What controls the net direction of molecules?

Molecular diffusion is the process of mass transfer as a result of a concentration difference in a mixture.

This process is related to the thermal movement of the particles and their velocity, which is a function of the temperature, the viscosity of the fluid and the size of the particles. This is how diffusion controls the net direction of the molecular flow from a region of higher concentration to one of lower concentration, as long as there is a difference (or gradient) of concentration.

specific latent heat of fusion, lf, of a substance is defined as the amount of heat required to change a unit mass of the substance from solid to liquid state, without any change in the temperature.

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