Solubility of Solutes and Aqueous Solutions

When we talk about the mixing of two or more substances together in solution we must consider solubility. Simply defined, it is a measure of how much solute will dissolve into the solvent. Not all substances will dissolve in all solvents. Understanding solubility properties will provide a basis for understanding the golden rule of solubility...Like dissolves like!

FACTORS AFFECTING SOLUBILITY
	Temperature will affect solubility. If the solution process absorbs energy then the solubility will be INCREASED as the temperature is increased. If the solution process releases energy then the solubility will DECREASE with increasing temperature.
	Molecular Size will affect the solubility. The larger the molecule or the higher its molecular weight the less soluble the substance will be. Larger molecules are more difficult to surround with solvent molecules in order to solvate the substance. In the case of organic compounds the amount of carbon "BRANCHING" will increase the solubility since more branching will reduce the size (or volume) of the molecule and make it easier to solvate the molecules with solvent.
	The Polarity of the solute and solvent molecules will affect the solubility. Generally polar solute molecules will dissolve in polar solvents and non-polar solute molecules will dissolve in non-polar solvents. The polar solute molecules have a positive and a negative end to the molecule. If the solvent molecule is also polar, then positive ends of solvent molecules will attract negative ends of solute molecules. This is a type of intermolecular force known as dipole-dipole interaction. All molecules also have a type of intermolecular force much weaker than the other forces called London Dispersion forces where the positive nuclei of the atoms of the solute molecule will attract the negative electrons of the atoms of a solvent molecule. This gives the non-polar solvent a chance to solvate the solute molecules.

DETERMINING THE PROPERTIES OF SOLUTES FOR PROBLEM SOLVING
Three intermolecular forces determine what is soluble in water. Before selecting an answer for a problem, you must identify which of these are possessed by the solute.
Dispersive Forces are net attractive forces generated by temporary charge imbalances among molecules. Every molecule has dispersive forces.
Dipolar Forces describe the interaction between molecules with dipole moments. To ascertain which molecules have a dipole moment, you must be able to label it as either polar or non-polar. The first step is to draw the correct Lewis Structure. (For information on drawing Lewis Structures, click here) Next, you must draw dipole moments based on electronegativity differences between atoms. If the dipoles do not cancel each other out the molecule has a dipole moment and possesses dipolar forces. (For Help In Determining The Polarity Of A Molecule Click Here).
Hydrogen Bonds are special bonds between an atom of hydrogen and either flourine, oxygen, or nitrogen. Again, the Lewis Structure must be drawn to ensure the hydrogen is bonded directly to one of these atoms. Water possesses all of these intermolecular forces. For a substance to dissolve in water, it must possess them as well. Also, when dealing with substances other than water, it must have the same set of forces to be soluable. Remember, like dissolves like!

SAMPLE PROBLEMS
Is carbon tetrachloride, CCl4, soluble in water?	Yes	No
Is dimethyl ether, CH3OCH3, soluble in water?	Yes	No
Is ethanol, CH3CH2OH, soluble in water?	Yes	No
Is hydrogen fluoride, HF, soluble in water?	Yes	No
Is propane, CH3CH2CH3, soluble in water?	Yes	No
Keeping in mind that "Like Dissolves Like", which of the following compounds would be the most soluble in a nonpolar solvent such as carbon tetrachloride, CCl4?
A) H2O B) CH3OH C) CH3CH2CH2CH2CH2OH D) CH3CH2CH2OH

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Last Revised : Tuesday, December 9, 1997

This page was created by Seth Martin as a Chemistry 1201/2 class project.