Co2
All gases are compressible (even all fluids and solids are, though much lesser), so there is no special name needed for this group because it is not special.
Butane gas IS compressible.
no salt is not compressible
Mercury, aka: "Quick Silver"
Yes, air is a compressible fluid. Water is not a compressible fluid.
Sponge is compressible because it is porous.
Compressible, Low Viscosity (compared to a liquid)
Sodium chloride is compressible.
All gasses are compressible
The correct name for the graduation hat is a mortarboard.
anything is compressible. it depends on how hard you compress it.