Freezing CO2 Name: Frank V. Status: Educator Age: 20s Location: N/A Country: N/A Date: April 2002 Question: I saw the answer to the Coldest Recorded Earth Temperature at a chilly -128.6F (89.6C). This is 10 degrees (C) colder than the freezing temperature of Carbon Dioxide at atmospheric. My question is: "During Ice Ages, did the temperature at the poles ever get cold enough to freeze CO2?" Replies: The vapor pressure of CO2 = 760 mmHg = 1 atm @ 194.7 K. At 184 K = -89.2 the vapor pressure of CO2 = 300 mmHg = 0.4 atm (approx.). So unless the partial pressure of CO2 during the Ice Ages exceeded 300 mmHg = 0.4 atm the CO2 would sublime. I doubt that the partial pressure of CO2 would be that large. Vince Calder Updated -- Summer 2009 Frank, The freezing temperature of pure carbon dioxide at one atmosphere of pressure is -78.5 C (-109.3 F). In the Earth's atmosphere, at sea level, carbon dioxide constitutes only about 0.0004 atmosphere of partial pressure. Partial pressure refers to the amount of force (which is basically equivalent to weight in the Earth's atmosphere) that the gas exerts in air. At such a low partial pressure, a temperature of less than about -140 C is needed for carbon dioxide gas to be converted to solid carbon dioxide. At such low temperatures and one atmosphere pressure, carbon dioxide can not exist as a liquid, only as a solid. Since the lowest recorded temperature on Earth is -89.2 C at Vostok Station, Antarctica, it can be safely said that carbon dioxide gas has not frozen out of the air anywhere on Earth in recorded history. Furthermore, since Vostolk is not at sea level, the atmospheric pressure there is less than one atmosphere, resulting in less dense air, a lower partial pressure of carbon dioxide than at sea level, and thus an even lower temperature being required to convert carbon dioxide gas to a solid. The temperature being at "freezing", the temperature usually defined as resulting in a phase change from pure liquid to pure solid at one atmosphere of atmospheric pressure, does not imply that the solid phase of carbon dioxide will result at that temperature in the atmosphere. The partial pressure of the gas must be high enough for "saturation" of the air to occur (where the air can no longer hold all of the element as gas without some changing to the liquid or solid phase). At Vostok, Antarctica, the carbon dioxide content of the air is not high enough and the air temperature is not low enough for carbon dioxide gas to reach the saturation partial pressure (referred to as saturation vapor pressure for water); therefore carbon dioxide gas will not freeze out of the air. On Mars, the temperature is often low enough and the carbon dioxide content of the atmosphere is high enough (approximately 90% of the Mars atmosphere is composed of carbon dioxide gas) that carbon dioxide gas can "freeze out" of the atmosphere as a solid, even though the surface atmospheric pressure is much lower than it is on the Earth. David R. Cook Atmospheric Research Section Environmental Research Division Argonne National Laboratory Click here to return to the Environmental and Earth Science Archives

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