Why does adding solute particles decrease the freezing point?

In summary, the addition of solute particles to a solvent leads to an increase in boiling point due to a decrease in vapor pressure. However, when it comes to freezing point, the addition of solute particles leads to a decrease in freezing point as the liquid and solid states of the solvent exist in equilibrium and have a vapor pressure. The decrease in freezing point can also be explained by the presence of two different phases with different saturated vapor pressures, causing one phase to evaporate while the other condensates, leading to a depression in the freezing point. This phenomenon is not affected by the nature of the solute particles, as it is a result of the concentration of the solution. The concept of phase is more general than the state of matter,
  • #106
Borek said:
First, the difference (understood as Vvap-Vsub) is getting lower and lower when we move to higher temperatures, see the first plot.
Yeah, I can see that but I was talking about the absolute value of the difference in vapor pressures.
Borek said:
Absolute value for the 1 molal solution for which the calculations were done has minimum at around 271.3 K (normal melting point minus cryoscopic constant). (Minimum value should lie on zero, just the plot is inaccurate).
Okay, I didn't notice that, I thought the minimum was at 273 K in the plot.
Borek said:
So, if we move from 273 down, the difference gets lower till it hits zero, it doesn't grow, at least initially.
Yes, I agree it doesn't grow initially.
Borek said:
what really matters for us here, is the absolute value of the difference (second plot), as as long as the pressures are not equal system is not at the equilibrium, and it is the equilibrium that we are interested in (as this is where the new melting point is)
Well, I've been talking about the absolute value for all this time. The absolute value of the difference in vapor pressures is increasing whether we go to lower/higher temperatures (except when it decreases initially when we reduce temperature from 273 K to 271.3 K, as you said).
 
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  • #107
Mr Real said:
Okay, I didn't notice that, I thought the minimum was at 273 K in the plot.

How could it me minimum at 273, when the difference of zero makes it possible for both phases - ice and solution - to coexist? And we are talking about the freezing point depression, so the zero (coexistence of liquid and solid) must be at some other, lower temperature?

Mr Real said:
except when it decreases initially when we reduce temperature from 273 K to 271.3 K

So you have missed the only part of the plot that matters for the whole discussion about the freezing point depression. Again, you are concentrating on things that are irrelevant to the discussion making me wonder if you understood the important part :frown:
 
  • #108
Borek said:
So you have missed the only part of the plot that matters for the whole discussion about the freezing point depression.
Sorry, but I didn't know that and it was kind of a minute detail in the graph.
Borek said:
Again, you are concentrating on things that are irrelevant to the discussion
You asked me to follow the plot, so I did and listed all my observations.
Borek said:
making me wonder if you understood the important part :frown:
I have understood the important part.
 
  • #109
Borek, can you please answer my doubt? Was I right when I said in post #87 that the 2 vapor pressures can become equal only if the decrease in vapor pressure is less for the solution (brine in our case) than for the solid state of the solvent (ice in our case)? And if I was right then does this hold for every solvent?
Thanks
Mr R
 
  • #110
Mr. Real - it seems to me that you have some kind of issue understanding what @Borek has done for you. It is a great explanation! If you cannot focus clearly enough to come to an understanding of the material, then some of us with less patience than Borek, like me, will simply close the thread.

Thanks in advance for your help. We want you to learn, not to parasitize someone's time and effort when you do not appear to make the effort yourself.
 
  • #111
Mr Real said:
Borek, can you please answer my doubt? Was I right when I said in post #87 that the 2 vapor pressures can become equal only if the decrease in vapor pressure is less for the solution (brine in our case) than for the solid state of the solvent (ice in our case)?
Strictly speaking, were it otherwise then the vapour pressures could become equal on heating rather than cooling.
Mr Real said:
And if I was right then does this hold for every solvent?
Well, helium (both isotopes) is unique for having the property to freeze on heating.
 
  • #112
jim mcnamara said:
Mr. Real - it seems to me that you have some kind of issue understanding what @Borek has done for you. It is a great explanation! If you cannot focus clearly enough to come to an understanding of the material, then some of us with less patience than Borek, like me, will simply close the thread.

Thanks in advance for your help. We want you to learn, not to parasitize someone's time and effort when you do not appear to make the effort yourself.
Sorry, Borek if I came across that way. I am not deliberately trying to waste your time by behaving stupidly, I am genuinely not clear with the things I asked.
jim mcnamara, I do recognise Borek's helping attitude and patience in trying to clear my doubts, and because of that most of my doubts have been cleared. Some doubts(which I mentioned in the last post) remain and I've really tried to answer this myself and in that regard have asked Borek's opinion on whether I'm right or not. I wholeheartedly agree his explanation has been really great, with analogies I've not even found in my textbook!
Thanks so much :smile: !
(and sorry
@Borek :frown:)
 
  • #113
Mr Real said:
Was I right when I said in post #87 that the 2 vapor pressures can become equal only if the decrease in vapor pressure is less for the solution (brine in our case) than for the solid state of the solvent (ice in our case)?

Isn't it clear from the plot? And as snorkack wrote, it is quite easy to realize it can't be other way, as it won't lead to the existence of the melting point at all.

And if I was right then does this hold for every solvent?

https://www.physicsforums.com/threads/depression-of-freezing-point.915216/page-5#post-5776336

(and don't worry about exotic cases, they definitely make nature more interesting, but they are rare enough to be ignored at the level you are now)
 
  • #114
Borek said:
And as snorkack wrote, it is quite easy to realize it can't be other way, as it won't lead to the existence of the melting point at all.

(and don't worry about exotic cases, they definitely make nature more interesting, but they are rare enough to be ignored at the level you are now)
Or more precisely, being otherwise would lead to the exotic case of melting on cooling rather than heating.
 
  • #115
  • #116
Thread came to a good ending. Thank you.
 

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