Diffusion is the movement of particles from a region of higher concentration to a region of lower concentration until they are evenly spread.
Diffusion is the movement of particles from a region of higher concentration to a region of lower concentration until they are evenly spread. Malay: resapan · Chinese: 扩散.
Diffusion happens because the particles of matter are in constant, random motion, as the kinetic theory describes. As they move about, they gradually mix until the concentration is the same everywhere. Diffusion is fastest in gases, where the particles move quickly and are far apart, slower in liquids, and negligible in solids because the particles there only vibrate about fixed positions.
Example. When a crystal of purple potassium manganate(VII) is placed in still water, its colour slowly spreads until the whole beaker is evenly coloured, the coloured particles have diffused among the water particles. In the same way, the smell of food diffuses across a room as the gas particles spread out. Diffusion is faster when the temperature is higher, because the particles then have more kinetic energy.
Confusion to avoid. Do not confuse diffusion with dissolving. Dissolving is a solid breaking up and mixing into a solvent to form a solution; diffusion is the net spreading of particles down a concentration difference, and it happens in gases and liquids alike. Diffusion is not the particles “trying to escape”, it is simply the result of their random motion.
The rate of diffusion depends on the state of matter and the temperature: gases diffuse fastest, and any substance diffuses faster when it is warmer because its particles then have more kinetic energy. Lighter particles also tend to diffuse more quickly than heavier ones. Comparing these rates is a favourite way for the exam to test whether you really understand the particle model.
Diffusion is important in this chapter because it is direct evidence that matter is made of tiny, moving particles, which is the central claim of the kinetic theory. Being able to explain a diffusion observation in terms of moving particles is a common exam task in the Matter and the Atomic Structure chapter.
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