Freezing Water With Salt

Most recent answer: 01/31/2015

Q:
In my science class, we did an experiment with ice, water, and salt. so, in this plastic container that was almost a similar size of a cup, we filled it mid- way with ice. However, in the middle, there was a vial filled with regular- temperatured water, and then there were two thermometers- one in the vial, and one in the ice. what my classmates and i did was, i added salt only in the ice- spreading it evenly and in a fine amount. The experiment lasted for twenty minutes or so, and after one minute, we would always check the temperatures. It would decrease. At the end, the water inside the vial sort of froze. What in the salt that made the water freeze?
- Roxy (age 13)
Jackson Heights New York
A:

Nice experiment. The pure water in the middle froze because it got colder, as you measured. There are two reasons that adding the salt to the ice water cooled it down. 

1) This first is a small effect, but simple. The salt NaCl dissolves in water forming Na+ and Cl- ions. This arrangement has a little higher energy than the separate water and NaCl had. So that energy came from all the jiggleng molecules, leaving them jiggling a bit less. That means they're colder.

Why would the NaCl dissolve if that means raising its energy? Isn't that sort of like falling uphill? The reason is that there are many more ways for the ions to arrange when they're spread out through the liquid than when they're arranged in a crystal NaCl pattern. So it is like falling uphill, which is exactly what air molecules do if you release a batch at ground level. They spread out through the atmosphere because that lets them reach many more possible states.

2. That leads to our big effect, which we've described before. () Adding salt lowers the freezing point of water. Some ice melts because it's above that new freezing point. It takes a lot of energy to pull a water molecule from a nice orderly ice crystal into the jumbled liquid, where it doesn't make such good contacts with its neighbors. So that big piece of energy also comes from the jiggling molecules, and leaves them colder. This happens until the ice is used up or the temperature falls to that lower freezing temperature.

You might ask why having salt in water lowers its freezing point. Say you had some ice in pure water, right at the freezing point. Forming a little more ice would reduce the number of states because the water molecules have to line up, but also increase the number because the energy released lets them jiggle around in more ways. At the freezing point, those two effects just balance. Now put some salt in the water. If some liquid forms, in addition to the balanced changes we had before, there's an increase in the volume of liquid for the salt to run around in. So now melting raises the total number of states. Therefore the ice melts.

These effects are all reminders that nature settles into forms with the biggest possible number of little internal states. We say that "entropy" gets maximized.

Mike W.


(published on 01/31/2015)