Dry ice and liquid nitrogen are two substances commonly used to achieve extremely low temperatures. Understanding the differences in their composition, properties, and temperatures is important for appreciating their specific applications and safe handling requirements.
Dry Ice: The Solid Form of Carbon Dioxide
Dry ice is the solid state of carbon dioxide (CO2), a naturally occurring gas that is part of the Earth’s atmosphere. This solid material maintains a fixed, intensely cold temperature of -78.5°C (-109.3°F) at standard atmospheric pressure. Its extreme cold makes it a convenient and effective chilling agent for shipping frozen goods or preserving perishable items when mechanical refrigeration is not available.
The defining characteristic of dry ice is the scientific process known as sublimation. Instead of melting into a liquid, like water ice, solid carbon dioxide transitions directly into a gas when it absorbs heat from its surroundings. This avoidance of a liquid phase is why it is called “dry,” and it prevents the messy residue associated with traditional cooling methods.
The gas produced during sublimation is carbon dioxide, which is heavier than air and often used to create dense, dramatic fog effects in theatrical productions.
Liquid Nitrogen: The Cryogenic Liquid
Liquid nitrogen (LN2) is nitrogen, the most abundant gas in the atmosphere, condensed into a liquid form at an incredibly low temperature. As a cryogenic liquid, it has a boiling point of approximately -196°C (-320.4°F) at standard pressure.
When liquid nitrogen encounters warmer air or a warmer object, it rapidly undergoes boiling and evaporation, converting back into a gaseous state. This process is highly energetic and produces a large volume of nitrogen gas, often seen as a thick white cloud of condensed water vapor.
Liquid nitrogen’s extreme cold and rapid cooling ability make it a staple in scientific and medical fields. Common uses for this substance include the preservation of biological materials, such as blood, reproductive cells, and tissue samples, and in cryotherapy for the removal of certain skin abnormalities. The rapid cooling it provides is also utilized in industrial processes for shrinking metal parts to fit them together in a process called shrink-fitting.
Comparing Temperatures and Safe Handling
Liquid nitrogen is substantially colder than dry ice, with a temperature difference of over 117°C (211°F) between their respective phase change points. While dry ice maintains a temperature of -78.5°C as it sublimates, liquid nitrogen’s boiling point is -196°C.
Both substances demand strict safety protocols due to the extreme temperatures involved. Direct contact with either dry ice or liquid nitrogen can instantly freeze skin tissue, resulting in severe frostbite or cryoburns. Therefore, insulated gloves and eye protection are always necessary when handling them.
A major hazard common to both is the risk of asphyxiation, as they both convert back into a gas and displace oxygen in the surrounding air. When a small volume of liquid nitrogen boils, it expands by a factor of nearly 700 times, which can quickly deplete the oxygen in a confined space. