The Ammonia Fountain
Purpose: To show how pressure differences affect a system until equilibrium is reached.
Can also be used to discuss solubility trends (i.e. likes dissolve likes, etc.).
Grade Level and Topic: - 12U chemistry: chemical systems and equilibrium
- 11U, 12C chemistry: solutions and solubility
- any grade when pressure is the topic
Theory: When the small amount of water is squirted into the flask containing ammonia
gas, the ammonia dissolves in the water. This reduces the pressure of the top flask,
drawing the water up the tube from the bottom flask. As more and more water travels
into the top ammonia flask, more and more of the ammonia gas dissolves creating a
larger pressure difference, thereby speeding up the flow of the water. Eventually, the
water becomes saturated and the remaining gas compresses, resulting in the pressure
reaching equilibrium, and the flow of water will stop. Note: the phenolphthalein is
added since the ammonia creates ammonium hydroxide in solution (basic) turning the top
water pink and making for a more spectacular display.
Materials: - round bottom flask (x2, 500mL)
- two hole rubber stopper (x2)
- rigid hollow glass or plastic tubing (~18-24 inches)
- eye dropper
- ammonia gas
- water (distilled preferred)
- phenolphthalein indicator
- retort stand and clamp
Procedure: If ammonia gas is not available it can be prepared easily. Fill a large test
tube about 1/8 with ammonium hydroxide. Place a one hole rubber stopper on the test
tube with a flexible piece of tubing attached. GENTLY boil the ammonium hydroxide
over a flame collecting the gas into the round bottom flasks (insert other end of tube into
flask and hold flask upside down. Note- ammonia is lighter than air and will force the air
from the flask. Stopper the flask when enough ammonia collected (30 sec - 1 min)
Apparatus – Fill a flask with water and add a few drops of indicator. Stick one end of the
tubing into a stopper and place it on the water flask (adjust tubing so it almost reaches
bottom of flask). Insert the other end of the tubing into the second stopper, along with
the eye dropper containing water. Have a retort stand handy to secure the top flask.
When ready for demonstration, unstopper the flask containing ammonia gas, and quickly
place it on top of the other flask (on the stopper, connecting the two flasks). After the
introduction, simply squeeze the eye dropper to expel the water into the top flask and
start the fountain.
Hints for success: Always have two or three flasks of ammonia gas handy, in case it
doesn’t work the first time. Depending on the level of the class, the follow up discussion
can be in depth or simplified.
Safety Considerations: Be extremely careful with the ammonia gas preparation, as the
ammonium hydroxide is caustic and will boil violently if heated too strongly or quickly.
If at all possible use a fume hood because of the powerful smell of ammonia. Ensure that
the stoppers are in good condition and will not leak the final basic solution. Be careful
when disassembling the apparatus, as the ammonia-water solution is basic. Do not leave
the apparatus assembled after the completion of the fountain, at least empty the two
flasks and leave them in the sink.
References: Associate teacher.