CINCINNATI - Ever ask, "What is that?" Or, "Why is that?" In our new "Cincy Science" feature, we talk with people who can answer those questions: The folks who do science in Cincinnati and the Tri-State.
When yet another blast of snow, sleet and freezing rain hit the Tri-state last weekend, salt supplies were dwindling for municipalities and on store shelves.
Spreading the briny substance has become second nature this winter. So just what makes it work so well?
Eddie Merino, assistant professor of chemistry at the University of Cincinnati, broke down the science for us.
Why does salt melt ice?
Merino explained in essence, salt dissolves in water, allowing the salt molecules intermingle with the water molecules. As salt’s freezing point is lower than water, it keeps the water from becoming a solid. As the salt particles continue to intersperse with the water particles the ice begins to melt.
“Salt melts ice because when you add salt it lowers the amount of concentration of water. It makes it harder to solidify.”
So you need to have water to melt ice?
Salt needs to mix with the water to achieve an equilibrium, or equal mixture of both. As the freezing point for a salt water solution is much lower than that of water at 32 degrees, ice begins to melt, he explained. The temperature of the salt water solution can drop to a low as 15 degrees before it will start solidifying again.
“So if it’s too cold and there’s no water in the ice, then there’s nothing to dissolve the salt. So what has to happen, is you have to have 50 percent water and 50 percent salt and it will start melting because then the concentration of the water is quite low.”
Why does salt fail to melt ice at a certain temperature?
“Salt water’s freezing point is about 15 degrees. It fails because as the temperature drops, the water particles move slower, so that means it’s easier for them to solidify because a solid basically has no movement or has less movement of particles."
Are there chemicals you can add to salt to enhance the melting process?
Other types of melting agents can be used such as Magnesium chloride, which will effectively melt ice to about 5 degrees. Or Calcium chloride which effectively melts ice up to -20 degrees. However, sodium chloride, or rock salt is far more common as it’s the least expensive.
“In general, it doesn’t matter what the other substance is that’s lowering the water concentration.It just matters that the concentration or fraction of the stuff that’s water is less. So it doesn’t matter what else is in there as long as there’s some other impurity making there be less water.”
Are there other household substances that will melt salt, say for instance sugar?
“It’s the same principle as salt. If you think about a solid, the ice is basically a solid block of water. One of the water molecules has to run into the block and to stop moving for it to turn into a solid. When you add anything else to that solution, it’s going to run into the solid block of water and prevent the water from actually running into itself and solidifying.
So any other molecule you add will work. It doesn’t matter what the other molecule is – it could be salt, it could be sugar.”
Is there difference between table salt and rock salt when it comes to melting ice?
“No, there’s no difference, but there are a few reasons why they choose to use the rock salt. The main one is that it’s bigger, so it’s easier to disperse and when it gets crushed, it turns into a lot more salt. It doesn’t matter what kind, it just matters how much salt gets put on the roads.”
Is there a downside to using salt in terms of its corrosive properties to vehicles and pavement?
“It will eat up the roads and the vehicles. I’m from California and the cars there last forever. It crystalizes on the substance and eventually causes corrosion.”
In the case of vehicles, basic chemistry states iron rusts overtime when combined with oxygen. Ions that dissolved within the salt help speed up the process by setting free hydrogen which combines with water to attack metal. Salt also proves damaging to pavement and sidewalks increasing cracks and potholes.
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Beside salt’s acid properties that damage chemical bonding agents, it also retains water leading to increased occurrence of freezing and thawing creating cracks.
Does deicing fluid (Glycol) for airplanes work using the same principle?
“They’re all called eutectic mixtures, so what they’re doing is they’re doing is they’re basically doing the same thing. They’re preventing the water molecules from either hitting the solid block of ice or they’re slowing them down sufficiently so they don’t hit as often.”
However, even de-icing fluid freezes as passengers found out this winter. During the much publicized ‘polar vortex’ in January, when thousands of flights canceled due to temperatures reaching -15 degrees in some cities. According the aircraft de-icing fluid maker D.W. Davies , temperatures can reach -22 degrees before the fluid will freeze,
but the mixture is 20 percent less diluted than the standard mix which freezes at -12 degrees.
Do you think this winter will ever end?
“I hope so. I’m tired of being inside. I’ve already watched all my TV shows, I’ve done all my crossword puzzles. I have nothing left to do.”
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