2-L round bottomed flask, 2-L beaker, 2-hole stopper, Glass tube (2 ft. in length), Ring stand and clamps, medicine dropper,
Dry NH3 gas or apparatus to produce NH3, Water, Phenolphthalein
Fill the 2-L flask with ammonia gas and seal it with a rubber stopper. Ammonia can be generated by gently heating a 1:1 mixture of ammonium chloride and sodium hydroxide.
Set up the apparatus as shown. The glass tube should be about 0.5 inch from the bottom of the inverted flask. Fill the beaker with 1.5 L of water and add a few ml of phenolphthalein solution.
Fill the medicine dropper with water. When ready squeeze the water into the flask. The ammonia will dissolve in it, which creates a partial vacuum.
The water rushes up to the top flask and the solution turns pink.
The first few drops of water that enter the upper flask absorb a little ammonia gas. When this occurs, a slight vacuum that forms sucks a little more water into the flask to balance the pressure. This water, in turn, absorbs a little more ammonia. This continuous exchange creates a chain reaction that causes the solution to come rushing into the top flask.
The solution turns pink because of the basic activation of the indicator in the water.
As an alternative, HCl gas could be used (bromothymol blue or methyl violet replace the phenolphthalein). SO2 and Cl2 gases could also be used to create acidic solutions.