What is the primary cause of diffusion?
a) that some substances can dissolve in solvents
b) heat added to a substance
c) random internal motion of atoms and molecules
d) attraction of opposite atomic charges

Answer
c). random internal motion of atoms and molecules.

Explanation
Diffusion is the movement of particles from a region of high concentration to region of low concentration. The rate of diffusion can be increased by increasing amount of temperature but this is not its primary cause. Diffusion is primarily caused by movement of molecules or atoms in a substances. Form the choices given, the correct answer is c. random internal motion of atoms and molecules. 

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