solubility

[Guest hydroman]

hey im new here and i was just thinking of this question and it puzzles me. Why does thet solubility of most solids increase when the temp. increases but the solubility of gases increases at a lower temp?

[akcapr]

gases are more soluable at low temps because the molecules are moving slower, and thus gives the gas less chance of escaping the liquid. Solids are more soluable at high temps becasue there is more energy to break apart the bonds in the solute. Hot solvents can hold more solute than cooler solvent because there is more room for the particles (i think..)

[Guest hydroman]

Hot solvents can hold more solute than cooler solvent because there is more room for the particles (i think..)

 

only when its hot though right? because when it cools down it will precp out unless it has the ability to become super saturated?

[akcapr]

no- solutes will dissolve in cold water, but slower, becasue the molecules arent movign as fast and equilibrium isnt reached fast. And yes, if a hot solution is saturated, when it cools, the stuff will precipetate out cuz there wont be as mch room.

[H2SO4]

I always thought of dissolving as the molecules of (lets just say NaCl) go in between the molecules of (lets just say, H2O). I always thought if the H2O is hotter, theres more space in between the molecules, thus more NaCl will fit in between. When it cools, there pushed out of there spaces. Please correct me if im wrong, i think it is more complicated. If it is wrong, it is a good, simle way to look at it.

[Mendelejev]

I don't think so. I've always learned that something (NaCl for example) will desolve a lot better in hot water, becouse the water molecules are moving more heavily and faster. But ... (think, think), in fact, maybe you are correct. Becouse my theory could explain the fact that the salt will be disolved a lot faster. But it doesn't explain why there can disolve more of the salt !!?

[H2SO4]

ya, i agree with you why things dissolve faster. That makes sense.

[chemi]

my theory could explain the fact that the salt will be disolved a lot faster. But it doesn't explain why there can disolve more of the salt !!?

 

when a solid is put into the water, it becomes hydrated(surrounded by water molecules), so the higher the temperature, the more kinetic energy the water molecules have to break apart more bonds in the solute--> solvents with higher temperature dissolve more solute because it has the energy to break more bonds of the solute (in less amount of time also).

 

The polarity of the solid also matters, the more polar the solute, the better it'll dissolve in water since water is polar (like dissolves like). so a hot water would dissolve more polar solutes than non-polar solutes.

[akcapr]

yes but why in our opinion does hot water hold more solute?

[Yggdrasil]

The difference in solubility paterns can be explained using thermodynamics. First, here is a simple explanation:

 

In general, processes tend to go toward increasing entropy (disorder) at high temperatures even at the expense of creating a high energy (unstable) system. The opposite is true at low temperatures; processes tend to go toward decreasing enthalpy (potential energy/stability) even at the expense of becomming more ordered.

 

For a solid disolving in water, entropy is increasing since the water breaks up the highly ordered crystal latice of the solid. However, breaking the intramolecular bonds holding the solid together increases the potential energy of the aqueous (disolved) solution. Therefore at high temperatures, the aqueous (more disordered) state is more favorable while the solid (lower energy) state is favored more at low temperatures.

 

The opposite is true of gasses. Gasses are much more disordered in the gas state than in the aqueous state. However, they are in a lower energy state in the aqueous phase. So in order to make the aqueous state more favorable, one should decrease the temperature in order to keep it in the lower energy aqueous phase.

 

In mathematical terms, this principle is reflected in the equation:

[math]\Delta G = \Delta H -T \Delta S[/math]

 

As the change in free energy [math]\Delta G[/math] decreases, a process becomes more favorable. Therefore, for a positive change in entropy,([math]\Delta S > 0[/math]), the process becomes more favorable as temperature increases. For a negative change in entropy, ([math]\Delta S < 0[/math]), the process becomes more favorable as temperature decreases.