Anyone who remembers “ice boxes” may recall the regular deliveries of massive blocks of ice that kept their meat and food cool inside the icebox, which was a precursor to the refrigerator. When those bricks melted and turned to liquid, however, they made a big mess.
With dry ice, you don’t have that difficulty. Because it’s exactly what it says on the tin: dry ice. And as it “melts,” it really transforms from a solid to a gas. Carbon dioxide gas that has been compressed to a high pressure is known as dry ice.
Carbon dioxide undergoes a solid-liquid-gas metamorphosis in response to pressure, similar to how the nature of water changes as the sea level rises, boiling at lower temperatures and pressures. CO2 is neither a liquid nor a gas at atmospheric pressure. It becomes a liquid when contained within the high-pressure chamber of a fire extinguisher.
To generate dry ice, liquid carbon dioxide is discharged from a high-pressure container, followed by fast evaporation of some of the gas into the air, resulting in near-instantaneous cooling of the remaining liquid to the freezing point. The ice/foam-like product is subsequently compressed, resulting in blocks of ice with a surface temperature of roughly -109F.
Dry ice is ideal for transporting perishables across long distances due to its high density and delayed evaporation following compression. Because any evaporation converts the dry ice to a gas that is discharged into the air, there are no messy puddles of water. As a result, dry ice should never be transported in a closed vehicle or stored in an enclosed space without enough ventilation. When CO2 levels in regular air reach to around 5%, they become poisonous.