ROCHESTER, NY (WROC) – Road salt is used throughout the winter season to help make our roads, sidewalks and other pathways safer for travel. It is often believed that common road salt is solely used to melt the snow and ice that covers our roads and sidewalks, but there’s actually more to it than that. The salt isn’t used specifically just to melt the ice as much as it is to prevent the additional formation of ice. The process involves a little bit of chemistry, so let’s break it down:
Salt lowers the freezing point of water, so when you mix the two together it takes the water longer to freeze.
Water freezes at 32°F or 0°C, but when salt is added to water it works to delay the freezing process as salt ions are broken down into sodium and chloride ions (yellow circles). It is these ions that help disperse the water molecules (blue circles) and spread them apart, making them harder to reform into ice, even at below freezing temperatures.
This method can lower the freezing point of water to around 15° or so, but get any colder than that and it won’t work as well, or not at all. Sand can be used as an alternative in this situation to help provide more friction between tires and the slick ice. Road salt works most effectively above this point, and when this salt is mixed in with liquid water.
As mentioned above, road salt is used more as an agent to prevent additional ice from forming. Adding salt to roadways helps create more friction between the tires and ice to make it safer and easier to drive on. The salt can also work more efficiently with a little help from the sun as it melts some of that existing ice into water, which then allows the salt to mix in with the water a lot quicker.
Fun Fact: There’s a very thin layer of water that constantly coats the surface of ice, so you don’t necessarily need a huge amount of water to mix with the salt for this process to work, but this certainly helps the salt do its job. This is also why ice is so slippery!
With that said, a pre-wetting agent is often sprayed along with the salt to help this process along when needed. Bridges are much more difficult to pre-treat than roads are since they become colder much quicker, hence why you see bridges freeze before road signs.
Types of Road Salt
Different types of salt are used depending on how cold temperatures are. One of the most common forms of road salt is Sodium Chloride, also known as halite, or the salt you find at your kitchen table. They are essentially the same thing, only road salt is mixed with other chemicals and particles. This is the most inexpensive form as pre-wetting agents are used along with the salt to help prevent it scattering, and the need for any additional salt to be used which lowers the cost.
Magnesium chloride is used in its liquid form as a pre-wetting agent, and can be obtained through the evaporation of salt lakes. It works as a melting agent for snow and ice as heat is released while it dissolves. Calcium Chloride can help melt ice at extremely low temperatures, but is expensive and not as widely used. Potassium Chloride is often mixed in with other materials as it’s not quite as effective at de-icing on its own.
Using salt on roadways is important to ensure safety while people are traveling and getting where they need to go, but it’s not the best for the environment. There are eco-friendly salts around that are naturally produced from salt lakes that can be used as an alternate option.
Road salts can also be harmful to pets and get stuck in their fur and between their paws. Be sure to always check them after they’ve been outside in the winter to ensure they aren’t tracking salt or possibly any other harmful chemicals around your home!
~Meteorologist Christine Gregory