The ammonia fountain reaction is a popular chemistry demonstration that vividly shows the solubility of gases in water and the basic nature of ammonia (NH₃). It also highlights concepts like gas pressure, solubility, and acid-base reactions. This demonstration creates a striking visual effect—a fountain of water rising into a flask due to the rapid dissolution of ammonia gas in water.

Materials Needed

1. A flask filled with dry ammonia gas (NH₃).
2. Water, typically colored with a few drops of a pH indicator (such as phenolphthalein or universal indicator).
3. Tubing to connect the flask to a water reservoir or container.
4. A stopper with a hole for the tubing to fit tightly into the flask.
5. A reservoir of water (usually in a beaker or a water trough).
6. Phenolphthalein indicator or universal indicator (optional but adds visual effect).
7. Dropper or pipette for initiating the reaction.

Procedure

1. Preparation of the Flask:

   • First, the flask is filled with dry ammonia gas. Ammonia is less dense than air, so it can be introduced into the flask by upward displacement of air (flushing it into the flask from the bottom).
   • The flask is sealed with a stopper connected to a piece of tubing. The other end of the tubing is submerged in the water reservoir.

2. Adding the Indicator:

   • A few drops of phenolphthalein (a pH indicator) are added to the water in the reservoir. This is used to demonstrate the basicity of ammonia, as phenolphthalein turns pink in the presence of a base.

3. Initiating the Reaction:

   • To start the reaction, a small amount of water is squirted or dropped inside the ammonia-filled flask (e.g., by using a pipette). This initial water acts as a solvent for the ammonia gas, dissolving it rapidly.

4. The Fountain Effect:

   • As ammonia gas dissolves in the water inside the flask, it reacts and forms ammonium hydroxide (NH₄OH), increasing the basicity. This reaction significantly lowers the pressure inside the flask.

     $$NH_3(g) + H_2O(l) \rightarrow NH_4OH(aq)$$

   • The rapid dissolution of ammonia reduces the gas volume inside the flask and creates a partial vacuum. Due to this pressure difference between the inside of the flask (low pressure) and the outside atmosphere (higher pressure), water from the reservoir is forced up through the tubing and into the flask, creating the appearance of a fountain.

5. Color Change (Optional but Common):

   • If phenolphthalein or another indicator is present in the water, as the water is drawn up into the flask and mixes with dissolved ammonia, the pH increases. Since ammonia is basic, the water will turn pink (in phenolphthalein) or a different color depending on the indicator used, further enhancing the visual effect.

Key Chemical Concepts

1. Ammonia’s Solubility in Water:

   • Ammonia is highly soluble in water. When it dissolves, it reacts with water to form ammonium hydroxide (NH₄OH), a weak base. This reaction is responsible for the fountain effect because it reduces the pressure inside the flask as ammonia gas is removed from the air.

2. Pressure Difference:

   • The demonstration exploits the concept of gas pressure. As ammonia dissolves in the water, the gas inside the flask rapidly diminishes, creating a low-pressure environment (partial vacuum). The higher pressure outside forces water to rush into the flask to equalize the pressure.

3. Acid-Base Reaction:

   • Ammonia (NH₃) reacts with water to form ammonium hydroxide (NH₄OH), which is basic. This basicity is revealed by the use of a pH indicator like phenolphthalein, which turns pink in the presence of a base.
   $$NH_3(g) + H_2O(l) \rightarrow NH_4^+(aq) + OH^-(aq)$$

4. Visual Effects:

   • The fountain created by the rapid inflow of water and the accompanying color change from the pH indicator create an impressive visual demonstration of both ammonia’s properties and basic gas solubility concepts.

Summary of Steps

1. Fill a flask with dry ammonia gas.
2. Add phenolphthalein to water in a separate reservoir.
3. Initiate the reaction by introducing water into the ammonia flask.
4. Watch as water is sucked into the flask due to the partial vacuum created, forming a fountain, while the color changes to pink or other colors depending on the indicator.

Conclusion

The ammonia fountain reaction is a fantastic way to visualize gas solubility, pressure changes, and basicity. It demonstrates how gases like ammonia dissolve in water, how pressure differences drive fluid movement, and how pH indicators can be used to detect chemical changes in a fun, engaging manner.

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