This activity introduces students to:
that control the solubility of gases in water
- Henry's Law and LeChatelier's Principle
of gases in water
of oxygen solubility
in lake ecosystems
This lesson introduces students to the principles that control the solubility
of gases in
water, and includes applications to the dynamics of oxygen solubility
in lake ecosystems. For chemistry (and physics) students this lesson
provides explanation and animation of the physical principles that control
the solubility of a non-polar gas in water, as well as a practical application
that allows students to observe these attractive forces in action in
a lake ecosystem. For biology students, this lesson demonstrates applicability
of chemistry to changes in environmental conditions and illustrates
the role organisms can play in altering the chemical condition of their
Students will be able to:
and diagram how nonpolar gases enter into solutions in water.
the roles that pressure and temperature play in altering the concentration
of gases in water.
the physical basis for oxygen supersaturation and how it is related
Biology - abiotic
factors in the environment, photosynthesis
Chemistry - chemical equilibrium, solubility, polar and nonpolar materials,
Thermal stratification; oxygen; aquatic respiration
Work through the following discussions with the students. This
could be offered as a demonstration and lecture using computerized
projections, or with students in a computer lab while stopping periodically
how oxygen dissolves and review the animation,
Water, as a polar molecule, induces an accumulation of
electron density(dipole moment) at one end of nonpolar gas molecules such as oxygen
(O2) and carbon dioxide (CO2). In animation, observe a polar water molecule
approaching a nonpolar O2 molecule. The electron cloud of O2 is normally
distributed symmetrically between the bonded O2 atoms. As the negative
end of the H2O molecule approaches the oxygen molecule, the electron
cloud of the O2 moves away to reduce the negative-to-negative repulsion.
As a result, a dipole (a molecule with positive and negative charges
separated by a distance) has been induced in the nonpolar O2 molecule,
causing O2 and H2O to become weakly attracted to each other. This intermolecular
attraction between the oppositely charged poles of nearby molecules
is termed a dipole- induced dipole force. The creation of these forces then explains
the mechanism by which gases dissolve in water.
the effects of pressure on oxygen solubility and review the animation,
Because dipole-induced dipole forces are very weak, the quantity of
nonpolar gases (such as O2) that will dissolve in a given volume of
water is strongly affected by temperature and pressure. Henry's Law
the effect of pressure on the solubility of a gas in a liquid. The law
states that the amount of gas that dissolves in a given volume of solvent
at a specified
temperature (usually 25°C for water) is proportional to the partial pressure
of the gas above the liquid. When gas under pressure contacts
a liquid, the pressure tends to force gas molecules into solution. At
a given pressure the number of gas molecules that will enter into solution
rises until equilibrium is reached. By definition, at equilibrium, the
number of gas molecules entering and leaving the solution is balanced
and the concentration of the gas in solution remains constant. If the
partial pressure of a gas increases, more gas enters into solution.
If partial pressure drops, gas comes out of solution and reaches a new
equilibrium. Illustrate this by opening a can or bottle of soda pop.
At sea level, total atmospheric pressure is 760 mm Hg. This means that
gravity-induced weight of the atmosphere generates enough force to move
sufficient volume of mercury (Hg) 760 mm up a tube. At sea level, approximately
20.8 percent of this air is oxygen gas (O2). The partial
pressure of oxygen at sea level is, therefore, 158 mm Hg (760 mm Hg
x 0.208 = 158.08 mm Hg). Oxygen has a Henry's Law constant of 1.7 x
10-6 molal/mm Hg when dissolved in water at 25°C.
Molality of O2 = (1.7 x 10-6 molal/mm Hg)
(158.08 ) = 2.687 x 10-4 m
From the above value the number of milligrams per liter of oxygen that
dissolve in 25°C water can be calculated.
2.687 x 10-4 moles/kg x 32g/mole x 1000 mg/g = 8.6 mg/liter
the effects of temperature on solubility: Le Chatelier's Principle
the computer animation) and refer to the table oxygen solubility).
Begin with a demonstration. Open two cans of soda pop, one warm and
cold (students can also work in small groups). It is easily observed
gas is released if the can is warm than when it is cold. Before
pour cold water into a glass. During the lesson students can observe
bubbles that formed inside the glass of water that had been poured
warmed up over time. Both of these demonstrations illustrate the
fact that the
temperature of a solvent (recall that water is the "universal solvent")
the solubility of gases.
that dissolve in solvents usually release heat in an exothermic
process as they dissolve .
+ liquid solvent ---> saturated solution + heat
process continues until saturation is reached. At this point gas
will still dissolve, but will be balanced by the gas that leaves
solution. If heat is added to a solution, gas is released in this
solution + heat ---> gas + liquid solvent
equilibrium, as many molecules come out of solution as dissolve
in a given time period.
+ liquid <------> saturated solution + heat
Chatelier's principle states that a change in any of the factors
equilibrium will cause the system to adjust in order to reduce
the effect of the change. Le Chatelier's principle predicts that
the solubility of a gas will increase as a system loses heat,
and will decrease as it gains heat.
how O2 supersaturation is
Due to the effects of hydrostatic pressure on gases in solution, water
can become supersaturated with oxygen and other gases (exceed 100% saturation).
The attractive forces that hold excess oxygen in solution similar to
the dipole-induced dipole forces discussed earlier, but a smaller number
of water molecules are available to induce dipoles in oxygen molecules.
This leads to weaker attraction of oxygen molecules when water is supersaturation
with oxygen. Hypothesize what conditions in lakes could cause supersaturation.
What clues could you look for in a lake that may indicate O2
supersaturation is occurring?
student pairs to complete the student Studying Lesson.
A.J. and C.R. Goldman. 1994. Limnology. McGraw-Hill
R.G. Limnology, 1983. W.B. Saunders Publishing.
J. and K. Purcell. 1991. Chemistry and Chemical Reactivity.
dissolved O2 levels in aquaria with and without plant life.
Graph the O2 concentrations over time. Describe the patterns