What simple property causes molecules to diffuse from a high concentration toward a lower concentration?

Random motion of molecules osmosis causes molecules to diffuse from a high concentration towards a lower concentration.

Explanation:

In a solution various molecules are present, Movement of each molecule is random and not dependent on other molecules.

Osmosis is the movement of water molecules from high water concentration area to low water concentration area. The amount of energy required is very less in this activity. So it a kind of passive transport which means when movement takes place without any energy but on the other hand active transport needs energy.

Basically movement of molecules take place from high concentration to low concentration to make the concentration equal. Like a potato with high concentration of starch if soaked in water will swell because the water will flow into potato and the concentration of water and potato will become equal.