How does road deicer work?

[Goldfish4209]

We were doing the typical middle school salt in ice experiments and the teacher couldn't really get deep into how the deicer really works. Deicer is magnesium chloride, and that supposedly breaks into ions in water, so how does that reduce the freezing point of water? To me, it makes no sense where the extra energy goes. Is it some kind of endothermic chemical reaction?

 

[PolymerTim]

Well, I'm no expert in this area, but I think I can get you started.

The property you are referring to is known as a colligative property, which means that it is a property of solutions that depends on the number of particles (or ions) and not the mass concentration. This is because the effect is related to the physical bonds formed between the solute and the solvent. The two most commonly known are freezing point depression and boiling point elevation. So when you add salt to water, it both increases its boiling temperature and decreases its freezing temperature at the same time.

You can learn more about solutions in general here. Pay specific attention to the link for colligative properties. Also to consider with solutions is the heat of solution. You asked about where the energy goes: basically it is a balance between the form of heat and that located within bonds. Here is a good link on enthalpy of solution.

Essentially, every bond, whether physical or chemical, has an energy associated with it. When you add salt to water, you can consider a few things happening: 1) Some of the water molecules have to separate from each other to make room for the new molecules (endothermic); 2) The solute molecules have to separate from each other (endothermic); 3) solvent-solute bonds are formed (exothermic). Some solutions will heat when you mix them while some will cool based on how these 3 magnitudes with opposing signs add up.

Hope this helps!
-Tim

 

[CycloWizard]

PolymerTim did a pretty good job of describing it all. I'll describe it a little differently to give you a different perspective.

When pure water freezes, its molecules line up in a highly ordered fashion. The molecules tend to align themselves in this manner at low temperatures because they are very cold and don't move around much, so the polarity of the water molecules aligns them. By adding ions to the solution, the polarity of the water molecule is somewhat offset by the added electrical effects of the ions.

A more technical explanation is that there are two effects in freezing water: kinetic energy (directly related to temperature), which tells you how much the molecules are moving around, and electromagnetic energy (related to charges and polarity of molecules and independent of temperature), similar to energy between magnets of different polarities. If the magnets are close together and not moving very quickly, the magnetic force will bring them together and lock them in a certain orientation. If, on the other hand, the magnets are moving around very quickly, the magnetic forces will not be able to overcome the kinetic forces and the magnets will not interlock. In water, the kinetic forces become very low relative to the electromagnetic forces near the freezing point so the molecules can easily align and interlock. Increasing the temperature increases the kinetic energy and makes the molecules bounce off each other rather than linking up. Adding ions changes the likelihood that the magnets can interlock by making the magnets attract to more things. This is like having a bunch of magnets in space. If you suddenly add a magnetized box around them (i.e. deicer ions), then the magnets won't always end up running into other magnets. Some can run into the sides of the box. Maybe not the best explanation, but it's Saturday night and my brain is turned off for the weekend. 😛

 

[PolymerTim]

Hey , that's a good one. I learned about an effect called screening that is just what you're describing I think. The salt ions "hook up" with some of the water molecules through this ionic (electromagnetic) bonding and the water molecules actually prefer them over the other water molecules. The problem is that when the water molecules try to crystallize, they have to work around these ions and it takes more energy. I can see it kind of like laying a brick wall. Imagine you get half way up and someone gives you a couple of round bricks to stick in. No matter how you work it, they just don't fit well and you end up having to use a lot more cement to make your wall.

I'm sure there is a role for entropy here as well, but I'm definitely not the person to ask about that.

 

[CycloWizard]

Originally posted by: PolymerTim
Hey , that's a good one. I learned about an effect called screening that is just what you're describing I think. The salt ions "hook up" with some of the water molecules through this ionic (electromagnetic) bonding and the water molecules actually prefer them over the other water molecules. The problem is that when the water molecules try to crystallize, they have to work around these ions and it takes more energy. I can see it kind of like laying a brick wall. Imagine you get half way up and someone gives you a couple of round bricks to stick in. No matter how you work it, they just don't fit well and you end up having to use a lot more cement to make your wall.

I'm sure there is a role for entropy here as well, but I'm definitely not the person to ask about that.

Actually, the effect you just described plays a role in the entropy explanation. I didn't want to go into that because I was already long-winded in my previous post. 😛 On an atomic level, entropy can be defined as a measure of the number of conformational states that a molecule can achieve. Since you have added the strangely shaped brick to the equation, the number of potential conformational states (and, therefore, entropy) has increased. Freezing will only occur once the entropy has receded to a certain level, so to do that the temeprature again must be lower.

 

[PolymerTim]

Originally posted by: CycloWizard

Originally posted by: PolymerTim
Hey , that's a good one. I learned about an effect called screening that is just what you're describing I think. The salt ions "hook up" with some of the water molecules through this ionic (electromagnetic) bonding and the water molecules actually prefer them over the other water molecules. The problem is that when the water molecules try to crystallize, they have to work around these ions and it takes more energy. I can see it kind of like laying a brick wall. Imagine you get half way up and someone gives you a couple of round bricks to stick in. No matter how you work it, they just don't fit well and you end up having to use a lot more cement to make your wall.

I'm sure there is a role for entropy here as well, but I'm definitely not the person to ask about that.

Actually, the effect you just described plays a role in the entropy explanation. I didn't want to go into that because I was already long-winded in my previous post. 😛 On an atomic level, entropy can be defined as a measure of the number of conformational states that a molecule can achieve. Since you have added the strangely shaped brick to the equation, the number of potential conformational states (and, therefore, entropy) has increased. Freezing will only occur once the entropy has receded to a certain level, so to do that the temeprature again must be lower.

