A demonstration illustrating the solubility of ammonia gas in water. A review of relative pressure and acid/base properties is also noted.
An inverted, stoppered one liter round bottom flask is presented to the audience – the audience is asked to speculate as to the contents of the flask. The flask contains the colorless gas ammonia, NH3, not to be confused with solutions of ammonia in water which are commonly present as cleaning solutions commonly found in the grocery store. A rubber bulb with a plastic union is filled with water. It is mated to an apparatus consisting of a two holed rubber stopper and two glass tubes. The rubber bulb is attached to the short tubing which is a bent eye dropper tapered to a narrow upper opening. Another piece of longer glass tubing is in the other hole of the stopper. The stopper in the flask is removed and the resulting apparatus is then securely fitted to the flask in its place. The flask is then lowered so that the long glass tubing extends into the two liter beaker containing a clear colorless liquid consisting mostly of water. The water in the rubber bulb is then squeezed into the flask. As soon as the slightest amount of water is introduced, a fountain effect results as the water from the beaker is drawn into the upper round bottom flask. Momentarily disregard the purple color of the liquid in the upper flask. How did the liquid get into the flask above? What property did the ammonia have to make the “fountain” work? This will not work with air. The reason is that ammonia is highly soluble in water. As the ammonia dissolves, the pressure of the remaining gas decreases. The outside pressure (atmospheric pressure) is now higher than the pressure inside the flask, and the outside pressure pushes the water into the flask via the long glass tubing. This will work with any highly soluble gas in water – two such gases are ammonia and hydrogen chloride, HCl, a corrosive colorless gas when pure. The purple color of the water now inside the round bottom flask is the result of an acid base indicator phenolphthalein which was also introduced into the large beaker. Phenolphthalein turns pink when in basic solution – it is colorless in neutral or acidic solution. This shows that ammonia dissolved in water results in a basic solution consisting of ammonium ions and hydroxide ions – this concept is covered during acid base lectures later in the course. The demonstration works without the indicator, but is introduced here for a more dramatic visual effect.