Factors that Affect Solubility

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The property of solubility is generally thought of as the condition of molecular miscibility involving matter in liquid form.  However, homogeneous solutions of solids and gasses also exhibit this property. For solubility to take place, a solute (solid liquid or gaseous element or compound) must become fully dissolved in the dissolving medium(solvent).

Sometimes a very fine particulate can appear to be dissolved by a solvent, but in reality is only suspended in it. The pigments used to color paint are a good example of this and the reason paint must be continuously stirred to keep its components from separating. Is such a colloidal suspension, the suspended substance can usually be driven out or separated from the solvent with a device called a centrifuge, but for a true solution the solvent and solute cannot be separated in this way. This attribute of a solution points to the first and primary factor affecting solubility, that the solute an solvent must electrochemically react through formation of ionic, covalent or hydrogen bonding.

There is a class of compounds called lyophpobic colloids which closely mimic the properties of solubility and are difficult to separate. The reason is that the lyophobic colloidal molecules are attracted by opposing static electric charges on the other particles of the liquid. Irregardless, the seemingly homogeneous miscibility of these chemicals form a suspension and not a solution.    

The bonds holding solvent and solute in solution are usually weak ones and reversible or metastable in character. For instance, common glass cleaner is a solution of water(the solvent) with ammonia and ethanol(the solutes) dissolved in it. The way this particular solution works is really pretty amazing because each of its constituent components perform a separate cleaning function, but that is a little off topic. What is pertinent, is that if the glass cleaner is left in a warm place with the top off it will not work very well the next time you try to clean windows with it. The reason is, that as the solution warms up the metastable hydrogen bonds holding the solutes and solvent in their chemical embrace begins to break down and some of the ammonia and ethanol are liberated from the solution. If the glass cleaner solution is heated above 170 °F, pretty much only water would be left. The reason, is that hydrogen bonds cannot exist at temperatures above 170 °F and water boils at a higher temperature(212°F at sea level). This points to the second factor affecting soluble solutions, temperature.

Remarkably, the affect of temperature on solubility of a solution is not the same in all cases. For instance, if you want to dissolve sugar(a solid) in water, the warmer the water the more sugar you can dissolve in it. Conversely, the cooler a liquid solvent is, the greater amount of a gas solute that can be dissolved in it. There are some exceptions to this rule, but generally involve exotic cryogenic circumstances. A good example of a solution that reaches saturation at the lowest temperature at which the solvent remains in liquid state would be that of carbon dioxide dissolved in water. This can be  demonstrated with a can of soda pop  placed in a freezer and cooled to around 32 °F. When the can is opened it will not effervesce or fizz, but allow the can to stand at room temperature until the solution in it warms up 10 degrees or so, and when the top is popped it will fizz with gusto.

Interestingly, if a can of soda pop at room temperature were place in a containment vessel, and the air pressure inside the vessel around the can is increased to two or three atmospheres, when the top on the can is popped opened, just as when it was cooled, the water and carbon dioxide solution will not effervesce. This demonstrates another factor affecting solubility, pressure. The 19th century English Chemist, William Henry, defined a law called Henry's Law with respect to the effect of pressure on the solubility of gases in liquids. It states: “The solubility of gasses in liquids is directly proportional to the pressure of the gas above the liquid.”

Molecular weight is a last  factor affecting solubility to consider in this discussion. As a general rule, the closer in molecular weight a solute is to the solvent,  the greater degree of solubility possible. As we have seen, both carbon dioxide and ammonia have an electrochemical affinity for water, but you can dissolve a whole lot more ammonia in water than you can carbon dioxide. The reason is, water has a molecular weight of 18  and ammonia  17, while carbon dioxide has a molecular weight of 44.

In summary, we can now state that there are four factors which affect solubility, electrochemical affinity, temperature, pressure and molecular weight.

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