Diffusion

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What this model shows

A strip turns blue when enough ammonia has dissolved in its water to make it alkaline. When each strip turns comes from a model of one puff of ammonia spreading through still air in a closed box, so the order and the pairs are exact; time is schematic, not seconds or minutes, and the chart draws each run to its own time scale.

The magnified view shows far fewer, larger and slower particles than real, and far more ammonia among the air. Each change of direction stands for a collision.

Once you have looked closer, violet dots in the tank show where the ammonia particles are: far fewer and far larger than real, with the air not drawn. They move at one speed the whole time; after the run they go on spreading until the tank is evenly filled.

Concentrated ammonia solution is corrosive and its gas harms the eyes and lungs: a real version is a teacher demonstration in a fume cupboard.

Reduce motion

With reduced motion, nothing moves on its own: “Show the next change” turns the next strip, and the particles are drawn still.

Equipment

Select equipment to read its description in the card.

Step 1 · Diffusion

Load the pad

Ammonia is a colourless gas you can smell but not see. Select the bottle to drop some ammonia solution onto the cotton-wool pad.

Pad: Dry
Strips blue: 0 of 6

Pad at the end Pad in the centre

Equipment

Ammonia solution

Concentrated ammonia solution. It gives off ammonia gas, which is colourless and has a sharp smell. The solution is corrosive and the gas harms the eyes and lungs: a teacher uses it in a fume cupboard.