Many people think that oil floats on water because it is lighter. This is kind of correct but lets look at exactly why it does so in more detail.
Why oil floats on water
The reason oil floats on water is not a question of its mass, but rather its density.
The density of a substance is how much mass there is in a given unit of volume. For example, air has a very low density while steel has a very high density – the same volume of each substance have very different masses.
Density is calculated as mass per unit volume, so the units used for density measurements are usually g/mL and kg/L for fluids, or g/cm3 and kg/m3 for solids.
It’s also easy to think that thicker fluids are heavier than thinner fluids, but this is actually a measurement of how easily a substance flows, or its viscosity.
Looking at oil and water, it would seem that oil would sink when poured into water. Water is thin, or non-viscous while oil is thick, or viscous. However, the oil stays on top of the water. This can be seen in oil and vinegar dressing – the oil is more viscous than the vinegar yet it stays on top. This is because oil is less dense than vinegar (which is mostly water).
Density is an important measurement because it compares equal amounts of substances. If someone says that oil floats on water because it is lighter, they’re forgetting the other half of density – volume.
Ten litres of oil is certainly heavier than one litre of water, but to be accurate they must be compared directly, with equal volumes in a density measurement. One litre of oil is lighter than one litre of water, because oil is less dense than water. And, because oil is less dense, it floats on top of the water.