The Ammonia and Hydrogen Chloride Tube Demonstration

Introduction

One of the most famous demonstrations of diffusion uses a long glass tube, two gases and a single white ring. Ammonia gas enters from one end, hydrogen chloride from the other, and after a few minutes a white band of solid appears inside the tube — but not in the middle. Where the ring forms tells us which gas moved faster, and linking this to particle mass is a classic exam question.

Core explanation

The set-up in outline. A long, dry glass tube is clamped horizontally. At one end, a cotton-wool plug gives off ammonia gas; at the other end, a plug gives off hydrogen chloride gas. Both ends are sealed with bungs at the same moment, so the gases can move only by diffusion along the tube. There is no draught or stirring inside.

What is seen. After a few minutes a white ring of solid forms inside the tube. The ring is nearer to the hydrogen chloride end than to the ammonia end.

Why a solid forms. When the two gases meet, they react to form ammonium chloride, a white solid:

ammonia + hydrogen chloride → ammonium chloride

NH₃(g) + HCl(g) → NH₄Cl(s)

The white ring marks the place where the two gases first meet in high enough amounts to react.

Why the ring is off-centre. Both gases start at the same moment and at the same temperature. If they moved at the same speed, they would meet in the middle. The ring forms closer to the hydrogen chloride end, which means the ammonia particles have travelled further in the same time. Ammonia diffuses faster.

Linking to mass. The relative formula masses explain this:

- NH₃: Mr = 14 + (3 × 1) = 17 - HCl: Mr = 1 + 35.5 = 36.5

Ammonia molecules are lighter, so at the same temperature they move faster on average and diffuse more quickly. They cover roughly 60% of the tube's length while hydrogen chloride covers roughly 40%.

The tube also shows that diffusion is not instant. Gas particles move at hundreds of metres per second, yet it takes minutes to cross a tube less than a metre long. This is because each particle collides with air particles in the tube billions of times per second, constantly changing direction.

Safety. Both gases are harmful. Ammonia irritates the eyes and lungs; hydrogen chloride is corrosive and forms hydrochloric acid on contact with moisture. The demonstration is carried out by a teacher in a fume cupboard, wearing eye protection, and the tube stays sealed.

Step-by-step reasoning

To explain the ring position:

1. Identify the white solid as ammonium chloride formed where the gases meet. 2. Note that the ring is closer to the hydrogen chloride end. 3. Conclude that ammonia travelled further in the same time. 4. Compare Mr values: 17 for ammonia, 36.5 for hydrogen chloride. 5. Explain that lighter ammonia molecules move faster at the same temperature.

Visual explanation

Draw a horizontal tube. Label the left end "NH₃" and the right end "HCl". Draw many small arrows spreading from each end, with longer arrows from the ammonia side. Mark a thick white band about three-fifths of the way along from the left. The picture shows at a glance that ammonia has come further.

Real-world analogy

Two friends set off towards each other from opposite ends of a long corridor at the same moment. One walks quickly, the other slowly. They meet closer to the slow walker's starting point, because the fast walker has covered more ground. The meeting point reveals who was faster, just as the white ring does.

Real-world example

In laboratories and at ammonia storage sites, a leak of ammonia can be traced by holding a source of hydrogen chloride vapour nearby: white fumes of ammonium chloride appear where the gases meet. The same reaction explains the white film that can build up on glassware stored near bottles of both chemicals.

Why?

Why do the gases take minutes rather than a fraction of a second to meet? The tube is full of air. Each gas particle keeps colliding with air particles and changing direction, so its path is a long zigzag. Its overall progress along the tube is far slower than its actual speed.

Common misconception

"The ring forms nearer the HCl end because HCl is heavier and sinks." The tube is horizontal and gravity plays no real part. The position is set by speed: the lighter ammonia molecules move faster and travel further before the gases meet.

Worked example

Question: In a 100 cm tube the white ring forms 59 cm from the ammonia end. How far is it from the hydrogen chloride end, and what does this show?

Reasoning: 100 − 59 = 41 cm. In the same time, ammonia travelled 59 cm and hydrogen chloride travelled 41 cm. Ammonia travelled further, so it diffused faster, as expected from its lower Mr of 17 compared with 36.5.

Answer: 41 cm; ammonia diffuses faster because its molecules are lighter.

Quick check

1. What is the white solid formed in the tube? Answer: Ammonium chloride, NH₄Cl.

Exam focus

Learn the word and symbol equations and the Mr values 17 and 36.5. Questions often ask you to predict or explain the ring position, or to predict what happens if the tube is warmed: the ring forms sooner, but in about the same place, because both gases speed up.

Advanced insight

Graham's law predicts the ratio of distances as √(36.5 ÷ 17) ≈ 1.47, which places the ring about 59% of the way from the ammonia end. Real results often differ slightly because the gases are released unevenly from the plugs and the ring forms where the concentrations are high enough to react, not exactly where the first particles meet.

Summary

Ammonia and hydrogen chloride diffuse towards each other along a sealed tube and react to form a white ring of ammonium chloride. The ring forms nearer the hydrogen chloride end, showing that ammonia travels faster. This is because ammonia (Mr 17) is lighter than hydrogen chloride (Mr 36.5). The gases are harmful, so this is a teacher demonstration in a fume cupboard.

Practice questions

1. Write the word equation for the reaction in the tube. Answer: Ammonia + hydrogen chloride → ammonium chloride. 2. Explain why the white ring is not in the middle of the tube. Answer: Ammonia molecules are lighter (Mr 17) than hydrogen chloride molecules (Mr 36.5), so they move faster and travel further in the same time. 3. Why are the ends of the tube sealed at the same moment? Answer: So that both gases start diffusing at the same time and no draughts move the gases, making the comparison fair. 4. Why does the ring take several minutes to appear, even though gas particles move very fast? Answer: The particles collide with air particles in the tube very often and keep changing direction, so their overall progress is slow. 5. Suggest why this is done as a teacher demonstration in a fume cupboard. Answer: Both gases are harmful to breathe and can damage the eyes, so they must be contained and removed safely.