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I'm not talking about normal physics here, as it would be extremely hard to get such a situation to arise, this is more a hypothetical question.

If air was the same density as water, while still keeping all of it's current properties as a gas, would you be able to swim in it?

Furthermore, would the buoyancy be equal to water, or would it be lower due to being a gas and not a liquid?

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Yes, but not very well.

If air were dense enough to swim in, assuming it remained in its gaseous phase, its compressibility would play a significant role. Even if air was the same density as water, its properties as a liquid make it a lot less compressible, since gas compressed will undergo a phase transition to a liquid but water will not compress into ice. Even at 150 ATP water compresses <1% (source).

In aerodynamics we see that we lose a lot of the properties of smooth flying when gases no longer behave ideally (e.g. incompressible), which would make resistance and drag forces more unpredictable than in water in terms of fluid dynamics.

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The equations for buoyancy only take density of the fluid into account and not the phase of the fluid. Eg we use the same equations to calculate the buoyancy of a helium weather balloon in air, as we use for an object submerged in a liquid. I do not not know of any substance with the density of water, that is gas at normal pressure. However, it might be possible to compress certain gases to a high enough density and temperature, without them becoming a liquid. The high pressure would compress the lungs of a human to such an extent that the reduction in their volume would decrease their buoyancy and they would probably sink. Gases generally have a lower viscosity that a fluid, making it harder to propel the gas away from yourself in order to propel yourself forwards or upwards.

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  • $\begingroup$ @SolomonSlow That is not what I was thinking. In fact I mention the words "submerged in a liquid". I deliberately avoided discussion of buoyancy at the surface because then we have to deal with buoyancy in air and in liquid at the same time. $\endgroup$
    – KDP
    Commented Jul 11 at 3:22

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