what is the principle used in separatio of mixtures ?

There are different methods which can be used to separate mixtures. The particular technique chosen for any given mixture depends on the nature of the constituents. For example:

Evaporation is based on the principle that molecules of liquid substances on heating become gaseous and are lost from the surface. The other component (which is required), is then collected.

Distillation is based on the principle that when liquids are heated to their boiling points, they become gaseous, and when the gases are cooled, they change back to the liquid.

Dissolution is based on the principle that some solid substances are soluble in certain kind of solvent, while others are not.

Filtration is based on the principle that the particles of liquid are small enough to pass through the filter material while those of solids are not.

There are other methods also which involve different principles according to the nature of the mixture.