posted
With the winter season rolling around, and many of us rolling around in big unwieldy machines (also known as automobiles) I felt that some people here would be interested in why putting salt on the road makes it safer driving conditions.
First off let’s take a look at the end result of salting the road. As most of us probably know the final effect of dumping salt is lessening the amount of ice on the road. This is why it’s safer of course, less ice means better traction for the tires, and thus, safer driving. So, clearly what we need to figure out is how adding salt to ice gets rid of the ice.
There are two common ideas for how this works, one is that salt increases the road temperature and thus warms up the ice to melting point, the second is that adding salt lowers the temperature at which water freezes (normally water freezes at 32 degrees Fahrenheit, or 0 degrees Celsius, the idea is that the temperature would have to be lower for the water to freeze when there’s salt in the system). Both of these concepts are … not completely inaccurate, but at best vague generalizations that hide the real mechanism by which it works.
The important thing to understand is how freezing works, the real mechanics of it. And the way to understand that is realize that ice is not just ice sitting there, it’s some ice and some water, where part of the water is freezing and part of the ice is melting at equal rates; it’s called dynamic equilibrium. To explain it again, there’s some amount of liquid water in they system (a chemist’s name for the big picture) and some other amount of ice, but 20 minutes later it probably wont be the same ice, just the same amount of ice.
Now, this would be a good time to jump right into how salt affects this dynamic equilibrium, and thus reduces the total amount of ice, except there’s one more thing that’s important to understand before that: temperature. Now temperature is often descried as a measure of the movement of air, the more movement the higher the temperature. This is, once again, only vaguely accurate (and only when you’re talking about temperature in the air).
There’s many ways to accurately define temperature, and obviously I’ll use the definition most helpful for understanding this scenario. Temperature is the measure of the amount of energy per volume, in every, say, square meter, there’s a certain amount of energy, probably contained in the movement of air (which is why our original definition is not entirely inaccurate).
OK, so now let’s walk through the process. Thee freezing temperature of water is defined at 0 degrees Celsius, this does not meant that at one degree Celsius there’s no ice at all, or that at minus one degree Celsius there’s no liquid water. So as we approach 0 degrees small ice crystals begin to form in the water. Water is continually turning into ice, releasing energy, which, combined with the energy in the system (the temperature), continually melts the ice. If the temperature stays constant then the amount of water freezing and ice melting will be the same; but as the temperature dropped that means energy is taken out of the system, not enough energy is there to re-melt the ice, so more water will freeze than ice melting, and the crystals begin to grow. At 0 degrees most of the water has been converted into ice, but there’s still some water there freezing and ice melting.
That’s the freezing process without anything else in the system, but we’re interested in the change adding salt to the situation makes, so let’s get down to it.
Salt, as you probably know, is NaCl, sodium and chloride bonded together in what’s called an ionic bond (technically this is just one kind of salt, but the definition of a salt is basically that it will all act this way for the same reasons). When salt is added to water the sodium and chloride break their bond, and attach to the water molecules. These bonds between the water and the ions (sodium and chloride separated are ions) have negative energy stored in them. In other words, it takes energy to separate the salt from the water.
Now when water freezes, there can be nothing in it. Any ice is purely water, sometimes it freezes around other substances, but the ice crystal itself is pretty much composed entirely of water. So for this salt water to freeze, first the salt has to come out of solution, which, as we described above, will take energy.
So here we are with some ice and some water, mostly ice, and the water is freezing and ice is melting at the same rate. Now we add in some salt, this dissolves in the water (even if there’s only a small amount of water there). Because there’s still some energy in the system (a non – absolute zero temperature, which happens to be anything above -273 degrees Celsius) the ice keeps melting at the same rate, but some of the water is bonded to the salt, and it takes up a lot more energy to get that water to freeze, since first the salt has to fall out of solution, then the water has to freeze. This means that the amount of water freezing is disproportionably smaller than the amount of ice melting, and now we’ve got ourselves less ice.
