The Sound of Science - "Why Does Salt Melt Ice?"

Oct 26, 2018

Sam: I’m Sam Watt from NIU STEM Outreach and this is the Sound of Science on WNIJ. I have a question from Andy today and Kate Powers, our resident chemist, will help me answer. Kate, Andy wants to know why we put salt on our sidewalks and roads to melt ice in the winter?

Kate: That’s a great question Andy. It does seem kind of magical that a solid, salt, turns another solid, ice, into a liquid. Sam, to start answering Andy’s question I have a question for you: what happens when you put table salt into water?

S: Um, I suppose it disappears right? It dissolves?

NIU STEM Outreach

K: Exactly. Table salt is made up of two types of atoms: sodium and chlorine. When these atoms bond together they are called ions. Sodium is attracted to chloride just like the north pole of a magnet is attracted to the south pole of a magnet. And when sodium and chlorine come together they form table salt. Once table salt goes into water, the ions separate and float in between the water molecules. So when you dissolve the salt in water the two ions in the salt come apart and start to get in between water molecules. The salt ions space out the water molecules.

S: So, if the water molecules spread out, does that mean they can’t group together and freeze into a solid?

K: Exactly right! The more salt you have in the water the more energy would have to be removed for the water to freeze. That is called a freezing point depression. The freezing point of a liquid can be lowered by adding something that separates out the molecules.

S: Does it have to be table salt?

K: No, it can be almost anything. In fact, cities don’t usually use table salt -- they use something called calcium chloride. Can you guess why? I’ll give you a hint: table salt is sodium chloride, one sodium ion and one chloride ion. Calcium chloride is one calcium ion and two chloride ions.

S: Hmm… Well if the ions get in between the water molecules, and you have more ions, that would disrupt more water molecules making the water even more difficult to freeze? So, the calcium chloride would produce three ions instead of sodium chloride’s two ions? More ions means even further freezing point depression?

K: Yep! You get three ions for the price of one molecule of salt. That was a great question Andy. Keep your questions coming to NIU STEM Outreach at stemoutreach@niu.edu. This has been the sound of science on WNIJ.

S: Where you learn something new every day.