whenyouwishuponapaperstar-deact asked: Why is oil lighter than water ?
It’s not lighter than water.
Oil is heavier than water, one molecule of oil weighs (depending on the oil, it can have hundreds of carbons and many other organic molecules) much more than a single molecule of H2O! However, one must consider density.
Water is far more dense than oil, i.e. there are more molecules of water per unit volume than oil!
This means it will float on water, even though water is “lighter”.
This also explains why ice floats on water (think icebergs and ice in drinks; as you freeze water, the molecules take up more space, hence when you freeze a bottle of water, it will expand and sometimes break, the same effect can be seen with milk — it is, after all nearly all water).
This fact can be extrapolated from the phase diagram due to the negative slope of the solid-liquid line which is unique to water.
This means you have the same amount of water, but it is of a larger volume and hence it will float as the density has decreased!
Remember, weight can have very little to do with density!