Gases in Motion
Learning Objectives
Students will be able to:
- Explain how gas molecules move (random motion, collisions)
- Define diffusion and predict how gases spread
- Describe factors that affect the rate of diffusion
- Model how pollutants disperse in indoor spaces
What is Diffusion?
Diffusion is the movement of particles from an area of higher concentration to an area of lower concentration, caused by random molecular motion.
Kinetic Molecular Theory
Gas molecules:
- Are in constant, random motion
- Collide with each other and walls
- Have large spaces between them
Higher temperature means:
- Faster molecular motion
- More collisions
- Faster diffusion
Factors Affecting Diffusion Rate
| Factor | Effect | Explanation |
|---|---|---|
| Temperature | Higher temp = faster | Molecules move faster |
| Molecular mass | Lighter = faster | Less mass, more speed |
| Concentration difference | Bigger difference = faster | More "push" from high to low |
| Distance | Longer = more time | Same rate, more ground to cover |
Classroom Demonstrations
Demo 1: Perfume Diffusion
Spray perfume at the front of the room. Time how long it takes students at different distances to smell it.
- Students at 2 meters smell it first
- Students at 4 meters smell it next
- Students at 8 meters smell it last
Demo 2: Food Coloring in Water
Drop food coloring in three containers:
Slowest spreading
Medium spreading
Fastest spreading
Indoor Air Quality Scenarios
Scenario 1: The Classroom
"30 students are exhaling CO2. The windows are closed. What happens?"
CO2 spreads from students throughout the room. Concentration rises everywhere until fairly uniform (but elevated).
Scenario 2: New Furniture
"A new bookshelf releases formaldehyde (a VOC). Where is concentration highest?"
Highest near the bookshelf, decreasing with distance. Ventilation helps disperse it faster.
Scenario 3: CR Box in a Room
"A CR box fan is running. How does this change pollutant spread?"
The fan creates air movement (convection), making mixing much faster than diffusion alone. Pollutants are pulled to the filter.
Key Takeaway
Pollutants don't stay in one place—they spread! Understanding diffusion helps explain why ventilation is so important. Without air movement, pollutants spread slowly by diffusion alone. With ventilation or filtration, we can control where pollutants go.