Which two gases can be used for the demonstration of the fountain experiment?

Analysis

The fountain experiment is a classic chemistry demonstration used to show the high solubility of certain gases in water. For the experiment to work, the gas must be extremely soluble in water so that when a small amount of water is introduced into the gas-filled flask, it dissolves rapidly, creating a vacuum that sucks more water into the flask, forming a "fountain."

Concept

The principle relies on Henry's Law and the polar nature of molecules. Gases that are highly soluble in water (often due to their ability to ionize or form strong hydrogen bonds with water) are the best candidates.

• Ammonia ($NH_3$): Highly soluble in water because it reacts to form ammonium hydroxide ($NH_4OH$).
• Hydrogen Chloride ($HCl$): Highly soluble in water because it dissociates completely into $H^+$ and $Cl^-$ ions.

Step-by-Step Solution

1. Evaluate Option A ($SO_4$ and $NH_3$): $SO_4$ (sulfate) is an ion, not a gas. This option is chemically incorrect.
2. Evaluate Option B ($SO_3$ and $NH_3$): While $NH_3$ is highly soluble, $SO_3$ is a gas that reacts with water to form sulfuric acid, but it is not the standard pair used in the classic fountain experiment demonstration.
3. Evaluate Option C ($HCl$ and $NH_3$): Both $HCl$ and $NH_3$ are gases that are extremely soluble in water. In a laboratory setting, these two are the standard gases used to demonstrate the fountain effect because they create a dramatic pressure drop almost instantly.
4. Evaluate Option D ($SO_2$ and $CO$): $CO$ (carbon monoxide) is only slightly soluble in water, so it cannot create the vacuum required for a fountain.

Conclusion

The gases $HCl$ and $NH_3$ are the most common examples used in chemistry textbooks and laboratory demonstrations to show high solubility in water.

Exam Tip

Whenever you see a question about the "fountain experiment," look for gases that are highly polar or highly soluble in water. Remember: $NH_3$ (basic) and $HCl$ (acidic) are the "star players" for this experiment!