What is the diffrence between osmosis and diffusion?

Answer 1

Diffusion is the spreading of a substance in another substance effected by the thermal movements of the molecules or other forces acting between the molecules. The spreading happens from an area with high density into areas with originally lower density.

For osmosis to occur you must have a solvent in two rooms separated by a membran that lets the solvent molecules pass. In one of the rooms there is a dissolved substance that cannot pass the membrane in a higher concentration than in the other room.

This makes solvent pass over to the room with the highest concentration of the substance so that the pressure gets higher in that room and the solution gets less concentrated. The process stops when the pressure is high enough or the concentration of the dissolved substance is equal in both rooms.

The mechanism behind osmosis is as follows: The solvent molecules are always moving because of their termal kinetic energy and are steadily colliding with each other and also with molecules of the dissolved substance. All the time some solvent molecules pass through the membran between the two rooms also, and in both directions.

But the dissolved substance will all the time collide with some of the solvent molecules and block their passage through the membran. Solvent molecules in the room with most dissolved substance will however be blocked more often from passing than solvent molecules in the other room. Thus there will be most passings from the room with the least dissolved substance.