Why does oil float on water?
Did you realize that a large log of wood floated on water but a comparatively smaller piece of rock sank?
How is this even possible?
This is possible because of density.
It is the measure of the mass of an object per unit volume.
In the first case, the log of wood floats on water because it has less density than water.
This means that the amount of mass present in unit volume of wood is less than the amount of mass present in unit volume of water.
However, in the second case, the piece of rock sinks because it has more density than water.
This means that the amount of mass present in unit volume of rock is more than the amount of mass present in unit volume of water.
Do you know why the helium filled balloon is rising up in the air?
This is again because of density.
The balloon consists of helium gas which has less density than the surrounding air, hence it rises up.
Here, have a look.
Oil is floating on water.
Are you assuming that this is because of density?
Yes, you are right.
Oil has less density than water.
Hence, it floats.
However, there is another reason due to which oil floats on water.
The reason is that both oil and water are immiscible liquids, that is, they do not mix with each other even if we shake them.