When there is a fear of roads freezing, towns often put salt on the road presumably to depress the freezing point of water and make it harder to freeze. Yet, when making ice cream, salt is added to ice to lower the temperature. I can’t get my head around this. I would like a chemical explanation if possible.
Also, at what temperature does salt water freeze? Why if the Northwest Passage frozen (though not for long), yet there is plenty of open water around Antartica? Does it have something to do with ocean currents?
I’m no chemist, but I did take AP Chemistry in high school.
If I recall correctly, the issue is “latent heat.” Latent heat is the energy absorbed or released when a substance changes from one state of matter to another. If you were to graph the temperature of a solid piece of ice, starting at, say, ten degrees below freezing, and apply a steady heat to it, the temperature graph would climb up in a linear fashion until it hit 0[sup]o[/sup] C. Then the graph would level out, showing that although energy was still being absorbed, the temperature was not changing – instead, the energy was being used to melt the ice, changing it into water. Once the ice was melted, the temperature can continue to rise again. That plateau on the graph shows the latent heat phenomenon.
The same thing happens in reverse. When salt is added to ice, the resulting mixture has a freezing point below 0[sup]o[/sup] C. The mixture melts, which you would think meanss it’s warming up. In fact, it’s drawing latent heat from whatever’s around it in order to have the energy to melt – what’s around it in the ice cream maker is, of course, the internal tub of cream mix which is to be frozen.
Why doesn’t this work on the streets?
It does. But on the streets, we don’t care about making the street a bit colder. We care about melting the ice, and the salt does that. Paradoxically, it also makes the street colder, because the melting ice-salt mix does draw latent heat from the street surface and the air in order to melt. But we’re not concerned with that – just with getting salty slush instead of ice.
It goes without saying that I welcome any corrections by those with more impressive chemistry credentials than Mr. Henry’s AP Chem class.
It comes down to heat flow. Heat always goes from a place of themperature to to one of lower temperature.
Suppose it was real cold durig the night and the road surface is at -10 C and the air temperature this monring is - 3 C. Heat will flow from the air to the road surface. However the ice will not melt because all temperatures are below 0 C. Add sodium or calcium chloride and the melting point is now lowered below -3 C. The heat flowing from the air to the ice can now melt it.
Likewise with ice cream. Withough salt the water in the freezer can’t go below 0 C. Add salt and the water can now get down to a much lower temperature provided the ice is at least that cold.
Salt water freezes at 0 F. Or rather, salt saturated water freezes at 0. The concentration of salt will affect the the exact point. So, when there is ice and snow on the road, salt is applied, lowering the freezing point to some temperature below the ambient temperature, which means the ice melts. If the ambient temp is too cold, this will not work. Most de-icing salt is not sodium chloride, but some other chemicals that will depress the freezing point even further. The result of applying the salt, as Bricker notes, is that you get colder slush. This is why home made ice-cream uses salt. You cannot freeze the cream with just ice- the cream will melt the ice. You need to get the ice colder than the freezing point of the cream to absorb the latent heat. The way this works on a molecular level is that water is a polar molecule, with the two hydrogens sticking off of the oxygen, looking much like Mickey Mouse. The O is greedy with the electrons from the H, so that end (the chin) has a somewhat negative charge, while the Hs, being single protons, give the ears a somewhat positive charge. The chins wind up aligning with the ears (this is what makes water “sticky” and is the basis for adhesion and cohesion and why it works so well as a solvent). When frozen, these polar bonds are just a bit stronger than in liquid form. The positive and negative ions from the salt have stronger charges than each end of the water molecules, and can break these frozen bonds, causing the ice to become water.
As for your second paragraph, there is plenty of frozen water around Antarctica, called ice shelves. But there is a limit to how far north these can extend, due to the currents and climate.
Another effect is the endothermic process of solvation. It takes energy to break the lattice energy and the solvent interactions resulting in a drop in temperature. This can be observed when you mix sodium chloride with water the temperature of the water will decrease.
Just a point of clarification: the first sentence is correct, but the second one is not. Ice at 0 C will cool to below 0 C with the addition of salt (or to be more specific, the resulting salt-ice-water solution). This is due to the absorption of the latent heat of fusion that we all learned about in Mr. Henry’s chemistry class.
But the salt-water combination will not go below the melting point at that concentration.
My second sentence was at least misleading. If we have ice and water both at 0 C and we add salt the brine can indeed get colder than than the ice down to the new melting point.
The problem with this statement is that the water will be at 0C, but the ice will be colder than that. Adding salt doesn’t just spontaneously cause the ice to get colder. If it did, we could just keep adding salt forever, and the ice would never melt.
Thanks for the great answers.
As for the quote above, I’m wondering how breaking the ice crystal apart actually causes the temp to go down. If temperature is the average energy of the particles and, in this case, entropy is increasing, how does the temperature go down?
Actually, due to the enthalpy of solvation and the enthalpy of melting (latent heat) both processes will lower the temperature of the ice/salt water mixture. Breaking up ice crystals and breaking up salt crystals requires energy, so both processes are endothermic. i can guarantee that if you add room temperature salt to ice at 0˚C you will end up with a mixture that is lower than 0˚C.
You can add salt until you reach saturation, but the ice will be constantly melting. Ice/salt baths can get amazingly cold.
Let’s say you want to make ice cream. So you add the sugar, cream, and whatnot to the canister and you start cranking the handle. But the ingredients need to be kept cold during the process. So you say to yourself, “Ah, I have an idea. Ice is cold. I’m going to pack ice around the canister!”
You’re correct that ice is cold. The temperature of the ice will be equal to the set point temperature of your freezer, which is typically around -18 °C (0 °F). This is certainly cold enough for making ice cream. But there is a problem with using ice: when you pack ice around the canister, you will find that there isn’t much “contact area” between the ice and the canister. There will also be lots of air spaces between the ice particles. So while it’s true the ice itself is very cold, the thermal resistance between the ice and the canister is too high to allow for efficient cooling of the canister.
So then you say to yourself, “I know how to fix this problem. I’ll melt the ice until there’s an ice-water slurry. Since a slurry allows 100% contact area, this will create a low-resistance heat transfer path.” This is an O.K. plan, but the problem is that this will result in a slurry temperature of 0 °C (32 °F). While 0 °C is pretty cold, it would be nice if it were colder.
This is where salt comes in. When you add salt to ice, it will melt at a lower temperature than without the salt. You’ll end up with an ice-water slurry (which is good for efficient heat transfer) at a temperature around -16 °C (which is much better than °0 C).
Actually, that was kind of the point of my post: adding salt to ice DOES spontaneously cause the ice to get colder, because it uses the heat of the ice as energy to transition itself from solid to liquid. It seems funny to speak of “heat” of ice, but compared to, say, -10[sup]o[/sup] C., 0 [sup]o[/sup] C. is warm. So the heat in the ice is decreased by the physical change that happens when the ice melts. That’s another counter-intuitive thing happening here: you hear “ice melts” and you think it’s getting warmer. In fact, it’s melting and getting colder.
