Given the task definition and input, reply with output. Given a scientific passage and an answer, generate a question for the given answer.

Passage: Pressure is given in units of millimeters of mercury. We can either convert this to atmospheres or use the value of the ideal gas constant that includes the mmHg unit. We will take the second option. Substituting into the ideal gas law,. Answer: pressure
What property of certain states of matter can be given in units of millimeters of mercury?