice. Students should be given samples of the three salts, a gram of each, with which they will weigh out a small amount, and then place it on top of the cups of snow. These can be set aside for the moment and checked periodically. The results will be obvious as to which deicer is most effective.
While the snow is melting, another demonstration should be set up, helping to explain conductivity and how deicing salt affects this property in waters. A cup of the dirtiest snow available should be taken and spread out inside a zip bag to melt. Similarly, a cup of cleaner snow, taken away from deicing materials, should also be spread out inside of a zip bag and left to melt.
Road crews also have another option on their hands to help drivers: traction agents. Distribute salt and wood ash to the students and have them stir the cup. Measure the conductivity of this mixture. The students will be asked to observe the clearness of the water with these traction devices in it. What does this say about the negative effects of these substances? Sand sinks down to the bottom, not making the water turbid, like the wood ash will. These substances can also be mixed with deicers for a combined effect.
Now, take time to observe the three salts. Which has melted ice the fastest? The advantage of CaCl2 is that it melts ice at lower temperatures and is less hard on concrete because its low temperature effectiveness prevents multiple freeze-thaw cycles present with the other two that are not as effective at cold temperatures. It also absorbs moisture from the atmosphere, meaning that it has an easier time getting into solution. The downside of this is that it must be stored in moisture-proof containers.
The dirty and clean snow should be melted at this point. Pour them into cups and measure the conductivity. Now is a good time to discuss how things flow downhill into watersheds, and how salt flows into streams and lakes after the spring snowmelt. The students should then receive a short lesson on the effects of salts on soils, waters, and living things like plants and animals.
Soil is one of the things most affected by salt. Nutrient cations are replaced with sodium, if sodium chloride is used, a much less useful ion. The structure of the soil can also be affected by salt. Through soil structure rearrangement, the soil can become more impervious, meaning that water cannot pass through. There are also many other complicated effects, but these are the basic ones. Unlike pH, salt cannot be neutralized. Salt sits in the soil until water comes to flush it out down into lower soil layers, which is also a problem with contaminating groundwater over time. In order to see how to get rid of salt from soil, an experiment will then be done to show that water can flush salt out of soil; indeed it often is the only course of action for farmers with saline fields.
For the saline flushing experiment, mix two tablespoons of salt into a cup of potting soil. Place this soil inside of a coffee filter, and then place it inside of a funnel. Place a beaker under the funnel and run 140 milliliters of water through the soil a couple of times. Each time, measure the conductivity of the water in the beaker. It will grow less with each trial, as the soil is flushed out. This can also be tried with the light bulb circuit, it should grow dimmer whenthe less salty water is tested from the flushing.
Salt negatively impacts waters through changing the salinity of water. This can potentially make it undrinkable and corrosive to pipes. Saline water also is more dense, and will sink down to the lowest point of the lake, never mixing with the rest of the water column in order to be exposed to the air, therefore it contains less oxygen and can prohibit aquatic life from living in it. As stated before, it can also leach down through soil over time and contaminate groundwater, leading to a new source of water needing to be found. Salt affects animals negatively as well. Some animals, notably amphibians and aquatic animals, simply cannot withstand high salt levels and die. With larger animals, the salt can attract them to roads to lick it off, increasing vehicle collisions and causing animal dehydration. It harms domestic pets’ paws and can lead to dehydration if pets consume salty water. In humans, the sodium can have a negative impact and cause hypertension, much like excessive salt consumption from junk food. Plants also cannot tolerate excessive salt, it dehydrates them and results in dead leaves and branches and sometimes even death to the entire plant. Salt tolerant, non-native plants can move in. It also harms their reproductive process.
Students should be informed of a couple of more factors into their decisions. The cost of road salts varies. Pure sodium chloride is cheapest. Potassium chloride is 1.6 times more expensive. The calcium chloride is 5.7 times higher. Other, more environmentally friendly salt is up to twenty times more expensive. Keep in mind, though, that with a salt like calcium chloride, less salt may be needed due to its high effectiveness. The chloride ions increase the rate of corrosion in metals, leading to structural unsoundness of bridges, roadways and cars, among other things. This cost has been estimated to be between 16 and 19 billion dollars per year. This is often a grossly underestimated problem. More shoveling and more effective salt spreaders on trucks are also things that can decrease the use of salt deicers.
From this information, students can make the final decision about what salt or mixture they will use for the town. A variety of locations can enable discussion. What would they use at a hospital, a rural route or residential areas, etc?
Extra Activity
A standard curve of solutions with known salinity can be used to estimate how much salt is in a solution of unknown salinity. Add differing amounts of salt and measure the conductivity. Graph this, and find out the equation of the line. This can then be solved to find out the salt concentration of an unknown solution.
