why oil floats on water

Oil floats on water because it is less dense than water. Density is the mass per unit volume of a substance. When a substance is denser than water, it sinks to the bottom, but when it is less dense, it floats on the surface.
Oil is less dense than water because it consists of long chains of hydrocarbon molecules that are relatively light and spread out over a large volume. Water, on the other hand, is made up of smaller, more tightly packed molecules. When oil is poured into water, it will float on the surface because the water is denser and pushes the less dense oil upwards.
Additionally, the molecules in water are polar, meaning they have a slight electrical charge, which causes them to attract each other and form cohesive bonds. This makes water more difficult to penetrate, which is why oil does not mix with water and instead forms a layer on top of it.