As the temperature increases, why do some solubilities of substances increase while others decrease?

Answer 1

Simply put, when you heat up a substance you give it energy, this energy is dissapated within the substance as vibrations between the atoms.

These vibrations cause the atoms to be not as close together as they originally were before you heated it, therefore other smaller atoms can fit in between, thus soluability is increased.

You can try this with water (using the same volume of water), see how much sugar you can dissolve in cold water (20 celcius) then slightly warmer (50 celsius) and then hot (95 celsius)

You should notice you can dissolve more sugar in the 90 celsius water.

Answer 2

Consider the free energy of dissolution:
dG = - RT ln K, where K is the equilibrium constant.

If the free energy of the dissolution is negative, then
K = exp (-dG/RT) increases with increasing temperature, i.e. more of the substance will dissolve if the temperature increases.

If the free energy of the dissolution is positive, then K = exp (-dG/RT) decreases with increasing temperature , so less of the substance will dissolve as you increase the temperature.

Typically, entropy increases as you dissolve something, so a positive dG is generally possible when the heat of dissolution is endothermic:

dG = dH - TdS (dS >0, so dG is +ve if dH >0 => endothermic)

Answer 3

this depends on whether their reaction with water is exothermic or endothermic. If the reaction is endothermic heating would encourage more substance to dissolve. If the reaction is exothermic, heating would suppress the reaction from happening and substance would precipitate out