That’s pretty much the full thing, but before we’re through, I’ll explain how the two general ideas people have relate to the actual phenomena. The first idea, that the salt raises the temperature, is pretty much totally inaccurate. The one thing is, technically it does raise the temperature, it’s just that this effect is completely negligible, you can try it yourself, dump a whole lot of salt into s cup of water and see if you can tell if the temperature raises (you wont be able to).
The second idea, that adding salt decreases the freezing temperature of water, is much more accurate, because what happens is that as the temperature keeps dropping, the lack of energy allows for more and more of the salt to fall out of solution and the water freezes, until eventually you’ve just got a lot of salt on the road and plenty of ice. Now the effect of adding salt is that it will take a lower temperature to freeze water, but that hides all the interesting physics/chemistry behind it!
posted
Coolness. Just got home, and have to take off again. Just had time to read it. I'll post more when I get to work.
Posts: 1132 | Registered: Jul 2002
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posted
I thought that was fascinating. I love it when people can explain scientific stuff without using all the scientific terms that I won't understand. Good times.
Posts: 1261 | Registered: Jun 2002
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Next time can you explain why I should bother sprinkling a little salt into a pot of water being heated up like my mom tells me to? Will it really boil that much faster?
posted
Hmmm. I don't know if I agree. If the salt and the chlorine required energy to be separated, that would pull energy out of the chemical milieau, making it colder. I think the sodium chloride remains in solution. Otherwise, wouldn't salting ice result in the release of chlorine?
I know there is something called fusion something or other, maybe temperature of fusion. Are you describing that or is this something you reasoned out?
What I am familiar with in the behavior of salt in water is that a salt molecule will gather water molecules to itself. 52 water to one salt. Which is why salt makes the body retain water. But the application of this is not always intuitive. There are minerals that tend to gather water into smaller, weaker groups and the salt breaks these up. Salt is used in a water softener to pull molecules off the supramolecular clusters. I think a bigger molecule cluster = harder water. At least, that's what I think I understand.
From Ask a scientist:
quote: There is a physical constant called "molar heat of fusion" that somehow controls the melting point temperature. When you have salty water, the salts in the water make the molar heat of fusion, (and the melting point) lower, and this means that the ice inside this salted water will melt only at a lower temperature. And the ice cubes will melt slowly...
What's getting me here is the "somehow controls". I'm thinking that it is more important that the temperature at which water freezes is lowered, as evidenced by my extensive ice-cream making experiments.
I'm saying the mechanism by which this happens is that the water molecule-grabbing property of the salt challenges the ice-forming process, rather than the salt molecule being broken.
[ December 24, 2004, 07:06 AM: Message edited by: Trisha the Severe Hottie ]
Posts: 666 | Registered: Dec 2003
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posted
Salt water boils at a lower temperature than pure water. It will boil faster, but it will be cooler when it does.
Posts: 26077 | Registered: Mar 2000
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posted
Katharina: That's what I thought, but how salty would it have to be to make any appreciable difference? And maybe boiling cooler isn't necessarily a good thing? I just wondered. But I should stop derailing Hobbes' thread. Mostly I just wanted him to love me as much as he loves Raventh, WheatPuppet, Kirk, and Flyby.
*Drives to work and admires the salt all the way there*
Posts: 1990 | Registered: Feb 2001
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posted
Salt water has a higher boiling point, not lower. But the amount of salt you add probably doesn't make a huge difference.
Posts: 1810 | Registered: Jan 1999
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quote:Hmmm. I don't know if I agree. If the salt and the chlorine required energy to be separated, that would pull energy out of the chemical milieau, making it colder. I think the sodium chloride remains in solution. Otherwise, wouldn't salting ice result in the release of chlorine?
Actually, disolving salt into water is an exothermic process, just not by a whole lot, and the energy I was talking about is not the amount required to disolve the salt, but the energy required to pull it out of solution and reform it. Yes the ions seperate, but the things is, when they come out of solution, to keep the water's charge balanced they do so together, and it's just like it never happened. Also, to keep in mind, chloride and chlorine are two seperate things, but don't worry, niether of them will form when you dump salt into water and then pull it out again.
quote:I know there is something called fusion something or other, maybe temperature of fusion. Are you describing that or is this something you reasoned out?
