What is the natural tendency of molecules to spread from an area of high concentration to an area of low concentration?

Diffusion

Explanation:

Diffusion is a process by which a substance or certain molecule moves from a region of high concentration to a region of low concentration due to the concentration gradient and continues till the concentration become uniform.  

This is a passive mode of transportation.  

For example – Nutrients move across the cell membrane through a lipid bilayer of a cell membrane by the process of diffusion

he movement of air through earth's or any planet's atmosphere is called wind, and the main cause of earth's winds is uneven heating by the sun. this uneven heating causes changes of atmospheric pressure, and winds blow from regions with high pressure to those with low pressure.

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