Today’s Wonder of the Day was inspired by Sydney from IL. Sydney Wonders, “Why does salt melt ice?” Thanks for WONDERing with us, Sydney!

Brrrr! It’s a cold day here in Wonderopolis. Is it cold where you are? If you live in a place that gets a good deal of snow and ice each winter, you already know what the next few months may hold. We hope you’re ready for snowball fights, hot chocolate, and maybe even . . . snow days!

Of course, areas with heavy snowfall welcome another seasonal visitor around this time of year: the salt truck. At the first hint of snow or ice in the forecast, drivers will be out spreading salt on the roadways. You may also see people spreading salt on their sidewalks and driveways. 

Why do people put salt on roads in winter? It helps prevent ice from forming on the pavement, making it much safer to drive during winter weather. The salt spread on roads and sidewalks is essentially the same as table salt people add to their meals. However, it’s not purified like table salt. That’s why it often appears gray or brown.

Are you WONDERing how exactly salt makes the roads safer? You may have heard people say salt melts ice. However, the process is a bit more complicated than that. It has to do with a concept called freezing point depression.

Liquid water becomes frozen ice at 32°F (0°C). This is its freezing point. When salt is added to water, it makes it harder for water molecules to form ice crystals. In effect, this lowers the freezing point of water to about 15°F (-9.4°C). Below that temperature, even roads that have been salted may be covered in ice.

The process works best when salt is already mixed with water when it meets the road. That’s why many cities use a brine solution instead of rock salt. In very low temperatures, you may see sand on icy roads. Sand doesn’t melt the ice, but it does offer a less slick surface for driving.

Salt offers a cheap and effective way of making roads safer during winter months. However, recent studies have shown that it’s having a negative effect on the environment. Rock salt used to treat roads ends up in local bodies of water and even in ground water. In large quantities, this can reduce the amount of freshwater in an area and negatively impact plants and animals that rely on it.

Salted roads may also be the cause of increased chloride levels in U.S. lakes and rivers. This chemical can be toxic to plants and animals. In addition, the salt is corrosive and can damage roads, bridges, and vehicles. These impacts have led some groups to advocate other solutions for icy roads, like sand. Some even suggest using molasses or beet juice in place of salt.

Are you looking forward to winter weather? For many families, it’s the best season of the year! However, driving during cold months can be dangerous with or without salt on the roads. Talk with your family about how you can stay warm and safe this winter.

Standards: NGSS.PS1.A, NGSS.PS1.B, CCRA.R.4, CCRA.L.3, CCRA.L.6, CCRA.R.2, CCRA.R.10,CCRA.R.1, CCRA.R.2, CCRA.W.2, CCRA.W.4, CCRA.W.9, CCRA.W.10, CCRA.L.1, CCRA.L.2, CCRA.W.3, CCRA.SL.1

Wonder What's Next?

Tomorrow’s Wonder of the Day is kind of dark, but that’s OK. Stop by and we’ll see what develops.