That's just another name for melting point, so yes, I was kind of describing the reverse process (freezing). Normally it's reffered to as "heat of fusion" because then it's a measure of how much energy is required to break the freezing/non-freezing barrier.
quote:What I am familiar with in the behavior of salt in water is that a salt molecule will gather water molecules to itself. 52 water to one salt. Which is why salt makes the body retain water. But the application of this is not always intuitive. There are minerals that tend to gather water into smaller, weaker groups and the salt breaks these up. Salt is used in a water softener to pull molecules off the supramolecular clusters. I think a bigger molecule cluster = harder water. At least, that's what I think I understand.
Well I'm not too framilar with the water softener techniques, nor have a I heard the 52 before, though it's entirley possible, but yes, salt will gather water to itself, and it does this because both the sodium and choloride ions, when seperate from each other are, well ions. They're charged, and water is not equally charged; one side is positive, one negative (the weak pull of the hydrogen atoms on their electrons allows for what is called "hydrogen bonding", a much stronger version of intramolecular force. This hydrogen bonding will occur on both ions (though they'll probably stay close together) seperatly, since one is positive (Sodium) and one is negative (choloride), that's why it dissolves, and that's why water dissolves any salt which is pretty much just anything with an ionic bond.
quote:I'm saying the mechanism by which this happens is that the water molecule-grabbing property of the salt challenges the ice-forming process, rather than the salt molecule being broken.
Yes, it is that property, which causes the need for the salt to then let go of all those water molecules before they can freeze that does it, it's just that to do it, the salt has to break up first.
quote:Salt (or other solutes, like sugar) can easily dissolve in liquid water. However, taking the solute out of the water and putting it in the gas phase (air) requires a lot of energy. At temperatures around the water boiling point, these solutes stay in the liquid.
Now the total pressure in the liquid and the air at the boundary are the same- otherwise one would push the other into a smaller space. Part of the pressure in the liquid comes from the solutes, not the water. So the pressure due to the water alone is reduced compared to that of pure water at the same temperature. The vapor pressure, that is, the pressure of water vapor that would stay in equilibrium with the liquid, is reduced by the same amount because of the solutes.
Water boils when the vapor pressure of the water gets to be as big as the pressure of the atmosphere. At that point, vapor bubbles in the water can grow. You have to heat the liquid with solutes up more to get the vapor pressure in it to equal the atmospheric pressure, so it has a higher boiling point.
A very similar argument explains why solutes also lower the freezing point. Since the solutes are almost completely excluded from the solid (like from the gas) they stabilize the liquid. A search of this site will turn up some answers about freezing salt water.
Adding salt extends the liquid range at both ends.
And the explanations for raising the boiling point were wonderful too.
I recall from my Chemistry classes that the bond strengths (thermodynamics) are the important thing in this as Hobbes has pointed out. I vaguely recall some mention of lattice structures (crystals) and sort of distributed ions (in liquids and gasses) being important too. The point being that a solid of salt water doesn't form as readily because the ions no longer line up in a nice crystalline structure. As was also true for the now dissolved salt crystal as well. The two in combination just sort of "don't fit" a nice lattice or something like that.
But really, it's the story of bond strengths that makes the most sense. I mean, if the bond strengths between NA+ and O-; or H+ and CL- ions (*or the various larger ions) were stronger, the whole thing would form ice readily (i.e., it would become a solid).
The other thing that's sort of cool about thinking about chemistry of biologically important molecules like NaCl and H20 is that the bond strenths are sort of "just right" for super-important chemical reactions to occur. Sure, maybe it works this way because it couldn't have evolved life without these preconditions. Or, maybe it was designed that way.
I'm not going to quibble on Christmas Eve. I'm just going to marvel at it and think how studying science can reveal the divine if you want to think of it that way.
Posts: 22497 | Registered: Sep 2000
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posted
What's funny about this is that I had asked my dad this same question a month or two ago before he moved. He gave me the lattice work explanation.