Wait, I think its all coming back to me now...

something about energy...

and a funny guy named Gibbs...

...

...

G = H - T * S

Gah! I thought I would never see that again!
{stumbles into the darkness mumbling something about never-ending derivatives}

PS - If we're not careful, we might even bump into the interaction parameter (chi). That one is really big in polymer solution theory.

 

[Gibsons]

Originally posted by: PolymerTim
Well, I'm no expert in this area, but I think I can get you started.

The property you are referring to is known as a colligative property, which means that it is a property of solutions that depends on the number of particles (or ions) and not the mass concentration. This is because the effect is related to the physical bonds formed between the solute and the solvent. The two most commonly known are freezing point depression and boiling point elevation. So when you add salt to water, it both increases its boiling temperature and decreases its freezing temperature at the same time.

You can learn more about solutions in general here. Pay specific attention to the link for colligative properties. Also to consider with solutions is the heat of solution. You asked about where the energy goes: basically it is a balance between the form of heat and that located within bonds. Here is a good link on enthalpy of solution.

Essentially, every bond, whether physical or chemical, has an energy associated with it. When you add salt to water, you can consider a few things happening: 1) Some of the water molecules have to separate from each other to make room for the new molecules (endothermic); 2) The solute molecules have to separate from each other (endothermic); 3) solvent-solute bonds are formed (exothermic). Some solutions will heat when you mix them while some will cool based on how these 3 magnitudes with opposing signs add up.

Hope this helps!
-Tim

It isn't listed on the wiki article, but I can say from personal experience than anhydrous magnesium chloride generates a lot of heat when added to water (hexahydrate doesn't, obviously...) . I remember making a 1M solution and being worried that the glass bottle was going to break from heat. It was too hot to touch for more than a few seconds, so I'd guess it reached 70C or so.

 

[PolymerTim]

That would be related to the enthalpy of solution (see my link above). I have dealt with hydrochloric acid and potassium hydroxide a lot myself and they also get very hot. A few times I have dissolved about 2 kg of potassium hydroxide in about 2 L of water and the temoerature got well over 100 deg C! (but of course didn't boild due to boiling point elevation). The link to wikipedia has a few common heats of solution listed. Note that a negative sign means heat is released into the water while a positive sign means heat is removed from it (get colder).

 

[Goldfish4209]

Awesome! Thanks, guys! Extra credit, here we come...

 

[Paperdoc]

Firstly, don't get too sidetracked by the exothermic Heat of Solution. While it is true that many of these salts "react" with water when they dissolve and release heat, that has little to do with their "ice melting" ability. "Ice melting" in this context usually refers not to the original melting of solid ice, but rather to the fact that the salt/water mix produced stays liquid rather than crystallizing back to ice.

The key is the colligative property, as mentioned by others, that ANY solution of a solute in a solvent will have a freezing point LOWER than that of the pure solvent. How much lower depends on which solute, and on how much is present. And the "how much" part must be measured in MOLARITY - that is, how many MOLES of the solute are dissolved per liter of solution. In reference books you can find tables of the Molar Freezing Point Depression Constants for many solutes in water.

Now, at any temperature below the freezing point, there is a dynamic equilibrium between solid water (ice) and a bit of liquid water. When you sprinkle a salt on the ice, it immediately starts to dissolve in the water phase, and the resulting solution now has a much lower freezing point, so it will not re-crystallize unless the temperature is reduced. This disturbs the equilibrium, so now more of the solid phase melts to liquid, and this keeps up until the liquid phase becomes more diluted to the point that its melting point is close to the ambient temperature. Note, however, that if it really is cold enough outside, even this salted liquid phase will have a freezing point higher than the really cold outside, and it will freeze anyway. In other words, salt still will not melt the ice and keep it slushy if the temperature is too cold.

I trained in Physical Chemistry, but I cannot remember clearly the molecular-level explanation of these colligative properties. It is very tempting to talk about the presence of ions in the water interfering with the normal hydrogen bonding between water molecules, thus reducing their ability to align into a rigid crystalline structure. (And yes, I did use the term "hydrogen bonding". This is neither ionic nor pure covalent bonding, but a form of weak covalent bonding involving hydrogen atoms almost shared between molecules.) The problem with that explanation is: how does this account for boiling point elevation? If the ions' presence interferes with attractions between water molecules, should that not make it EASIER to convert liquid water into gas? That is, one might predict that the Boiling Point would get lower, but in fact it goes higher when you dissolve a salt in water!

 

[CycloWizard]

Originally posted by: Paperdoc
The problem with that explanation is: how does this account for boiling point elevation? If the ions' presence interferes with attractions between water molecules, should that not make it EASIER to convert liquid water into gas? That is, one might predict that the Boiling Point would get lower, but in fact it goes higher when you dissolve a salt in water!

The addition of ions decreases water's ability to bind to itself since water also has an affinity for the ions floating around. The same goes for elevation of the boiling point: water is now clinging to other waters and to ions, so it is more reluctant (in an energetic sense 😛) to change phases to a vapor. It simply has too much energetic baggage to make the change. If you hold its feet to the fire and let the fire get hot enough, it will come around and move on with its life.

 

[Acanthus]

Just to add to the discussion, a salt-ice bath will get down to about -20C in ideal conditions from the endothermic reaction between the salt and ice.

 

[Mday]

Originally posted by: Acanthus
Just to add to the discussion, a salt-ice bath will get down to about -20C in ideal conditions from the endothermic reaction between the salt and ice.

and cool a can of beer in a few mins ;-)
(assuming crushed ice is used and not ice cubes)