Road salt lowers the freezing point of water by causing the formation of a salty solution that freezes at a lower temperature than pure water. This allows for melting snow and ice on roads in winter.
When you put salt in water, what you are essentially doing is separating the salt molecules (sodium chloride, NaCl) into two elements: chloride ions (Cl⁻) and sodium ions (Na⁺). Why does this happen? Because water molecules have small positive electric charges on one side and negative on the other—in short, they are polar. This polar side attracts sodium ions (positive) towards the negative end of the water, and chloride ions (negative) towards the positive end. The result is that the salt ions release their initial partner to be surrounded and separated by these water molecules. And that’s how salt dissolves in water.
When salt is added to water, it dissolves and disrupts the natural organization of water molecules. For water to freeze, its molecules must arrange themselves into a specific structure called ice crystal. Salt acts as a disruptor: it prevents water molecules from easily arranging themselves, thus lowering their normal freezing temperature. The result is that the freezing point of water drops below its usual 0 °C. This means that salty water remains liquid even below zero, which explains why snow and ice melt when roads are salted during winter. The more salt is dissolved in the water, the lower the freezing point.
When you sprinkle salt (sodium chloride, NaCl for short!) on snow, it dissolves into two ions: sodium (Na⁺) and chloride (Cl⁻). These ions insert themselves between the molecules of frozen water, disrupting their neat arrangement. As a result, water struggles more to remain in solid form. In fact, this lowers its freezing point. Instead of freezing at 0°C like pure water, it will remain liquid even when it’s colder. This salty water, now liquid, then mixes with the surrounding snow, speeding up its melting and facilitating the clearing of roads.
Salt is particularly effective at melting snow when temperatures are not too low, typically down to about -7 to -9 °C. The colder it gets, the more salt is needed to melt the same amount of ice. Below approximately -10 °C, salt becomes significantly less effective at preventing water from refreezing, and from -15 °C, its effectiveness is almost negligible. At very low temperatures, it is better to mix salt with sand or other abrasive materials to maintain good tire grip. Humidity, wind, and the thickness of the snow layer also drastically influence the performance of salt: a thin layer of wet snow will melt much faster than a thick layer of very dry snow.
The salt spread on the roads every winter ends up in the soils or in waterways when the snow melts. It then causes a salinization of freshwater, which is rather harmful to plant and animal species sensitive to overly salty water. This increase in salt also disrupts the health of trees along the roads, whose roots absorb a too high concentration of chloride. In addition, salt accelerates the appearance of rust and corrosion on vehicles regularly operating in these conditions, damaging bodies, chassis, and metal parts. Even road infrastructures are not spared: concrete surfaces degrade faster due to the effects of salt.
Did you know? The sand sometimes spread on snowy roads is not used to melt the snow, but rather to increase vehicle traction on ice by creating a rough surface.
Did you know? The saltwater used to de-ice roads can have a harmful impact on the surrounding vegetation by altering the mineral balance of the soils and making it difficult for plants to grow.
Did you know? It is estimated that one ton of salt spread on the roads can pollute up to 1 million liters of freshwater, which is why some countries are actively seeking more environmentally friendly alternatives.
Did you know? Historically, the use of salt to melt ice on roads dates back to the 20th century, but it became widespread after World War II with the significant increase in automobile traffic.
Sodium chloride is effective down to about -9 °C. Beyond this temperature, it becomes significantly less effective, and special mixes such as calcium chloride should be used, which is effective down to about -30 °C.
Yes, repeated exposure to road salt can accelerate the corrosion of certain metal parts of vehicles or damage the leather of shoes. It is advisable to wash vehicles frequently in winter and to protect shoes with suitable products.
Absolutely! Alternative solutions exist, such as sand, sawdust, or even natural abrasive materials. Additionally, some municipalities use mixtures based on beet juice to reduce environmental impact.
Some cold regions, where the temperature consistently falls below the effective limit of salt, prefer to use abrasives or more specific products such as gravel or sand. Additionally, environmental or economic concerns can also play an important role in this choice.
Generally, it is sodium chloride (rock salt), but in some cases alternatives such as calcium chloride or magnesium chloride may be used, as they are effective at even lower temperatures.
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