Posts: 2283 | Registered: Dec 2003
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posted
I, for one, would enjoy that immensely. I didn't do well in Chemistry until I got past Organic Chem (and into strict Biochem). Then it all started to make sense because I could see it working in real ways.
Your road salt explanation is excellent because it makes the chemistry understandable in that same way -- through something familiar.
posted
OK, can someone extend this information to explain why those supersaturated heating pads with a clicky-disc in them work? These are the ones that are clear plastic, filled with a hard, waxy substance. You nuke it until it's entirely liquid. When you want it to generate heat, you flex a metal mesh disc inside and crystals form while a lot of heat is released. As the pad cools, the viscosity goes from liquid to slushy to solid.
I know why the cryatals form - supersaturated solutions don't give up their solute until there's something present the crystals can form on. You can do the same thing with sugar or salt on your stove.
But where is the heat stored in this scenario, and why does the solidifying of the crystals release it?
posted
Forming a crystal lattice usually releases heat, especially when it doesn't have to come out of solution (though technically a supersaturated solution still does, it's a lot less energy).
posted
Let me recheck some of your facts; it doesn't feel quite right. I just spent a full week cramming for an IB chemistry exam and my chem brain is weakly exclaiming that something's wrong . . .
Posts: 1735 | Registered: Oct 2004
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quote: So is there a better way to do all that stuff that doesn't rust my car?
Move to Arizona.
I read the 52 molecules thing in the Encyclopedia Britannica.
So in your boiling explanation, did you say that in order for water to boil away, it makes the solution more dense in salt, and so it is fighting diffusion?
Posts: 666 | Registered: Dec 2003
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posted
The slight charge difference caused by the stronger pull of the Oxygen atom than the Hydrogen atom is, while very noticable, still much smaller than the charge of a sodium ion, or chloride ion, and chances are the thing that really limits the number of water molecules to 52 per is the surface area of the ions, not the charge differential. 52 is a totally believable number and I wasn't questioning it at all. m:)
posted
I'm less concerned with the chemistry of the melting process than with the polluting aspect of spreading salt all over the place.
Seems to me I read somewhere that the oceans are measurably more briny than they were before we started doing this, and you know rivers and streams are not normally briny. So how much damage are we dong to the ecosystem, when what we ought to be doing is taking our cue from nature and staying home until the roads clear? What's the rush all about anyway? Stay home and go sledding with the kids. Build a snowman.
Posts: 3735 | Registered: Mar 2002
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That's not to say I'm not interested in the chemistry, but I have different priorities.
Salt on the roads is just one more example of the frog in hot water effect. We just don't seem to notice how we are destroying ourselves.
Posts: 3735 | Registered: Mar 2002
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Now, the Great Lakes, or other inland water sources wouldn't surprise me too much. I've always thought this seems potentially dangerous. Historically, irrigation has turned a lot of arable land barren by increasing salinity. Doesn't seem implausible we could do that to our large fresh water supplies, either.
posted
Actually the more immediate effects would have less to do with increasing salinity of water, as soil. About 15 years ago I bought my Trek 2300, which is carbon fiber bonded to aluminum. I had heard that salt can cause corrosion in the metal to carbon bonding, so I avoided riding it in the winter.
The first spring that came along, I waited for the white crystal-y stuff to go away, and was startled to discover that it never did. There seems to be a permanent layer of "Frost" on the shoulder of the road now, and a quick taste shows that the "frost" is salt. I would have thought it would wash away with the April Showers, but it doesn't. I gotta believe that much salt in the soil isn't good.
Another thing: There's a bridge nearby that was built out of a kind of steel that was supposed to make a protective covering out of rust. That is, it rusts to a certain extent, and then wasn't supposed to rust any further, so it wouldn't need to be painted. Back in the 70's they announced that it was rusting badly, because of the salt, and so they wound up painting it, at great expense. And now there are signs on the entrance ramps "Danger: Reduced salt area."
Salt is a nasty chemical. It just seems bizzarre that nobody seems to think twice about dumping tons of it all over the place.
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