Diffusion in Gases

Introduction

You can often tell what is for dinner before you reach the kitchen. Particles released from the cooking food escape into the air and travel through the house to your nose. In still air, this happens by diffusion. Gases diffuse far faster than liquids, which is why smells are such a quick and useful warning signal — from burning toast to a gas leak. Yet diffusion in air is much slower than you might expect from the very high speeds of gas particles.

Core explanation

How a smell spreads. Many substances release some particles into the air as a gas or vapour. Near the source, the concentration of these odour particles is high; further away it is low. Air particles and odour particles move randomly and quickly. As a result, there is a net movement of odour particles from high to low concentration through the air. When some reach the receptors in your nose, you detect the smell.

Why gases diffuse quickly. Gas particles are far apart and move fast, with negligible attractions between them. There is plenty of empty space for particles to move into, so they mix readily with the air. This makes diffusion in gases much faster than in liquids, where particles are crowded together.

Why diffusion is still quite slow. Air particles at room temperature move at around 500 m/s, so an odour particle might be expected to cross a room in a fraction of a second. In reality, in perfectly still air, a smell may take minutes to cross a room by diffusion alone. The reason is collisions . Air contains so many particles that an odour particle travels only a tiny fraction of a millimetre — about 0.0001 mm — before hitting another particle and bouncing off in a new random direction. It makes billions of collisions per second, so its path is a tangled zigzag, making very slow overall progress in any one direction.

Draughts help. In real rooms, smells usually spread faster than diffusion alone would allow, because air is rarely still. Draughts, breathing and convection currents — warm air rising from people, radiators or cooking — carry the odour particles along in bulk. Diffusion then finishes the job of spreading them evenly.

Two gases mixing. A classic demonstration uses two gas jars placed mouth to mouth: the lower one contains a coloured gas such as bromine, which is brown and denser than air; the upper one contains air. Even though bromine is denser, the brown colour slowly spreads upwards until both jars are evenly coloured. Because bromine is toxic, this demonstration is only carried out by teachers in a fume cupboard, and it is widely shown on video instead.

Diffusion into a vacuum. If the upper jar is emptied of air first, the bromine fills it almost instantly. With no air particles to collide with, bromine particles travel freely at their full speed. This comparison shows clearly that collisions are what slow diffusion down.

Step-by-step reasoning

To explain why a smell reaches the far side of a still room:

1. The source releases odour particles into the air, giving a high concentration nearby. 2. Odour and air particles move rapidly and randomly. 3. Net movement carries odour particles from high to low concentration. 4. Frequent collisions make progress slow, but eventually particles reach every part of the room.

Visual explanation

In the simulation, release a small cloud of coloured particles into a box of grey "air" particles and trace one coloured particle. Its path is a dense scribble of short straight lines. Now remove the grey particles: the coloured particles shoot across the box almost at once.

Real-world analogy

Imagine trying to cross a packed railway concourse at rush hour. You can walk quickly, but you bump into someone every step and get knocked in random directions, so reaching the far side takes ages. In an empty concourse you would stride straight across in seconds.

Real-world example

Natural gas used in homes has almost no smell of its own, so gas companies add a small amount of a strong-smelling sulfur compound. If gas leaks, the odorant spreads through the air by diffusion and air currents, warning people to ventilate the area, avoid sparks and call the emergency gas service.

Why?

Why do smells spread faster in a warm room than in a cold one? At higher temperatures, the gas particles move faster, so diffusion is quicker. Warm rooms also tend to have stronger convection currents, which carry the smell around more rapidly.

Common misconception

"Because gas particles travel at hundreds of metres per second, a smell should cross a room instantly." Particles collide billions of times a second with air particles, so each one follows a random zigzag path, and its overall progress is much slower than its speed.

Worked example

Question: A jar of brown bromine gas is placed beneath an inverted jar of air, with the lids removed. Describe and explain what is seen over about half an hour.

Reasoning: Bromine particles move randomly and are at high concentration in the lower jar. Air particles in the upper jar slow their progress through collisions.

Answer: The brown colour gradually spreads upwards until both jars are the same pale brown. Bromine particles diffuse from high to low concentration while air diffuses downwards; the process is gradual because of frequent collisions with air particles.

Quick check

1. Why does diffusion of a gas into a vacuum happen almost instantly? Answer: There are no other particles to collide with, so the gas particles travel freely at high speed.

Exam focus

Explain gas diffusion using random motion and net movement from high to low concentration. If asked why it takes time, mention collisions with air particles. Know the bromine demonstration and that bromine spreads upwards even though it is denser than air.

Advanced insight

The distance a particle travels between collisions is called its mean free path . In air at sea level it is about 70 nanometres; high in the upper atmosphere, where particles are far more spread out, it can be metres or more. That is why engineers must treat gases very differently in space vacuum chambers.

Summary

Smells spread through air because odour particles diffuse from high to low concentration by random motion. Gases diffuse faster than liquids because their particles are far apart and move fast. Diffusion in air is nevertheless much slower than particle speeds suggest, because of countless collisions. Air currents usually help spread smells faster than diffusion alone.

Practice questions

1. Explain how the smell of perfume reaches someone across a still room. Answer: Perfume particles evaporate and then move randomly; there is net movement from high concentration near the source to low concentration across the room, until some reach the person's nose. 2. Give one reason why gases diffuse faster than liquids. Answer: Gas particles are further apart and move faster, so there is more space to move into and fewer obstructions. 3. Why does a smell take much longer to cross a room than the speed of gas particles suggests? Answer: The particles collide constantly with air particles, changing direction randomly, so their overall progress is slow. 4. In the bromine demonstration, why does the brown colour rise into the upper jar even though bromine is denser than air? Answer: Diffusion depends on random motion and concentration differences, so bromine particles move into the region where their concentration is lower, whatever its position.