Dissolution of salts and gases
It's a practical observation in chemistry that, usually, the solubility of salts increase with temperature, while the solubility of gases decrease with temperature, T (excepetions does exist). Although it may look like a simple pratical observation, there's a thermodynamical explanation for this temperature dependence. What is it?
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1 solution
The fundamental equation in chemical thermodynamics is the Gibbs Energy equation, dG=dH-TdS. Since a spontaneous chemical process must have dG<0, the equation tell us that all chemical process with an increase in entropy will become more favourable as we increase the temperature (cause dG becomes more negative). Salts have a rigid and defined cristalline structure, and thus its dissolution in water increases entropy (we passed from solid to solution). Gases, on the other hand, are a bunch of free molecules, poorly interacting with one another. When we dissolve it, solvation occurs, decreasing the entropy (we passed from gas to solution). Interactions between gas molecules, salt ions and water molecules contributes too, but it will contribute to the value of the enthalpy, dH. Since enthalpy is pratically temperature independent, it couldn't explain the phenomena.