Solubility of Gases In Liquids

Sticha, Frank J.                   Saint Ignatius College Prep
                                   421-5900
                          

Objectives:

Through this demonstration lesson, students are expected to construct 
the relationships of temperature and pressure to the solubility of 
gases in liquids.  Using their discovery, the students are then 
presented with the "ammonia fountain" as a phenomenon to interpret. 

Apparatus/Chemicals needed:

       hot plate or other heat source (2, if possible)
       ice or other cold source
       500 mL thick-wall round-bottom flask (2)
       1000 mL beaker or other see through container (1)
       50 mL graduated cylinder (1)
       one-hole rubber stoppers to fit flask (2) and bottle (1)
       8 inch length glass tubing (2)
       20 inch length rubber tubing (1)
       ring stand with clamp for flask (2)     
       small paper cups (enough to hold contents of 4, 16oz bottles)
       metal tray (2)

       Coca Cola or other carbonated beverage (4 bottles, 16oz size)
       40 mL concentrated hydrochloric acid, HCl
       30 grams ammonium chloride, NH4Cl
       15 grams sodium hydroxide, NaOH
       3 mL phenophthalein indicator
       water

Recommended strategy:

Uncap two of the soda bottles and pour the contents into paper cups.  
Place 1/4 of the cups on a metal tray and set the tray on a hot plate 
(warm setting) about an hour before class begins.  Set one capped 
bottle on the same tray (be careful the hot plate is not too hot!).  
Place the other cups on a metal tray and set this tray in a 
refrigerator or in an ice water bath, again about an hour before class 
begins.  Set one capped bottle in the same refrigerator.  Right before 
class, take the two trays and place all of the warm cups and twice as 
many cold cups on a table in your classroom.  
Offer the students a cup of soda as they enter class.  Students with 
the warm soda will complain that their soda "tastes flat".  Have 
students observe each others' soda, and let those with the warm soda 
come up and get a cold soda for comparison.  As they compare, ask the 
students questions such as those below to lead them to discover the 
relationships of temperature and pressure to the solubility of gases in 
liquids. 
          Why do the liquids taste different? 
          What is different about them? 
          Which soda contains more "bubbles", warm or cold?  (cold)
          What gas makes up the "bubbles"?  (carbon dioxide, CO2)
          Which of the two unopened bottles do you expect to bubble
                 more when opened, the warm of cold one?  (try it,
                 carefully -- warm bubbles more)
Cover the top of the cold soda bottle you just opened, shake it, and
then release your finger (be careful with your aim).
          What is happening?  Can you explain it?
          If "soluble" means "will dissolve", what can you say about 
                 the relationship between temperature and the 
                 solubility of a gas in a liquid?
          What can you say about the relationship between pressure and 
                 the solubility of a gas in a liquid?

In light of their initial discovery, the students should now be able to 
offer plausible interpretations regarding the ammonia fountain 
experiment. 

Place the ammonium chloride, sodium chloride and the 10ml water in the 
thick-wall 500ml flask and cover it with a 1-hole rubber stopper that 
is connected by tubing to a 1000ml beaker containing 960ml water, 40ml 
concentrated HCl and 3ml of phenophthalein indicator.  Ammonia gas is 
immediately produced in the flask.  Within time, the gas flushes out 
the air that was present in the flask so that the flask consists mostly 
of ammonia gas.  Since this gas is highly soluble in water, it soon 
dissolves in the water present in the beaker creating a partial vacuum 
in the flask.  This partial vacuum allows the atmospheric pressure to 
push the water contained in the beaker through the tube and into the 
flask, causing a dramatic color change.  The reactions involved are 
listed below: 

          NH4+  +  OH-   -----------  NH3(g)  +   H2O   
     
          NH3(g)  +  H2O   -----------  NH4+  +   OH-

 phenophthalein (colorless)  +  OH-  -------  phenophthalein (pink)

After the students discover the phenomena at work in this presentation, 
(solubility of gas in liquid creating a partial vacuum), students can 
be led through a variety of other investigations/discussions involving 
acid/base chemistry, equilibria, etc. 

References (in no particular order):

ChemMatters.  "What's That Fizz?" by George Poscover.  
The Chemistry of Beverages by Carl Torkko, United Graphics Inc. 1978. 
Chemical Demonstrations:  Vol. 2 by Basaam Shakashiri (this is one of 
     the better references on the ammonia fountain demonstration,
     though almost any chemical demonstration book will contain some
     reference to it).