Winter is coming and with it, low temperatures, snowfall and frost. The arrival of the coldest season forces many places condemned to deal with these adverse weather conditions to prepare so that the maximum that the “white Christmas” holds for them is nothing more than a beautiful picture to be able to photograph, without major worries. .
It is a typical image of these dates to find operators spreading large amounts of salt on the roads with shovels that allow cars to circulate safely and reduce the risk of their tires not adhering to the layers of ice that form.
Now, for what reason do you throw salt on the roads?
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A Simple Chemical Reaction
The reason why roads are salted to melt ice is chemical. Contrary to what many may think, salt does not melt ice (as heat would); but it dissolves it. Let’s see why this simple reaction occurs:
Union and separation of atoms
Salt (NaCL or “sodium chloride”), contains crystals formed by atoms of chlorine and sodium, hence its name. The chlorine will lose an electron that the sodium will “borrow” . In this way, the first will be negatively charged and the second positively (anion and cation, respectively).
The same thing will happen when sodium chloride interacts with water (H20), whose components, hydrogen and oxygen, are also charged.
Hydrogen bonds
This expression is known as the link between molecules that generate partial charges . Generally, these bridges work best in water, since the liquid element increases the force of attraction between the hydrogen itself and a partially charged molecule.
When temperatures are above 0º, these unions cannot finish linking completely, so their state will continue to be liquid, since the molecules move too quickly to change state. On the contrary, if the temperatures are below 0, the speed decreases, hydrogen bonds are created and the water begins to solidify.
When you add salt, it dissolves in water, so that its sodium ions (positive) and chlorine ions (negative) float in it. Each attracts hydrogen and oxygen molecules from the water according to its charge, breaking hydrogen bonds. When it breaks, the water remains liquid, since the freezing temperature of the resulting solution with the salt drops to -21º. Simply put, it will take much colder for ice to form.
Thanks to the fact that the freezing point of the mixture of water and salt is so low, the passable roads remain clear of ice, since it will hardly be so cold that the mixture also freezes.
Throw salt before or after it freezes, it doesn’t matter
Although it is always advisable to anticipate snowfall and add salt beforehand just in case; there will be no problem doing it once it has fallen. That is, the snow will dissolve just as sooner or later, although it may take a little longer. However, what we will not know is if by acting a posteriori , this will generate some mishap for the drivers; After all, the consequences of a snowfall cannot be minimized.
By spreading salt on the road first, it is achieved that the snow that is going to fall does not congeal when it accumulates on the ground, but rather liquefies due to the action of the salt. Likewise, do not panic: the salt will dissolve the ice even if it has already snowed. As we said, the freezing point of the water + salt solution drops to 21 degrees below zero, far from the 0 degrees of the liquid element alone.
This technique will work both for snowfall, frost and even when it rains or there is fog, since having sprayed this anti-ice solution on the pavement, at night, when temperatures drop the most, the chances of generating ice will be reduced.
In short, what salt does is increase the safety of vehicle traffic, since it increases the adherence and traction to the asphalt of the same.
Ways of applying salt on roads
There is usually the custom of throwing shovels of dry salt in the form of crystals, although it is not uncommon to find that it comes already dissolved in water, so as not to wait for it to mix with the snow that is in the ground or have to fall. In this way, the roads are sprayed with the solution. However, it must be taken into account that, depending on the concentration of salt, the freezing temperature will go down or up , so it is advisable to increase its proportion if extreme cold is anticipated.
To ensure greater effectiveness, sand can also be added to the mix ., which will help car tires adhere better to the surface.
Cons of salt
Despite its effectiveness, salt is a very aggressive agent. It is not uncommon that, when road workers spread sodium chloride on the road, it can accidentally fall on unpaved areas, which causes the earth to dry out, preventing anything from growing afterwards.
In addition, it is a powerful corrosive that oxidizes the metals that the signs, guard rails, poles or even the underside of the cars themselves are made of.
Alternatives to salt
Precisely because of what we discussed in the previous point, many countries use alternative remedies against frost on roads.
For example, in places like Switzerland or Sweden, where snowfalls are frequent, these salt compounds are not allowed to be used continuously due to their impact on the environment. Instead, antifreeze products, such as calcium, magnesium, or potassium acetate, are used . The downside is that they are more expensive than salt. Another option that they also tend to take is to throw gravel in pedestrian areas, as well as forcing drivers to circulate with snow tires on.
However, in areas where it snows sporadically, salt is still used, precisely because of its ease of production.
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