Hydrogen chloride gas and ammonia can be used to demonstrate the fountain experiment because they are

1. Analyze the question

The question asks for the specific physical property of hydrogen chloride (HCl) gas and ammonia ($NH_3$) gas that makes them suitable for the "fountain experiment."

2. Explain the concept

The fountain experiment is a classic demonstration in chemistry used to show the high solubility of certain gases in water.

• How it works: A round-bottom flask is filled with the gas (HCl or $NH_3$). A glass tube is inserted into the flask, and the other end is placed in a beaker of water (often containing an indicator like phenolphthalein).
• The mechanism: When a small amount of water is introduced into the flask, the gas dissolves almost instantaneously. This creates a partial vacuum (a sudden drop in pressure) inside the flask. Because the atmospheric pressure outside is higher than the pressure inside the flask, water is forced up the tube into the flask, creating a "fountain."

3. Show the solution

• Hydrogen Chloride (HCl): This is a highly polar covalent compound. It dissolves extremely well in water to form hydrochloric acid.
• Ammonia ($NH_3$): This gas is also highly polar and reacts with water to form ammonium hydroxide ($NH_4OH$). Its solubility is so high that one volume of water can dissolve hundreds of volumes of ammonia gas.
• Evaluation of options: • A & D: Being "light" or "heavy" (density) does not cause the pressure drop required for the fountain effect. • B: While both gases are volatile, volatility refers to the ease of turning into a gas, which is not the primary reason for the fountain effect. • C: The high solubility is the critical factor. Without the gas dissolving rapidly into the water, the pressure inside the flask would not decrease, and the water would not be drawn up.

4. Identify the answer

The correct option is C. Both gases are chosen specifically because they are extremely soluble in water, which allows them to create the vacuum necessary for the experiment to work.

5. Exam Tip

Whenever you see the "fountain experiment" in an exam, immediately associate it with high solubility. Remember: the faster the gas dissolves, the more dramatic the fountain will be!