Question: @Yannis Why does oil float on top of water is there a chemical in it or is there just lots of oxygen in it?

Gonzalo Delgado-Pando answered on 7 Nov 2017:

Two things are happening here. First one is the oil cannot get mixed with the water even if you stirr it, this is because their chemical composition. Oils are not miscible with water and that is why you can see the oil.
The other important thing is called density (as Yannis has said), lower density always on top. Density has nothing to do with weight, if you throw a small stone to the sea it won’t float but you can see really big boats floating because they are less dense, although they weight a lot more than a stone!
Density measures how much “stuff” is packed in a certain amount of volume, the less stuff the less dense. In the case of the oil if you weight a full glass of oil and compare it to the weight of a full glass of water (using the same glass and filled the same) you will see that the weight is lower for the oil than for the water. In the same volume (the glass) the weight is lower, it means lower density.
In the case of the boat and the stone the liquid they are submerged in is the same but there are big differences in weight but also volume. The stone has low weight (in comparison with the boat) but also low volume. However, the boat has high weight but much more high volume and then the density is going to be lower.