What is oil?
The answer to the question "What is oil?" can depend on whether you are talking about animal, vegetable or mineral.
ANIMAL OIL
Animal oil or fat is sometimes used in cooking or for other purposes, such as boot grease in years past. In the olden days, the only oil available for most cooking was lard. It was commonly used for frying, baking, and pastry and even as a bread spread.
VEGETABLE OIL
Vegetable oil is also used in cooking and baking. They are comprised of triglycerides and may be edible or inedible. Vegetable oil is also used in other products as well. Manufactured vegetable oil products include such things as soaps, paints, cosmetics, electrical insulators, synthetic motor oil, and hydraulic fluid. With today's technology, to say "what is oil?" may mean vegetable oil as easily as hydrocarbons. Vegetable oils such as linseed oil and castor oil have been used for lubricants.
One major problem with vegetable oil is their tendency to turn rancid. Since they are biodegradable, they are not as harmful to the environment as the petroleum based oils. Biodiesel is a type of vegetable oil which is used as fuel.
In terms of vegetable oil, what is oil refining technique which is effective? Newer methods use chemical extraction which is more effective and allows more utilization of the vegetable matter. The chemical used is often petroleum-based hexane. Health conscious people prefer to stay away from the petroleum processing and instead use the traditional extraction method which is typically either a screw press or a ram press.
MINERAL OIL
In one sense petroleum or crude oil can be considered a vegetable oil, because it is formed of highly compressed and heated organic materials. What is oil when it comes from drilling into vast reservoirs in the earth? It is usually string or formations of hydrocarbons along with some impurities such as sulfur and sulfides. These hydrocarbons removed from the underground formations using oil drilling rigs. The crude oil, as the liquid oil is called when it comes from the earth is sent to a petroleum refinery for one or more processes whereby the crude oil is separated out based on its rate of volatility.
The major components in determining what oil is include the heavy oils, also known as paraffins, the aromatics, naphtha and the gases. Each step in determining what oil is drawn off at a particular point is dependent upon the temperature at which the product begins to cool enough to condense.
All the petroleum products with the exception of the gases are referred as oil in one terminology or another, i.e. diesel oil, fuel oil, aviation oil, motor oil but the more general term usually refers to any lubricant whether vegetable or mineral or animal.
So, in the broadest sense, the question "What is oil?" can be answered with a response of "Any animal, vegetable or mineral substance which provides lubrication or energy to be expended.
It will be a long time before the question is fully answered because chemists are unlocking new secrets in the hydrocarbon molecules constantly.