Cotton wool soaked with aqueous ammonia is placed at one end of a glass tube and cotton wool soaked with concentrated hydrochloric acid at the other. As the vapors diffuse toward each other, they form a visible white ring of ammonium chloride closer to the HCl end, showing that ammonia diffuses faster due to its lower molar mass.

1. Place a long glass tube horizontally on a retort stand and secure it with clamps.
2. Soak a small ball of cotton wool with aqueous ammonia and insert it a few centimeters into one end of the tube, sealing with a rubber bung.
3. Using rinsed tweezers, soak another cotton ball with concentrated hydrochloric acid and insert it into the opposite end of the tube, also sealing with a bung.
4. Leave the tube undisturbed and observe over 15–20 minutes.
5. Watch for the formation of a white ring of ammonium chloride where the two gases meet. Note its position relative to each end.

Ammonia and hydrogen chloride diffusion experiment - Nigel Baldwin:

📄 Diffusion of ammonia and hydrogen chloride gas - Institute of Physics: https://spark.iop.org/diffusion-ammonia-and-hydrogen-chloride-gas

• Try using tubes of different lengths to see how the time for ring formation changes.
• Instead of a tube, put ammonia and hydrochloric acid in two separate small beakers, then cover them with an inverted larger beaker.
• Leave the experiment overnight in the fumehood.

• Wear safety goggles and a lab coat.
• Conduct the experiment in a well-ventilated lab or outdoors, as both ammonia and HCl vapors are irritating and corrosive.
• Handle concentrated hydrochloric acid and concentrated ammonia carefully; avoid skin and eye contact.
• Dispose of cotton wool safely in a fume hood after use.
• Rinse tweezers after handling each cotton wool sample to avoid corrosion or cross-contamination.

• Why does the white ring of ammonium chloride form closer to the hydrochloric acid end? (Because ammonia molecules, being lighter, diffuse faster than hydrogen chloride molecules.)
• How does this experiment provide evidence for the particle model of gases? (The movement and eventual meeting of invisible particles is inferred from the appearance of the solid ring.)
• Why do lighter molecules move faster at the same temperature? (Because the average kinetic energy is the same for all particles at a given temperature, so lighter particles must have higher speeds.)
• What is the balanced equation for the reaction observed? (NH3(g) + HCl(g) → NH4Cl(s).)