Solutions Intermolecular Forces Compounds  Elmhurst College
Review of Solutions: Solubility of Polar Solutes Temperature & Pressure  Chemistry Department
Solubility of Salts Non-polar Solutes    Virtual ChemBook

Temperature and Pressure Effects on Solubility

Effect of Temperature on Solubility:

The solubility of solutes is dependent on temperature. When a solid dissolves in a liquid, a change in the physical state of the solid analogous to melting takes place. Heat is required to break the bonds holding the molecules in the solid together. At the same time, heat is given off during the formation of new solute -- solvent bonds.

CASE I: Decrease in solubility with temperature:

If the heat given off in the dissolving process is greater than the heat required to break apart the solid, the net dissolving reaction is exothermic (energy given off). The addition of more heat (increases temperature) inhibits the dissolving reaction since excess heat is already being produced by the reaction. This situation is not very common where an increase in temperature produces a decrease in solubility.

CASE II: Increase in solubility with temperature:

If the heat given off in the dissolving reaction is less than the heat required to break apart the solid, the net dissolving reaction is endothermic (energy required). The addition of more heat facilitates the dissolving reaction by providing energy to break bonds in the solid. This is the most common situation where an increase in temperature produces an increase in solubility for solids.

The use of first-aid instant cold packs is an application of this solubility principle. A salt such as ammonium nitrate is dissolved in water after a sharp blow breaks the containers for each. The dissolving reaction is endothermic - requires heat. Therefore the heat is drawn from the surroundings, the pack feels cold.

Solubility of Gases vs. Temperature:

The variation of solubility for a gas with temperature can be determined by examining the graphic on the left.

As the temperature increases, the solubility of a gas decrease as shown by the downward trend in the graph .

More gas is present in a solution with a lower temperature compared to a solution with a higher temperature.

The reason for this gas solubility relationship with temperature is very similar to the reason that vapor pressure increases with temperature. Increased temperature causes an increase in kinetic energy. The higher kinetic energy causes more motion in molecules which break intermolecular bonds and escape from solution.

This gas solubility relationship can be remembered if you think about what happens to a "soda pop" as it stands around for awhile at room temperature. The taste is very "flat" since more of the "tangy" carbon dioxide bubbles have escaped. Boiled water also tastes "flat" because all of the oxygen gas has been removed by heating.


Quiz: Thermal pollution is merely waste heat that has been transferred to water or air. How is the concentration of dissolved oxygen in water be effected by thermal pollution?  

Gas Pressure and Solubility:

Liquids and solids exhibit practically no change of solubility with changes in pressure. Gases as might be expected, increase in solubility with an increase in pressure. Henry's Law states that: The solubility of a gas in a liquid is directly proportional to the pressure of that gas above the surface of the solution.

If the pressure is increased, the gas molecules are "forced" into the
solution since this will best relieve the pressure that has been applied.
The number of gas molecules is decreased. The number of gas molecules
dissolved in solution has increased as shown in the graphic on the left.

Carbonated beverages provide the best example of this phenomena. All carbonated beverages are bottled under pressure to increase the carbon dioxide dissolved in solution. When the bottle is opened, the pressure above the solution decreases. As a result, the solution effervesces and some of the carbon dioxide bubbles off.

Quiz: Champagne continues to ferment in the bottle. The fermentation produces CO2. Why is the cork wired on a bottle of champagne?  

Deep sea divers may experience a condition called the "bends" if they do not readjust slowly to the lower pressure at the surface. As a result of breathing compressed air and being subjected to high pressures caused by water depth, the amount of nitrogen dissolved in blood and other tissues increases. If the diver returns to the surface too rapidly, the nitrogen forms bubbles in the blood as it becomes less soluble due to a decrease in pressure. The nitrogen bubbles can cause great pain and possibly death.

To alleviate this problem somewhat, artificial breathing mixtures of oxygen and helium are used. Helium is only one-fifth as soluble in blood as nitrogen. As a result, there is less dissolved gas to form bubbles.

Quiz: If a diver had the "bends", describe how this can be treated.  

Another application of Henry's Law is in the administration of anesthetic gases. If the partial pressure of the anesthetic gas is increased, the anesthetic solubility increases in the blood.

Quiz: The amount of dissolved oxygen in a mountain lake at
10,000 ft and 50oF is __?_ than the amount of dissolved oxygen in a lake near sea level at 50oF.
 
A Coke at room temperature will have __?_ carbon dioxide in the gas space above the liquid than an ice cold bottle.  
 Hyperbaric therapy, which involves exposure to oxygen at higher than atmospheric pressure may be used to treat hypoxia (low oxygen supply in the tissues). Explain how the treatment works.