Oils

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An oil is a substance that is in a viscous liquid state ("oily") at ambient temperatures or slightly warmer, and is both hydrophobic, (immiscible with water, lit. water fearing) and lipophilic (miscible with other oils, literally "fat loving"). This general definition includes compound classes with otherwise unrelated chemical structures, properties, and uses, including vegetable oils, petrochemical oils, and volatile essential oils. Oil is a nonpolar substance.

Etymology
Oil is a non-scientific term used to refer to certain diverse and unrelated compounds sharing the same physical properties (such as viscosity and a hydrophobic nature), while ignoring related compounds. The compounds found in cooking oil are chemically very similar, almost identical, to those found in butter and very different from those found in diesel fuel, but while diesel is an oil, butter is not. Indeed diesel is once again very similar to natural gas, but gas is certainly not oil. This disparity stems partly from the fact that oils must be liquid at room temperature, and thus only certain liquid chemicals in many unrelated families are recognised, collectively, as 'oil'. Scientists, instead of using the term 'oil', adopt the terms lipids and other terms to denote them instead.

Mineral oil
All oils, with their high carbon and hydrogen content, can be traced back to organic sources or space. Mineral oils, found in porous rocks underground, are no exception, as they were originally the organic material, such as dead plankton, accumulated on the seafloor in geologically ancient times. Through various geochemical processes this material was converted to mineral oil, or petroleum, and its components, such as kerosene, paraffin waxes, gasoline, diesel and such. These are classified as mineral oils as they do not have an organic origin on human timescales, and are instead derived from underground geologic locations, ranging from rocks, to underground traps, to sands.

Other oily substances can also be found in the environment, the most well-known being asphalt, occurring naturally underground or, where there are leaks, in tar pits.

Petroleum and other mineral oils, (specifically labelled as petrochemicals), have become such a crucial resource to human civilization in modern times they are often referred to by the ubiquitous term of 'oil' itself.

Organic oils
Oils are also produced by plants, animals and other organisms through organic processes, and these oils are remarkable in their diversity. Oil is a somewhat vague term to use chemically, and the scientific term for oils, fats, waxes, cholesterol and other oily substances found in living things and their secretions, is lipids.

Lipids, ranging from waxes to steroids, are somewhat hard to characterize, and are united in a group almost solely based on the fact that they all repel, or refuse to dissolve, in water, and are however comfortably miscible in other liquid lipids. They also have a high carbon and hydrogen content, and are considerably lacking in oxygen compared to other organic compounds and minerals.

Food oils
Many edible vegetable and animal oils, and also fats, are used for various in cooking and food preparation. In particular, many foods are fried in oil much hotter than boiling water. Oils are also used for flavoring and for modifying the texture of foods e.g Stir Fry.

Health advantages are claimed for a number of specific oils such as omega 3 oils (fish oil, flaxseed oil, etc) and evening primrose oil.

Trans fats, often produced by hydrogenating vegetable oils, are known to be harmful to health.

Fuel
Almost all oils burn in air generating heat, which can be used directly, or converted into other forms of energy by various means, for example, heating water into steam which is funneled into a turbine which turns a huge magnet. This spins and generates electricity. Oils are used as fuels for heating, lighting (e.g. kerosene lamp), powering combustion engines, and other purposes. Oils used for this purpose nowadays are usually derived from petroleum, (fuel oil, diesel oil, petrol (gasoline), etc), though biological oils such as biodiesel are gaining market share.

Heat transport
Many oils have higher boiling points than water and are electrical insulators, making them useful for liquid cooling systems, especially where electricity is used.

Lubrication
Due to their non-polarity, oils do not easily adhere to other substances. This makes oil useful as lubricant for various engineering purposes. Mineral oils are more suitable than biological oils, which degrade rapidly in most environmental conditions.

Painting
Color pigments can be easily suspended in oil, making it suitable as supporting medium for paints. The slow drying process and miscibility of oil facilitates a realistic style. This method has been used since the 15th century.

Petrochemicals
Crude oil can be processed into plastics and other substances.

Other Usages
Sulfuric acid has been called oil of vitriol in pre-scientific times, due to its syrupy consistency. Even in modern times, sulfuric acid is sometimes called vitriolic acid, and caustic personalities are called "vitriolic." Sulfuric acid is not a petrochemical, and in modern parlance, is not an oil.

Religion
Oils have been used throughout history as a fragrant or religious medium. Oil is often seen as a spiritually purifying agent. It is used in religious ceremonies, such as chrism or baptism.