Under normal atmospheric pressure, carbon dioxide does not have a liquid phase. When the temperature falls below -78.5º C or -109.3º F, the gas transforms directly into solid through deposition. In the other direction, the solid, also known as dry ice, does not melt into a liquid but sublimates directly into a gas. Liquid carbon dioxide only exists at pressures above five times normal atmospheric pressure. Converting between gas and liquid, therefore, depends entirely on pressure. For this calculation, convert between gas at one atmosphere of pressure around the freezing point of water, and liquid at room temperature and 56 atmospheres of pressure.

Determine the mass of the CO2 gas. At one atmosphere and 0 degrees C, the density of CO2 gas is 1.977 grams per liter. Multiply the volume, in liters, by 1.977 to get the number of grams of CO2. As an example, consider 1,000 liters of CO2 gas. Under these conditions, it would have a mass of 1977 grams, or 1.977 kilograms.

Divide the mass by the density of liquid CO2. Liquid CO2 at room temperature and 56 atmospheres is 0.770 grams per milliliter. In the example, this calculation would give a result of 2,567.5 milliliters.

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Convert units so that they can be compared. Divide milliliters by 1,000 to get liters for a comparison with the gas phase. In the example, the result is 2.5675 liters of liquid from 1,000 liters of gas.