There’s the oil from the oil wells which is inflammable but it’s not in the form of petrol or diesel, which is also decided from the same oil, but petrol isn’t oily to the touch the way regular cooking oil is. But cooking oil isn’t as inflammable, irrespective of whether it comes from sunflowers, or groundnuts, or coconuts or some other source. However, the oil on our skin is oily, but again not inflammable.
Help me wrap my heart around all these different ‘oils’ – and why some are more oily to the touch vs those that are not, and why some are inflammable vs those that are not.
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In a nutshell, you start with a molecule called a “hydrocarbon” which is essentially a carbon atom with a bunch of hydrogen atoms linked to it. Carbon/Hydrogen molecules are quite stable which is nice, but both elements prefer being bonded to Oxygen. So if you give them access to a tiny bit of energy (heat) and some oxygen they’ll break the bonds and form Water (H20) and Carbon Dioxide (C02).
Now if you can start chaining more carbons and hydrogens together to get longer and longer molecules. Because chemists are naming geniuses; you tell how many carbon atoms are in a given hydrocarbon chain by it’s name, methane has (1), propane has (3), Butane has (4) carbons, Pentane has (5), hexane has (6), Octane has (8), diesel has (12), and on and on.
In general the lower carbon molecules, like methane and propane, are highly combustible gases. The mid-to-higher single digit molecules are highly combustible liquids, once you get into the diesel zone though the molecules become increasingly stable.
For example, if you light a match in a room filled with methane, you’re in for a bad time. If you light a match in a room full of gasoline vapor, you’re in for a bad time. But you can actually put out a match in liquid gasoline as it’s fairly stable (DO NOT TRY THIS), and diesel fuel can actually *put out* fires, you have to compress it first to use it as fuel.
This trend goes all the way to home heating fuel and ship fuel (so called “Bunker Fuel”) which is essentially sludge.
The point being that natural oil deposits contain a wide variety of all of the above hydrocarbons. While some might be heavier on methane and others lean towards heavier diesel, they are a blend. Before raw oil can be sold to the consumer as fuel it needs to be processed to separate the various gases and liquid fractions, which may even go on to receive further processing and separating for the consumer market.
Per your prompt, cooking oil isn’t inflammable, just google “home oil fire” and you’ll learn that it’s very, very much so flammable, it’s just stable to higher temperatures. Considering the above trends, most cooking oils are in the (18) carbon-and-higher range making them quite stable. But consider that cooking oil can be repurposed as bio-fuel, it’s actually not that far off from diesel in that regard.
Oil from wells is crude oil.
That stuff is split via a process called fractional distillation, producing everything from LPG (liquid petroleum gas, often butane or propane), kerosene (aviation fuel), gasoline (petrol), all the way to heavy stuff like bitumen (the tar they put in expansion cracks on roads).
The original crude oil and all of its products are hydrocarbons, they’re not actually oils in the same chemical sense as the others you listed, naming coincidence
Oil pressed from plant matter is generally comprised of a variety of natural acids and fats. The relative ratio of these in a mixture goes some way to determine its properties
The oil secreted by your skin is called Sebum, much like plant-based oils it’s comprised of fatty acids plus a load of triglycerides (which themselves contain more fatty acid). Honestly human skin oil is fairly close to vegetable fats.
I suspect (guess) that the ‘oily’ touch sensation is actually coming from the fats involved, but it’s not something I’ve ever examined (mainly Chem background)
Oil is chains of hydrocarbons. Biological oil is fat where each hydrocarbon chain have a lipid acid end and three of these lipid acids is connected to a glyceride molecule. They have a lot of the same properties as mineral oil but are still distinct.
The other major factor in the property of oil is the length of the hydrocarbon chains. Shorter chains is lighter which means they catch on fire much easier, flows much faster, etc. The shortest of these oils are natural gas which is technically the same thing but just very light. The longer the hydrocarbon chains the heavier the oil. They have a hard time catching fire (diesel) and runs much more slowly. Eventually they become tar, butter, pitch or tallow.
The lubricity of each oil is a lot more complex. All oils are in general slippery but there are a lot of variations. The oils are not always straight hydrocarbons but often have kinks and loops and stuff which change their property. And small trace elements of esters or sulphur compounts can make the oil even more slippery.
Oil wells generally contain a mix of different lengths of hydrocarbons while vegitable oil or animal fat is generally a lot more consistent. When we destill and process the oil from the wells this too becomes far more consistent. As for the flamability heavier oils like cooking oil or diesel is hard to burn in open air but if you heat it up enough it will catch fire. The easiest way though is to use a whick which helps it evaporate from the heat of its own flame. This is how old oil lamps worked and how a candle works. You can even make the oil in your skin into a candle if you want to.
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