The ammonia fountain
Demonstrations to capture the student's imagination by Adrian Guy of Blundell's School. In this issue: The ammonia fountain
The ammonia fountain is a classic experiment used to demonstrate the solubility of ammonia in cold water and the alkaline nature of the ammonia solution formed. The following method is a variation of the norm in which I initiate the reaction using body heat instead of water.
- 1000 ml volumetric flask with bung;
- 500 ml conical flask with bung;
- glass trough; glass tubing;
- phenolphthalein solution;
- 1 M sulfuric acid;
- concentrated ammonia solution - 35 per cent ('880');
- retort stand and clamp;
- Bunsen burner, heat-resistant mat, tripod and gauze.
Working in a fume cupboard, add 100 ml of concentrated ammonia to a 500 ml conical flask and insert a bung fitted with 20 cm of glass tubing. Using a retort stand, clamp the 1000 ml volumetric flask above the conical flask with the glass tubing from the conical flask reaching inside the volumetric. Heat the conical flask on a tripod and gauze so that ammonia gas produced is collected by the upwards displacement of air. Now heat the ammonia with a roaring flame until it appears to boil, owing to the dissolution of ammonia gas, and continue heating for ca 30 s (see photo (a)).
Remove the Bunsen, tripod, gauze and conical flask to leave the inverted volumetric of ammonia gas - ammonia is less dense than air so it will not escape in any significant quantities. Insert a bung - fitted with ca 45 cm length of glass tubing which has been reduced to a jet at one end (the internal end) - into the volumetric flask. Clamp the inverted volumetric flask above a glass trough filled with water to which 10 ml of 1 M sulfuric acid and 10 ml of phenolphthalein solution have been added. Ensure the end of the glass tube from the volumetric is submerged in the water trough.
(a)-(d): A pink fountain forms as phenolphthalein meets ammonia gas
Now, place both hands around the volumetric to warm the flask and contents. As the ammonia gas expands, bubbles can be seen emerging in the trough, and eventually the clear solution in the trough changes to a pink colour. When the bubbling stops, remove your hands and wait. The ammonia cools, the pressure drops, and very slowly the pink phenolphthalein solution rises up the tube. When the solution reaches the ammonia gas, the ammonia dissolves much more rapidly, 'sucking' more water up and producing a spectacular pink ammonia fountain (photos (b)-(d)).
Ensure all apparatus is dry before use. After heating the ammonia solution to produce the ammonia gas, note how cool the flask and contents are. The dissolution of ammonia is an endothermic process.
Ensure all bungs and glass tubing are air-tight because the slightest leak will cause problems - do not use cork for this reason.
Sulfuric acid (1 M, 10 ml) is added to the water in the trough to prevent bubbles of ammonia turning the phenolphthalein solution purple at this stage.
I use this demonstration with my Year 8 students when we discuss the properties of gases, but there are other uses, including cross-curricula links:
- the solubility of gases is greatly reduced at higher temperatures - relate to oxygen levels in fresh water ponds depleting when hot;
- increasing temperature increases the pressure and in this case the volume - relate to particle theory in physics;
- dissolving the ammonia gas in solution reduces the pressure inside the volumetric flask so that the external air pressure pushes the solution up into the flask - the idea of a pressure difference rather than using the word 'suck' is a better explanation of the movement of the phenolphthalein solution;
- ammonia is a weak base and forms an alkaline solution when dissolved in water - relate to definitions of bases and alkalis;
- ammonia is less dense than air and soluble - relate to method of collection, ie 'upwards displacement of air';
- the rapid dissolution of gases causes cooling - relate to cooling effect of perspiration or the physics of fridges;
- the use of phenolphthalein as an acid/base indicator.
Do all parts of the demonstration in a fume cupboard, wearing goggles and gloves. Ammonia is toxic by inhalation. Sulfuric acid (1 M) is an irritant. Concentrated ammonia solution (35 per cent) is corrosive and can burn the skin; it also has a toxic vapour. Phenolphthalein solution is highly flammable. Dispose of all products down the sink in the fume cupboard using plenty of cold water.