Diffusion:- means that the intermixing of solid and liquid, liquid and gas etc.
Example:- mixing of Water and Salt
Water is a liquid
Salt is a solid.
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The RANDOM movement of any substance from a region of high concentration to a region of low concentration is called diffusion. One great example of diffusion is the spreading of odour of food cooking throughout the kitchen. If you open the kitchen door, the smell of the food spreads into the next room. The movement of these molecules is said to be passive, which means that no energy is required for diffusion to occur. The kinetic energy present in the particles is enough to supply the energy for diffusion.

One more example of diffuison is the spreading of dye in water as you can see in the diagram below. The dye particles move from the high concenteration region to spread throughout the beaker.