Hey it’s professor Dave, let’s discuss the solubility of gases.
We know that solutions involve one or more solutes that have been dissolved in some solvent.
And typically, when we talk about solutions, we are looking at solids that have been dissolved
in a liquid to form a solution, such an aqueous solution of some ionic solid like sodium chloride.
But in actuality, solutions can involve any combination of phases.
One possible type of solution involves gaseous solutes dissolved in liquid solvent.
Different gases, like oxygen, hydrogen, methane, or carbon dioxide, will have greatly varying
solubilities in water, and these solubilities depend on the ability of the different gas
particles to make electrostatic interactions with water molecules in solution, just like
the way ionic solids dissociate in aqueous solution to make ion-dipole interactions.
Take for example, oxygen and helium.
Oxygen is completely nonpolar, and helium is monoatomic, so there are no dipoles present
for interaction in either of these substances.
That means that oxygen and helium are only able to interact with water molecules in the
way of dipole-induced dipole interactions, which are rather weak.
However, a molecule of oxygen is able to make much stronger interactions of these type than
a helium atom can, as it is much larger than a helium atom, and can therefore can exhibit
stronger momentary or induced dipoles.
That’s why oxygen is three times more soluble in water than helium.
But oxygen is still one hundred times less water soluble than chloroform, because chloroform
can participate in actual dipole-dipole interactions with water, which are possible because of
the formal dipole in the molecule.
This is a much stronger interaction, which results in a much greater solubility.
Remember the old saying, “like dissolves like”.
Since the dipole on every water molecule is interacting with the dipole on other water
molecules, substances that can best approximate these types of interactions will be able to
mix the most effectively amongst the water molecules, which means a greater solubility.
For this reason, we should also discuss the identity of the solvent, since that will make
a big difference.
Something like oxygen will be 20 times more soluble in hexane than in water.
This is because hexane molecules are unable to make dipole-dipole interactions like polar
molecules do.
Instead, they make induced dipole interactions just like oxygen molecules, though of a much
greater strength, since it is a larger molecule.
This means that when oxygen dissolves in hexane, it will simply replace some of these dispersion
interactions with other dispersion interactions, which is a relatively small discrepancy in
energy, rather if it is dissolved in water, where it has to disrupt the hydrogen bonds
in pure water without being able to provide an equally strong interaction to replace them.
Solubilities of various substances are also affected by temperature.
We know that increasing the temperature will typically increase the solubility of solids
in aqueous solution.
This is in part because the resulting additional kinetic energy can push the molecules in the
solid apart, but also higher temperatures increase the spontaneity of an entropically
favorable of a process, and dissolution is entropically favorable, since matter is being
dispersed.
However, contrary to this trend, increasing the temperature of an aqueous solution will
actually decrease the solubility of most gases in water.
This is because the gas particles will have more kinetic energy, so they are moving more
rapidly through the solution, and are therefore less capable of interacting with solvent particles.
This also makes them less likely to be contained in solution, as they are more likely to have
enough energy to escape the solution and move into the atmosphere.
As we might imagine, this fact has serious ramifications for our ecosystem, since in
addition to the slow and steady progress of climate change, some industrial processes
increase the temperature of nearby rivers and lakes, which reduces the solubility of
oxygen and thus reduces the concentration of oxygen in these bodies of water.
Aquatic life like fish need oxygen to breathe, so if enough oxygen escapes the water, many
fish can die, which can be catastrophic for that biosystem, since other organisms eat
those fish.
Beyond this, there can be great impact on the economy of the area, if those fish are
depended upon for food or profit.
As we have just discussed the dependence of gas solubility on temperature, let’s also
discuss the solubility of a gas as it relates to the partial pressure of that gas.
Specifically, the greater the partial pressure of a specific gas directly above a liquid
solution, the higher the solubility of that gas will be.
For example, carbonated beverages are prepared by exposing a liquid to a high pressured sample
of carbon dioxide gas.
Because there are so many carbon dioxide molecules that are colliding with the gas-liquid interface,
many more of these molecules will enter the solution, which results in a saturation of
the beverage with carbon dioxide.
This happens during the packaging of any can or bottle of soda.
Then, once the container is opened at home, much of that carbon dioxide will quickly escape,
which results in the familiar hiss we hear upon breaking the vacuum seal.
And now we know a bit about the solubility of gases.