Notes
The salt flushing experiment can be done as a demonstration to save time.
Sources
Conducting Solutions, University of Wisconsin Chemistry. Accessed 1/13/14 at
Electrolytes, University of Waterloo. Accessed online 1/13/14 at
Road Salt and Water Quality, New Hampshire Department of Environmental Services, 2011. Accessed online 1/15/14 at
Road Salt: Moving Toward the Solution. The Cary Institute, Kelly et al , December 2011. Accessed online 1/15/14 at
Managing Saline Soils, Cardon et al, Colorado State University Extension, May 2007. Accessed online 1/9/14 at
Water Cycle-Oceans Lab 4. Accessed online 1/27/14 at:
Deicing the Town
Student Worksheet
Name______
The Situation
As the manager for the town grounds crew, this winter has brought a lot of ice and snow to your community. When snow and ice strike the community, your job is to clean the ice and snow up in order to minimize accidents. The budget and the environment are important considerations to keep in mind. You have many methods and means at your disposal to choose from, but you have to make up your mind as to which way you will clean up the ice and snow based off of a couple of factors. One method of deicing the town is laying down salt. Salt is an electrolyte, meaning that it dissociates in water and dissolves. After winter ends, the salt moves with the snowmelt into soil and water. Having excessive salt in either can cause a whole host of problems, which will be discussed in more depth later.
Experimenting with Electrolytes
A light bulb circuit should be set up, with a gap in the circuit so that no electricity flow is occurring. If we a liquid is used to bridge the gap, electricity may or may not flow, depending on the liquid. Bridge the gap with a beaker of deionized water.
Does the light turn on? Circle one. Yes/No
Why do you think this is? Write your answer below.
Take the wires or probes out of the water. Now, try adding a teaspoon of salt to the beaker and stir it. Try the circuit again.
Does the light turn on? Circle one. Yes/No
Why does it work? Write your answer below.
Note how bright the light bulb is. Why? Because salt is an electrolyte. It dissociates in water, splitting into ions, atoms with an electrical charge, which are capable of conducting electricity. Add another teaspoon of salt to the water and stir. How bright is the light bulb now?______
It’s brighter. This illustrates a colligative property. A colligative property is a property of a solution that depends on the amount of solute in it. In this case, the water’s ability to conduct electricity depends on the amount of salt in the water. This is called conductivity, and it is measured using a conductivity meter. This measures how easily the water can conduct a charge, measured in Siemens per centimeter. We will be using conductivity a number of times in this lab, so it is important to understand it. It is also important to notice the units on this measurement in order that the correct prefix is recognized.
Now that we know about conductivity, let’s begin taking a look at one of the main options for deicing. Salts can be laid down in order to melt ice. This happens because the electrolyte salt dissociates and breaks the bonds in ice, allowing water to be formed. By doing this, it lowers the freezing point of the solution, meaning that the temperature must be colder in order for it to freeze. The salt solution can still freeze if it gets cold enough, if temperatures drop too low, the salt can be rendered ineffective. We have a couple of salts today with us. Rock salt (NaCl), calcium chloride (CaCl2), potassium chloride (KCl), and calcium magnesium acetate(CMA). We need to test them to see how quickly they can melt snow. Weigh out one gram of each of the salts on the balance and bring them back to your lab bench. Take four small cups of snow and label them with the names of each of the salts. When this is completed, pour the salt on top of each of the snow and set them aside for the moment. We will get back to this in a couple of minutes when the salts have progressed on the snow.
Every winter, tons of salt are laid down on roads throughout the area. This then gets plowed off into snow banks and sits until the end of the winter. Every body of water has what is called a watershed, an area from which water drains into that specific water body. When the snow banks melt, the salt flows down into a lake or stream. Chloride salts pollute the water, killing aquatic organisms and vegetation. Amphibians, which are already threatened, especially cannot tolerate salt. The salt dehydrates plants and is toxic to aquatic organisms, so having a lot in water is undesirable. The lake also is subject to unnatural stratification, the more saline water sinks to the bottom and does not get mixed with oxygen since it never comes into contact with the air.
Detecting and monitoring salt in the water is a process that many groups undertake, from lake watchdogs to the federal government. Testing water samples for the ions coming from road salt is expensive and time consuming. Thankfully, this is where conductivity comes in to play. Because salt has such a pronounced effect on water’s conductivity, we can monitor the conductivity of lake water in order to make sure that too much road salt isn’t getting in the lake. Today, we have two sample cups of snow. One is “dirty” and the other is “clean”. Take the samples and put them into a plastic bag, sealing them shut. Spread the snow out for even melting. We will get back to them in a second.