To demonstrate the solubility of ammonia in water, the most impactful and widely recognized method is the ammonia fountain demonstration. This classic experiment visually and dramatically illustrates ammonia's exceptionally high solubility, alongside its alkaline nature.
The Ammonia Fountain: A Vivid Demonstration
The ammonia fountain is a striking experiment that showcases how readily ammonia gas dissolves in water. When performed correctly, it creates a spectacular fountain effect inside a flask, making the concept of high solubility instantly understandable.
How it Works: The Science Behind the Spectacle
Ammonia (NH₃) is a polar molecule capable of forming strong hydrogen bonds with water molecules (H₂O). This strong attraction between ammonia and water is the fundamental reason for its very high solubility. When ammonia gas comes into contact with even a small amount of water, it dissolves almost instantaneously, reacting to form ammonium hydroxide (NH₄OH), which is a weak base.
NH₃(g) + H₂O(l) ⇌ NH₄OH(aq)
This rapid dissolution creates a significant drop in pressure within a sealed container, leading to the fountain effect.
Materials for the Demonstration
To set up an ammonia fountain, you will typically need:
- Dry Ammonia Gas: Contained within a round-bottom flask. It's crucial that the ammonia is dry to maximize the effect.
- Stopper with a Glass Jet/Tube: A tightly fitting stopper through which a long, thin glass tube or jet extends, reaching almost to the bottom of the flask. The other end of the tube dips into a beaker of water.
- Beaker of Water: Often containing a few drops of a pH indicator like phenolphthalein (which turns pink/purple in alkaline solutions) or universal indicator.
- Dropper or Syringe: To inject a small amount of water into the flask.
- Safety Goggles and Gloves: Essential for all participants and observers.
Step-by-Step Procedure
- Prepare the Flask: Fill a dry, round-bottom flask completely with dry ammonia gas. This is usually done by displacement of air, as ammonia is less dense than air. Seal the flask with a stopper fitted with the glass jet, ensuring the jet extends almost to the bottom of the flask.
- Submerge the Jet: Invert the flask and submerge the open end of the glass jet into a beaker containing water. The water in the beaker should ideally contain a few drops of a pH indicator.
- Initiate Dissolution: Using a dropper or syringe, inject a small amount (a few milliliters) of water into the flask through the stopper. This water will collect at the bottom of the inverted flask (which is now the top).
- Observe the Fountain: As the injected water dissolves the ammonia gas within the flask, a significant pressure drop occurs inside. The higher atmospheric pressure outside then forces the water from the beaker up through the glass jet, creating a spectacular fountain of water into the flask.
- Note the Color Change: Simultaneously, observe the color of the fountain water. If a pH indicator like phenolphthalein was added, the water will turn vibrant pink or purple as it enters the flask, clearly demonstrating the alkaline nature of the ammonia solution.
What the Demonstration Shows
- High Solubility: The rapid and forceful nature of the fountain clearly demonstrates that ammonia gas is extremely soluble in water. The large volume of water drawn into the flask indicates that a significant amount of ammonia gas has dissolved very quickly.
- Creation of Vacuum: The fountain effect is a direct result of the pressure difference created. As ammonia dissolves, the number of gas molecules in the flask decreases drastically, leading to a partial vacuum.
- Alkaline Nature: The color change of the indicator in the fountain water confirms that ammonia solutions are alkaline (basic). This is because ammonia reacts with water to form hydroxide ions (OH⁻), which increase the pH.
Why Ammonia is So Soluble
Ammonia's exceptional solubility in water is due to:
- Polarity: Ammonia molecules are polar, meaning they have a slight positive charge on the hydrogen atoms and a slight negative charge on the nitrogen atom. Water molecules are also polar.
- Hydrogen Bonding: The strong attraction between the slightly negative nitrogen atom of ammonia and the slightly positive hydrogen atoms of water (and vice versa) leads to the formation of hydrogen bonds. These strong intermolecular forces allow ammonia molecules to readily integrate into the network of water molecules.
Comparative Solubility of Gases in Water
To put ammonia's solubility into perspective, consider this brief comparison:
| Gas | Solubility in Water (vol/vol at 0°C) | Notes |
|---|---|---|
| Ammonia | ~1300 | Highly soluble due to hydrogen bonding and reaction with water |
| Hydrogen Chloride | ~500 | Highly soluble, reacts with water to form hydrochloric acid |
| Carbon Dioxide | ~1.7 | Moderately soluble, reacts slightly to form carbonic acid |
| Oxygen | ~0.049 | Sparingly soluble, essential for aquatic life |
| Nitrogen | ~0.024 | Very sparingly soluble, inert |
Source: General Chemistry Principles
Safety Considerations
Performing the ammonia fountain demonstration requires careful handling of chemicals and proper safety measures:
- Ventilation: Ensure the demonstration is conducted in a well-ventilated area or a fume hood, as ammonia gas has a pungent odor and can be irritating.
- Personal Protective Equipment (PPE): Always wear safety goggles to protect eyes from splashes, and gloves to avoid skin contact.
- Gas Handling: Ammonia gas should be handled with care. If preparing ammonia in situ, be mindful of the reactions involved.
- Glassware: Ensure all glassware is free from cracks or damage.
By executing the ammonia fountain demonstration, students can visually grasp the concept of high gas solubility and understand the chemical principles that drive such a dramatic physical phenomenon. It's a memorable and effective way to teach about intermolecular forces, gas laws, and acid-base chemistry.
