The fountain experiment demonstrates the solubility of gases in water. Hydrogen chloride gas and ammonia gas are used in the fountain experiment because they are both highly soluble in water. 

Hydrogen chloride: A liquid gas that is highly soluble in water and has an acidic nature. The acidic nature of hydrogen chloride is demonstrated in the fountain experiment when the blue litmus solution turns red. Also, Ammonia: A colourless gas with a pungent smell that is lighter than air