Oil floats on water because it is less dense than water. Density is defined as mass per unit volume. So, a cubic centimeter (cc) of oil has a much lower mass than a cubic centimeter (cc) of water.

In addition to density, the polarity of the molecules also plays a role in why oil floats on water. Water molecules are polar, meaning they have a positive end and a negative end. Oil molecules are nonpolar, meaning they do not have a positive or negative end.

Polar molecules are attracted to each other, while nonpolar molecules are not attracted to each other. This means that water molecules are attracted to each other, but they are not attracted to oil molecules.

When oil is added to water, the oil molecules are repelled by the water molecules. This causes the oil molecules to clump together and float on top of the water.

The buoyant force also helps to explain why oil floats on water. Buoyant force is the upward force exerted by a fluid on an object immersed in it. The buoyant force is equal to the weight of the fluid displaced by the object.

When oil is added to water, the oil displaces a volume of water equal to its own volume. The buoyant force of the water pushes up on the oil, causing it to float.

The combination of density and polarity explains why oil floats on water. Oil is less dense than water, and its nonpolar molecules are repelled by the polar water molecules. The buoyant force of the water also helps to keep the oil afloat.