Why can air be compressed but liquids can’t?
In: Physics
Water can expand up to 1700 times when evaporating. There will be significantly more empty space between the molecules that can be removed when it’s in gas form than when it is in liquid form.
You can compress them a little bit, but it’s generally much harder than with a gas.
One of the things that makes a liquid a liquid, and not a gas, is that the molecules in a liquid interact with each other strongly. In a gas, molecules mostly don’t interact unless a couple happen to collide and bounce off each other. But in a liquid each molecule is connected to its neighbors by intermolecular interactions. This is especially notable with water molecules hydrogen bonding to each other, but other things do it to.
These interactions are why if you put liquid into a container, it hangs out at the bottom instead of expanding to fill the whole container like a gas would. The molecules are basically stuck together
In order to have these interactions in the first place and be classed as a liquid, molecules have to already be pretty close to each other so you can’t easily squeeze them closer together. And the molecular forces try to keep them a certain distance apart, causing them to resist squeezing further. In contrast, there’s a lot of extra space with gasses and you can squeeze them until the molecules are much closer together (at which point they often start interacting and become a liquid).
So liquids are not very compressible, while gasses are very compressible.
This is precisely why you need a turbo charger to get more power out of your engine. (Four stroke internal combustion gasoline powered engine, that is.)
You can always squirt in more gasoline using the fuel injectors, no problem. Because the liquid basically does not compress.
But technically the “fuel“ needed by your engine is the mixture of both gasoline and air. And you cannot just squirt in more air. The only way to get more air in, when you squirt in more gasoline, is to first compress that air so there is enough of it to be matched with the additional gasoline.
This is how you increase the total “fuel“ injected into the cylinder prior to combustion. And this is why a turbo charged engine is more powerful, because it can use more fuel, by compressing the air.
Oh and by the way it’s called a “turbo“ charger because it has a turbine. (And the turbine spins by using the exhaust coming from the engine.) And that turbine is connected to a compressor, which is what compresses the air.
So this is a real-world, practical answer to your question: gasoline does not compress, air does, and you can see that in action in a turbo charged engine.
Latest Answers