dependence of rate of diffusion on molecular mass
Diffusion rate depends on several factors, these are:
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- Mass of the substance. The lighter the substance (lower Mr or Ar) the faster it diffuses
- Temperature. The more kinetic energy the particles have, the faster they move and diffuse.
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- Presence of other substance. Diffusion is faster when it occurs in an area where there are fewer particles of other substances present. This is why diffusion is extremely fast in vacuums. This is because the diffusing particles have less other particles to stand in their way.
- Intermolecular spaces. This is why gases diffuse faster than liquids and solids do not diffuse.
Diffusion of gases can be proved very easily. We can prove it by putting some bromine liquid in a cylinder and sealing it, then putting another inverted cylinder above it. When the bromine liquid evaporates, we remove the lid between the two cylinders, the brown bromine gas will diffuse upwards filling the available space.
We can also prove diffusion in liquids by a very simple experiment. Pour some water in a beaker, then add a drop of blue ink in the water. After a period of time, you will find that the black ink spread throughout the water and turned it into a blue solution. This was caused by diffusion.