le Ch‚telier's Principle: Gas Pressure

Carbonated beverages such as soda pop contain a saturated solution of carbon dioxide in water.  The gas is under pressure, and in equilibrium with the water in the drink through the following chemical reaction:

H2O (l)  + CO2 (g)  H2CO3 (aq)

H2CO3 (aq) or carbonic acid is a weak acid.

There are two reasons why we enjoy carbonated beverages: the bubbles of CO2 that are released in our mouth when we drink it cause a "tingling" sensation that we enjoy, and the weak acid makes the drink slightly sour, which is a taste that humans like.  However, we don't like drinking them when they go flat, which they will do if we leave the top off the container.

You will also notice that a drink will go flat (lose its carbonation) if you partially empty a resealable bottle.  Why is that, since the gas cannot escape from a sealed bottle?  When you partially empty a bottle, you increase the headspace (the volume above the liquid).  This headpace is filled with CO2 (g).  When the headspace is large, the volume available for the gas is larger.  This changes the gas pressure.

If you increase the volume of the CO2 gas, then its pressure will:
stay the same

Changing the volume will change the gas pressure in the container and the result can be predicted from le Ch‚telier's principle.  We can see neither the CO2 gas, nor the H2CO3 (aq).   However, we can detect the presence of the carbonic acid by using an acid-base indicator such as methyl red which changes color from red in weak acids (pH 5) to yellow when neutral.  This will allow us to determine the effect of changing the pressure on the equilibrium.

Considering the reaction equation with some methyl red indicator added:

H2O (l)  + CO2 (g)  H2CO3 (aq)

Since H2CO3 is an acid, we can predict that decreasing the pressure will make the solution
more acidic and thus more red.
less acidic and thus more yellow.
stay the same acidity.


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Wear safety goggles while doing this lab.

1. If your teacher has not already done so, add about 1 mL of methyl red acid-base indicator dye to the carbonated water, and note the color.  Stir very gently to mix the dye (you don't want to lose the carbonation at this point).
2. Measure 100 mL of the carbonated water, add it to each of the bottles, and seal them.
3. Shake the bottles vigorously for about 30 seconds.  This will increase the surface area in contact between the gas and liquid phases, so equilibrium is achieved more rapidly.
4. Observe the color of each solution.
5. Dispose of the solutions down the sink.


Observe the color of each bottle, and explain the results in terms of le Ch‚telier's principle.

1.Which bottle has the highest CO2 (g) pressure?
2 L      500 mL     100 mL

2. Methyl red indicator is pale yellow in neutral or slightly basic solutions.  It is red in acidic solutions.  As the acidity increases, the color will become more red, and less yellow.  The solution with the highest acidity, and thus the most H2CO3 (aq) was in which bottle?
2 L      500 mL     100 mL

3. The color change in this reaction is what you would expect from the prediction of le Ch‚telier's principle.
True      False

4. A device known as the "Fizzkeeper" has been marketed.  It is an air pump that fits the mouth of a carbonated beverage bottle.  It claims to work by letting you pump air into the bottle, therefore increasing the pressure and so keeping the beverage from going flat.  Will it work?
Yes      No