How does temperature affect solutions?

In summary: I don't know, as an actual chemist, I always felt like I needed to understand the exceptions to those rules.
  • #1
DaveE
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TL;DR Summary
Why can I dissolve more salt in hot water, than cold? Why does more air dissolve in cold water than hot?
- Why can I dissolve more salt in hot water, than cold?
- Why does more air dissolve in cold water than hot?
- What about non-polar solvents? Is it the water or the solute that does this?
- Are there combinations that do the opposite of the normal solid/gas dichotomy?

If you want me to understand you, I'll need a freshman undergraduate level explanation. Everything I learned about chemistry or thermodynamics has left long ago. Those were the classes I struggled to get through without wasting enough effort to really learn much. One thing I learned as an undergraduate was triage; those were the subjects that were mostly left to die. Plus, I have a whole diatribe about how chemistry was taught at my schools that you don't need to hear.
 
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  • #2
I am not sure I can make you understand by my explanation but the key word is Gibb's free energy in thermodynamics and statistical mechanics.

We add salt, NaCl, to lower the temperature of chilly water. It means that we need energy to solve NaCl and this process does not go spontaneously. On the other hand we observe the process of the ions leaving from crystal into water increase entropy of the system. It means the same process goes spontaneously. These plus-minus effects balance at a equilibrium which temperature of the system decides. In lower temperature energy wins more and equilibrium point goes less solvable. In increasing temperature entropy wins more and equilibrium goes more solvable.
 
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  • #3
Lithium carbonate has a solubility of 1.5g/100mL at 0°C and 0.7g/100mL at 100°C.
 
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  • #4
anuttarasammyak said:
I am not sure I can make you understand by my explanation but the key word is Gibb's free energy in thermodynamics and statistical mechanics.

We add salt, NaCl, to lower the temperature of chilly water. It means that we need energy to solve NaCl and this process does not go spontaneously. On the other hand we observe the process of the ions leaving from crystal into water increase entropy of the system. It means the same process goes spontaneously. These plus-minus effects balance at a equilibrium which temperature of the system decides. In lower temperature energy wins more and equilibrium point goes less solvable. In increasing temperature entropy wins more and equilibrium goes more solvable.
Thanks, this is helpful.

So would you say the same for N2 and water. The gas in water is the low entropy side, with weak binding to the water molecules, favored at low temps?
 
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  • #5
Ref to my post #2 I find much better explanation in web text https://chem.libretexts.org/Bookshelves/General_Chemistry/Book:_CLUE_(Cooper_and_Klymkowsky)/6:_Solutions/6.4:_Gibbs_Energy_and_Solubility
I hope it will help you.

As for gas we see cola pours out when we open the lid. We observe higher the pressure, more solvable CO2 gas. Higher the temperature, less solvable CO2.
For increasing pressure under constant temperature, we would fill more gas into the system of fixed volume vessel or shrink the vessel removing heat. So gas in water would increase.
For increasing temperature under constant pressure, we would remove gas from the vessel or inflate the vessel adding heat. So gas in water would also decrease.
I am not succeeding to connect above thought with Gibb's Free Energy. I appreciate advice of colleagues.
 
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  • #6
anuttarasammyak said:
Ref to my post #2 I find much better explanation in web text https://chem.libretexts.org/Bookshelves/General_Chemistry/Book:_CLUE_(Cooper_and_Klymkowsky)/6:_Solutions/6.4:_Gibbs_Energy_and_Solubility
I hope it will help you.

As for gas we see cola pours out when we open the lid. We observe higher the pressure, more solvable CO2 gas. Higher the temperature, less solvable CO2.
For increasing pressure under constant temperature, we would fill more gas into the system of fixed volume vessel or shrink the vessel removing heat. So gas in water would increase.
For increasing temperature under constant pressure, we would remove gas from the vessel or inflate the vessel adding heat. So gas in water would also decrease.
I am not succeeding to connect above thought with Gibb's Free Energy. I appreciate advice of colleagues.
Thanks again. That's a really helpful explanation. I've always had trouble with how you equate ΔS and energy, I've always understood that it takes energy to create order, but I never understood how much, how to use it. I think this is a result of all of the analogies of colored marbles mixing and such. It sort of teaches you about the concept of entropy, but no one ever talks about how you actually measure it, or the underlying processes (energy) require to sort those marbles.

