Density is a physical property of matter. It is the measure of the heaviness of objects. For example, a glass is more dense than a plastic cup. In chemistry, the density of water is often compared to many substances. Oil will float to the top of water because it is less dense. If an oil tanker leaks, then the oil will float on the ocean. Cleaning crews remove the oil by skimming it off the top of the water.
The water molecule is made up of two hydrogen atoms and one oxygen atom. The hydrogen in one water molecule is strongly attracted to the oxygen of another. Large oil molecules tend to clump together and bond. Oil molecules are made up of chains of atoms, which wrap around one another to cause the bond. Some oils are solid at room temperature. It is the molecular dissimilarity of the two substances and the strong mutual bonding between like molecules that cause oil and water to disassociate.
Water and oil both assist in the body's energy production. Fats and oils assist in long-term energy storage. Many seeds and fruit have energy stored as oil. Most oils that people eat come from soy and corn. A large part of the body's energy comes from a compound called adenosine triphosphate (ATP) -- the so-called energy current of life. ATP is like a rechargeable battery, and when you eat food or drink water, you are recharging your body.
Emulsion is the mixture of two or more unblendable liquids. An emulsifier is a substance that keeps the two separate parts of the emulsion combined together. Detergent is an emulsifier. It is attracted to both water and oil and helps them to blend together, which is how dishes are cleaned. The detergent removes oil from the plates into water. Mayonnaise is an example of an oil and water emulsion; egg yolk is the emulsifier.