[galpinj]

Hello everyone,

I have a couple questions about solubility, specifically for polar molecules. I am aware that "like dissolves like", but am having trouble understanding the gradient associated with this statement. Although it would make intuitive sense that molecules with larger ionic charges (i.e. Mg(OH)₂ has Mg²⁺ whereas the ions in NaCl both have a +/- 1 charge) are more polar/soluble, the ksp values for many such molecules says otherwise. As such, I can't seem to figure out what determines the degree of solubility; is it polar charge, radius, electronegativity difference?

 Likewise, would a more polar solvent be better equipped to dissolve a polar substance than a weakly polar substance? For example, both acetone and water are polar, but water is considered a much better solvent (in part due to h bonding).What characteristics about the solvent and solute will increase solubility?

Edit: I've read in a couple places that hydrogen bonding makes something more polar; however, this doesn't really make sense to me. I understand how a molecule must be polar to have hydrogen bonding, but I don't see how hydrogen bonding (an inter-molecular force) relates to polarity.

Thank you for the help

[bestsciencetutor]

Yep solubility is confusing mostly because so many factors can affect it.  You seem to be understanding a lot of it actually but it just looks like you have to break down each example you come across and apply all the rules you are talking about.  That can be difficult when you do not know what factors are more important than others.

When you talking about ionic compounds dissolving in non-ionic solvents (well at least partially non-ionic) like water then all the factors you mentioned need to be taken into consideration.  "polar charge, radius, electronegativity difference"

However, when you start to talk about a non-ionic solvent mixed with another non-ionic solute like your water and acetone example thinking about it in terms of the factors you described while correct is confusing.  I found a much better way to think about it is the MOLECULE SIZE versus the INTERMOLECULAR FORCES.  The smaller the molecule the more polar it is (assuming it has any polarity to begin with).  This actually not only helps you describe the difference between different molecules but also give you at least one clue as to why hydrogen bonding is treated different than dipole dipole.  It is partially because the hydrogens are so small.

So if we take your example acetone versus water then the size of acetone is rough 2 times that of water so that is one explanation of why acetone is less polar.  I find this is also the most useful explanation and the one that comes up the most often.

However, again it is not the only factor.  It is actually interesting that you bring up acetone because that example sets off a whole slew of possibilities of different factors that come into play if you change the molecule just slightly.  Some other factors that affect a molecule like acetone would be how many functional groups are attached to it.  If you do not know what functional groups are then just replace that word with oxygen and you will be able to see.  So if I attached another oxygen to acetone (some how).  Then it would become more polar because it has more bonds it can form with the second oxygen.  ANOTHER factor is how the molecule takes up 3D space (shape).  Longer / Larger molecules that are long and skinny (straight chains of carbons) are more soluble than the same number of atoms that forum chains that run in multiple directions.

If this was not complicated enough then you also have to factor in the inverse when you are talking about molecules that are generally non-polar.  The larger and longer the non-polar the molecule the more soluble it will be in a non-polar solvent.  This of course has limits but I am assuming we are talking about thinks all made of carbon and hydrogen and have no oxygen or nitrogen in them.

[Vidya]

Hello everyone,

I have a couple questions about solubility, specifically for polar molecules. I am aware that "like dissolves like", but am having trouble understanding the gradient associated with this statement. Although it would make intuitive sense that molecules with larger ionic charges (i.e. Mg(OH)₂ has Mg²⁺ whereas the ions in NaCl both have a +/- 1 charge) are more polar/soluble, the ksp values for many such molecules says otherwise. As such, I can't seem to figure out what determines the degree of solubility; is it polar charge, radius, electronegativity difference?

 Likewise, would a more polar solvent be better equipped to dissolve a polar substance than a weakly polar substance? For example, both acetone and water are polar, but water is considered a much better solvent (in part due to h bonding).What characteristics about the solvent and solute will increase solubility?

Edit: I've read in a couple places that hydrogen bonding makes something more polar; however, this doesn't really make sense to me. I understand how a molecule must be polar to have hydrogen bonding, but I don't see how hydrogen bonding (an inter-molecular force) relates to polarity.

Thank you for the help

When you dissolve an ionic compound into water then solubility depends on the amount of energy required to break the lattice and energy released during hydration ...This is true in solution making process ..first we need energy to break IMF of the solute and then energy is released when new IMF are formed between solute and solvent.This combined is going to determine if it is soluble or not soluble in the given solvent.

[mjc123]

See for example this thread: http://www.chemicalforums.com/index.php?topic=81896.msg297960#msg297960. There are several factors to account for, and the result can be a non-monotonic trend. And that's just the enthalpy of solution; it doesn't consider the entropy change.
And that's just alkali metal halides. Compounds with higher ionic charges tend to be less soluble, because generally the increase in lattice energy outweighs the increase in solvation energy.