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I am a high school student and I am confused about freezing point depression.

We say that when in winter's it gets even colder than normal freezing point Temperature then we use solute to melt ice that is there on the floor but we also say that the "dynamic equilibrium" between water and ice exists only at freezing point Temperature. These two statements are contradictory because the process by which the solute helps to melt ice is by disturbing the dynamic equilibrium between water and ice(as it doesn't let water molecules to for ice crystals , as a result more ice transition into water than water to ice). But at temperatures below freezing point, there was no water to begin with and hence no dynamic equilibrium between water and ice. So how the solute here can melt ice? The explanation they give in textbooks is "because it lowers the freezing point" but it doesn't make sense if we talk about the mechanism.

Virender Bhardwaj
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  • Have a look at the answer here https://chemistry.stackexchange.com/questions/116302/where-does-the-energy-come-from-to-lower-the-temperature-of-a-brine-solution/116316#116316 – porphyrin Jan 21 '24 at 08:37
  • I think my question is somewhat different from that one – Virender Bhardwaj Jan 21 '24 at 09:06
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    If it was not, it would be closed as a duplicate. // Consider ice vapor tension and hygroscopicity of salt at high relative humidity, leading to saturated salt solution. – Poutnik Jan 21 '24 at 09:28
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    Does this answer your question? Melting ice with salt - how does it start? – Mithoron Jan 21 '24 at 14:52
  • While there may not be dynamic ‘equilibrium’ initially, there are dynamics going on all the time. Add salt, and those dynamics will lead the system towards a new equilibrium. – Jon Custer Jan 21 '24 at 16:21
  • @Mithoron. The answer to that question explains it from a completely different POV. It creates more confusion from me because the answer on the internet and from the textbooks are always like "the salt no let let water to freeze, but the ice continues to transition into water, thereby it melts or we say the freezing point gets depressed. But according to that answer all of this is incorrect? – Virender Bhardwaj Jan 22 '24 at 12:36
  • @Poutnik in that answer , it is being explained that how the temperature of whole system drops down. I have watched some videos on it and it states that if we are using Nack, it takes more energy to dissolve it than the hydration energy, thereby it's an endothermic process which takes away energy from water and hence decrease it's temperature. But for salts like Cacl2, this is actually exothermic. Anyways my question is different. I am taking about ideal solute and the mechanism by which the ice melts on adding it even though there was no dynamic equilibrium (of ice and water) to begin with – Virender Bhardwaj Jan 22 '24 at 12:44
  • Endothermic is in the first place ice melting. For comparison, to melt 1 kg of 0 deg C ice needs the same heat as warming up 1 kg of water from 0 to 80 deg C. // NaCl dissolving in water is thermally almost neutral. // Dissolving of CaCl2 is exothermic, of CaCl2 . 6 H2O is endothermic. – Poutnik Jan 22 '24 at 13:36

3 Answers3

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There always is a dynamic equilibrium — because ice can evaporate, with measurable vapor pressure (between 100 and 600 Pa in the temperature range where saltwater is in equilibrium with ice: "salt" is taken here to mean sodium chloride). When the vapor recondenses onto salt particles, it creates a salt solution. This solution then becomes the liquid phase into which ice melts.

The implication there is a kinetic aspect to the melting of ice by salt. As the temperature drops so does the amount of vapor pressure (note the wide range quoted above) and thus the rate at which the dynamic equilibrium can operate. Thus we find salt losing effectiveness even at temperatures somewhat above the nominal thermodynamic limit. Those of us using Fahrenheit temperatures have a rule that sodium chloride is not really effective below $\pu{0 °F}$ (about $\pu{-18 ^\circ{}C}$).

Buttonwood
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Oscar Lanzi
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  • So the equilibrium between ice and water exists at all temperatures? And no matter how low the temperature gets there will be some water over it you trying to say?? But that is more problematic ,one can ask if that water (the whole ice +water system) is at that low temperature why it is not freezing – Virender Bhardwaj Jan 22 '24 at 12:39
  • Ice is always in equilibrium with water vapor. There is, however, a lower temperature bound for a liquid phase also to be in equilibrium. – Oscar Lanzi Jan 22 '24 at 13:00
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The dynamic equilibrium is between ice and water vapor [perturbed by atmospheric or mechanical pressure]. Addition of a salt or any solute dissolves a small amount of water. The resulting solution has a lower chemical activity and vapor condenses lowering the pressure. The heat of condensation melts some ice and some ice evaporates to attempt to raise the VP. Since the heat of sublimination is greater than the heat of condensation the temperature lowers until the vapor pressure of the solution and the ice are the same. In an adiabatic situation the result is freezing point lowering; in real life on the roads heat is added from the environment so there is no equilibrium. There is a slushy mess until the water and ice evaporate leaving the solute. Since addition of a solute lowers the vapor pressure of both solid and solution adding salt actually increases the time of slushy roads over simple plowing.

[This is happening on the right side of the water-ice-salt phase diagram not the left side. To examine the process without the atmosphere use the water-ice vertical line in the water phase diagram and imagine the results of drastically lowering the water activity.]

jimchmst
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The mechanism of ice melting in the presence of salt can be explained to high school students by using a human analogy (Sorry for the analogy). It may be stated that salt "wants" or "is happy" if it can make a solution in the presence of water. When salt touches ice, it "feels" that it is nearly water. It just needs a little bit of heat to make water and get dissolved. But there is no heat around. So salt feels a way of producing heat when there is no heat available. Each grain, each piece of ice is transformed into two parts : the outer layer and the core. The core (or inner part) gives heat to the outer layer, that melts, dissolves the salt and flows away. Of course the inner core gets colder, and soon becomes the outer layer. The same phenomena is repeated until the remaining inner part becomes too cold to melt, at about -$20$°C. Sorry for the "human" interpretation of the phenomena. It is wrong, I know. It is even contrary to the 1st principle of thermodynamics. Salt is never "happy" or never "wants" something. But in my personal experience, this explanation can be accepted or understood by high school students.

Maurice
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