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Name: Nadia
Status: student
Age: 14
Location: N/A
Country: N/A
Date: 12/10/2002


Question:
Why do solubilities of solids tend to increase with temperature while those of gasses tend to decrease?


Replies:
Here is a good way to think about it. The solubility of a substance in some solvent is its affinity for the liquid phase of the solvent, relative to its other options. A solid substance's principal other option is the solid phase; a gas's other option is the gas phase.

When temperature increases, a solid phase becomes less attractive than a liquid or gas phase, and a liquid phase becomes less attractive than a gas phase. This is because increased temperature means increased molecular motion, shaking apart the rigid connections in a solid and separating molecules in a liquid.

(The next three paragraphs are extra credit. You can just skip ahead to the end for the answer, and then come back for a more detailed explanation.)

When a solid is in contact with a liquid solvent, some of the solid's molecules will detach and mix into the liquid. As more and more of the "solute" molecules are present in the solvent, some of them will re-attach to the solid when they bump into it. When the limit of solubility is reached, the number of molecules detaching from the solid surface in any given period of time is equal to the number of molecules reattaching to it. If the temperature increases, the molecules move faster, making molecules on the surface of the solid more likely to detach and molecules already in solution less likely to attach to the solid. The limit of solubility will only be reached when the concentration of solute molecules in solution is higher.

Similarly, when a gas is in contact with a liquid solvent, some of the gas's molecules will dissolve in the liquid. Again, as more and more of the gas molecules are present in the solvent, some of them will re-enter the gas phase when they diffuse back to the gas/liquid contact region. The limit of solubility is reached when the number of gas molecules diffusing into the liquid in any given time period equals the number diffusing back out. So far, this is just like dissolving a solid in a liquid solvent, right?

The difference comes when the temperature changes. When the temperature increases and the molecules move faster, molecules from the liquid phase are more likely to escape when they contact the gas phase. By the same token, molecules from the gas phase are more likely to bounce off the liquid and stay in the gas phase instead of jostling for position in the more crowded liquid solution.

In both cases, the effect of higher temperature is to raise the chances that the solute molecules will be in the least restrictive phase. When the choice is between a solid and a liquid, the least restrictive phase is the liquid. When the choice is between a liquid and a gas, the least restrictive phase is the gas.

Richard E. Barrans Jr., Ph.D.
PG Research Foundation, Darien, Illinois


Solubility is a complicated process, and not well understood. That is, it is difficult to predict the solubility of a specific substance in a specified solvent, or its dependence upon temperature. Some generalizations are possible, however. 1. If heat is absorbed when one mole of solid is dissolved in an "almost saturated" solution of the substance (this is usually determined indirectly), then the solubility increases with temperature. Most salts have a positive temperature solubility coefficient, but there are many exceptions. One "problem" is that the solid phase that is in equilibrium with a solution may change with temperature. An example is sodium sulfate. Below 32.4C the equilibrium solid phase is Na2SO4(10H2O) and the solubility below that temperature increases very sensitively with temperature. Above 32.4C the equilibrium solid phase is Na2SO4 and the solubility decreases with increasing temperature. The solubility of KNO3 increases from about 15 gm/100 gm H2O at 0C. to about 180 gm / 100 gm H2O at 80C. In contrast, common table salt, NaCl, has a solubility of about 38 gm/100 gm H2O, and this does not change between 0C and 100C. The solubility of most non-reactive gases is between 25 and 50 ml gas/liter water, but gases that react with water, like CO2, H2S, SO2, and NH3 are a thousand times greater.

Vince Calder



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