How is gasoline different from diesel, and why does it damage the car if you put the wrong kind in the tank?
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You got some good explanations on the mechanical side, so I’ll chime in on the chemistry side. Gasoline and diesel are made up mostly of hydrocarbon chains. Carbon forms four bonds and hydrogen forms one, so these form the basis for an enormous amount of chemicals. Methane is the simplest hydrocarbon, which is a carbon with four hydrogens attached. If you pop a hydrogen off of one side of two methane molecules and attach them to each other, you have an ethane molecule. Pop a hydrogen off one end of either side, and you can continue adding links to the chain for a good long while; methane, ethane, propane, butane, pentane, hexane, heptane, octane, nonane, decane, etc.
Shorter alkanes (what the simple carbon-carbon chain structures are called) are obviously much lighter and more volatile, and increasing the chain length makes them heavier and less volatile: The first few are gases, becoming increasingly easier to condense into a liquid as they get heavier. The next few become liquid, but still evaporate pretty quickly. Once they get long enough, they start becoming pretty thick and viscous and don’t evaporate pretty quickly at all. Long enough, and they start becoming solid at room temperature and you get paraffin wax.
As this relates to your initial question of how gasoline is different from diesel; gasoline is compromised of shorter chains on average than diesel is. Keep in mind that the actual substances you will encounter in a practical setting have dozens-hundreds of different actual individual chemicals in them that are more complex than simple alkane chains, but this is the general idea behind why they behave differently despite being so similar.
They’re separated from crude oil via fractional distillation; I’m not sure exactly how it works, but it’s basically heating the whole mixture up in a giant container, and then collecting them from different sections of the container as the various densities cause the chains to settle into different layers.
Edit: Changed “fracking” to “fractional distillation”; turns out it’s actually short for “hydraulic fracturing”, which is the technique utilized to extract the crude oil from the ground.
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