Unfortunately, it also reaffirms why I didn't like or really understand chemistry. It always seemed to me that it was too complex to really understand. It always seemed like there were general rules that applied most of the time, but as soon as you think you have a method in hand, you run across and exception, then a different sort of exception. I always felt like the only way to understand it was to either learn about the quantum mechanics of every sort of chemical bond, or to give up and look up the answer in tables from someone else's experiment. If you really do understand it, it must be very difficult to teach to people like me that aren't happy with memorization in place of understanding.

In the example link, I completely followed their explanation. But, I have no idea why dissolving NaCl is endothermic and CaCl2 is exothermic. Something about the strength of the ionic bonds in these salts compared to the entropy change, I guess. Maybe because the Ca2+ ions have more charge than Na+ so it takes more energy to dissociate? Is the ΔH term essentially the energy of breaking the ionic bonds?
 
  • #7
DaveE said:
I have no idea why dissolving NaCl is endothermic and CaCl2 is exothermic.

There are two processes - one (endothermic) is breaking the ionic bonds in the crystal structure of the salt, the other (exothermic) is solvation of ions by water molecules. Overall effect is a sum of both, so in general it depends on the type of the crystal structure, sizes of ions involved and their charges.
 
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Related to How does temperature affect solutions?

1. How does temperature affect the solubility of a substance?

Temperature can greatly affect the solubility of a substance. In general, as temperature increases, the solubility of a solid solute in a liquid solvent also increases. This is because higher temperature means higher kinetic energy, which leads to more collisions between particles and a faster dissolution rate. However, for gases, the opposite is true - as temperature increases, the solubility of a gas decreases due to the gas molecules having more energy to escape from the liquid solvent.

2. How does temperature affect the rate of a chemical reaction in a solution?

Temperature has a significant impact on the rate of a chemical reaction in a solution. In most cases, an increase in temperature leads to an increase in the rate of reaction. This is because higher temperature means higher kinetic energy, which leads to more frequent and energetic collisions between reactant particles. As a result, more products are formed in a shorter amount of time. However, there are some exceptions where a decrease in temperature may actually increase the rate of reaction, such as in certain enzyme-catalyzed reactions.

3. Does temperature affect the concentration of a solution?

Temperature does not directly affect the concentration of a solution, as concentration is a measure of the amount of solute dissolved in a given amount of solvent. However, temperature does affect the solubility of a substance, which in turn can affect the concentration of a solution. For example, as temperature increases, the solubility of a solid solute in a liquid solvent also increases, which means more solute can dissolve in the same amount of solvent, leading to a higher concentration.

4. How does temperature affect the pH of a solution?

The effect of temperature on the pH of a solution depends on the specific chemical reaction involved. In general, an increase in temperature can lead to an increase in the rate of a reaction, which may affect the concentration of hydrogen ions (H+) and hydroxide ions (OH-) in a solution, and therefore the pH. For example, in the reaction between water and carbon dioxide, an increase in temperature can lead to more carbonic acid being formed, which increases the concentration of H+ ions and lowers the pH of the solution.

5. How does temperature affect the vapor pressure of a solution?

Temperature has a direct impact on the vapor pressure of a solution. As temperature increases, the vapor pressure of a solution also increases. This is because higher temperature means higher kinetic energy, which leads to more molecules escaping from the liquid phase and entering the gas phase. This is especially evident in solutions where the solute is a volatile substance, such as in a solution of water and ethanol. As the temperature increases, more ethanol molecules will evaporate, leading to an increase in vapor pressure.

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