Triple point constants carbon dioxide

The case of carbon dioxide, shown in Figure 12-28, differs from that of iodine in one important respect—the pressure at the triple point O is greater than 1 atm. A line at P = 1 atm intersects the sublimation curve, not the vapor pressure curve. If solid CO2 is heated in an open container, it sublimes away at a constant temperature of -78.5 °C. It does not melt at atmospheric pressure (and so is called "dry ice"). Because it maintains a low temperature and does not produce a liquid by melting, dry ice is widely used in freezing and preserving foods. 

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