Gravity plays an important role in the separation of liquids because it determines the volume of liquids, and therefore, it affects the density.
When you mix different liquids they often separate forming defined layers. This process is possible due to the differences in density (mass/ volume). In this way, the liquid with a lower density will float over the liquid with a higher density.
This phenomenon is affected by gravity or the force that makes everything on the Earth to be pulled towards the center of it. This is because gravity exerts pressure over bodies and affects the volume, which is one of the factors that determines density.
