Which of the statements are evidence that gases do not always behave ideally? co2co2 gas becomes dry ice (solid co2)(solid co2) at 1 atm1 atm and −78.5 ∘c.−78.5 ∘c. when two gases are mixed, they follow dalton's law of partial pressures. it is impossible to compress a gas enough so that it takes up no volume. at 4 k4 k and 1 atm,1 atm, helium is a liquid.