The gas that is commonly used to demonstrate the fountain experiment is

Analysis

The question asks for the specific gas used in the "fountain experiment." This is a classic demonstration in chemistry used to illustrate the high solubility of certain gases in water.

Concept Explanation

The fountain experiment relies on the principle of extreme solubility. For the experiment to work, a gas must be highly soluble in water. When a small amount of water is introduced into a flask filled with such a gas, the gas dissolves rapidly, creating a sudden drop in pressure inside the flask. The external atmospheric pressure then forces water from a reservoir up through a tube and into the flask, creating a "fountain" effect.

• Hydrogen chloride (HCl): This gas is extremely soluble in water. One volume of water can dissolve about 450 volumes of HCl gas at room temperature. This rapid dissolution creates the necessary vacuum for the fountain experiment.
• Hydrogen sulphide ($H_2S$): This is only moderately soluble in water and is not used for this demonstration.
• Dinitrogen(I) oxide ($N_2O$) and Nitrogen(II) oxide ($NO$): These gases have low solubility in water and cannot create the pressure differential required for the experiment.

Step-by-Step Solution

1. Identify the requirement: The experiment requires a gas that dissolves almost instantaneously in water to create a partial vacuum.
2. Evaluate the options: • HCl is highly polar and reacts with water to form hydrochloric acid, leading to rapid absorption. • $H_2S$, $N_2O$, and $NO$ do not possess the high degree of solubility required to displace the volume of air needed to pull water up a tube effectively.
3. Conclusion: Hydrogen chloride is the standard gas used in laboratory demonstrations to show this principle.

Exam Tip

Whenever you see "fountain experiment" in a chemistry question, immediately look for highly soluble gases like Hydrogen chloride (HCl) or Ammonia ($NH_3$). These are the two most common gases used for this demonstration because they are both extremely soluble in water.