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Name: Karina
Status: student
Grade: 9-12
Location: CO
Country: USA
Date: Winter 2009-2010


Question:
Hi... I am Karina and I am wondering how might you know whether a substance would dissolve in water without doing an actual experiment? I know that it has to be soluble and polar but how do you know if they are soluble and polar by just looking at them?



Replies:
Karina,

If you know the name of the substance (or its constituents) you can look up the formula, structure and solubilities in a current edition of the "Handbook of Chemistry and Physics." If you do not have access to a handbook in your class room, check out the library. Otherwise, Google the substance.

Warren Young


Hi Karina,

We can get a reasonable estimate on the polarity of a molecule by using what we know about likely shapes of molecules. Using such estimates and comparing these estimates to actual solubility data from experiments, we find that there is a trend with regard to the solubility of substances into water. We find that the more polar a molecule is, the more likely it would be soluble in water.

For example, a molecule like NH3 is known to have N in the middle with 3 H's attached to it. We could arrange the H's so that they form a triangle around the N, but we also know that N has what is known as a "lone-pair" that is also attached, like the H, to the N. So then the triangle of H's have to bend downward in order for there to be space to put the lone-pair. So the shape we get is like a pyramid with the N at the top and the 3 H's forming the base of the pyramid. This process is a combination of knowing Lewis Structures and electron domain (or VSEPR) rules.

At this point we apply what we know about bond polarity. We know that N is more electronegative than H, that is, if N were attached to H the N would pull electrons toward itself away from the H. So NH3 ends up with a lot more negative electrons near the N and the H's would be more positive because of the lack of electrons. This process is known as bond polarity and molecular polarity analysis.

Using all of these then, we know that NH3 is polar. - and we can expect that it would be soluble in an equally polar solvent like water.

Greg (Roberto Gregorius)
Canisius College


Karina,

If you are looking at a purely covalent molecule then you should consider two main factors: the molecular weight of the compound and the number of hydrogen bonding sites it has. These are the principles used by the pharmaceutical industry to determine the likelihood of a potential drug being soluble in the blood. Very large molecules tend to be insoluble due to strong intermolecular forces between solute molecules. Large numbers of H-bonding sites tend to increase the intermolecular forces between the solute particles and the solvent, so making dissolving more enthalpically favored. Most small molecular compounds with any sort of polarity or H-bonding capability tend to be soluble in water, to some extent.

A couple of other points to bear in mind:

-simply having an overall molecular dipole (being "polar") will not always give an appreciable solubility in water, for example dichloromethane CH2Cl2 ("polar") has a low solubility in water despite its polarity. H-bonding is a much stronger intermolecular force and has a larger affect on solubility than dipole-dipole interactions.

-all dissolving is a balancing act between the strength of the forces that must be broken (solute-solute and solvent-solvent) vs those made (solute-solvent) along with the change in entropy of the dissolving process at a certain temperature. This tricky balancing act makes it easier to explain solubility that you observe, rather than predict solubility from a molecular structure!!

Best wishes,

Tom Collins


Predicting solubility based solely on molecular structure is tricky. First, you need to define "solubility". Do you mean soluble at all concentrations? Or what other criterion. Common table salt dissolves in water but not at all concentrations. On the other hand, ethanol is miscible with water in all proportions. Predicting solubility is a tricky business.

Vince Calder



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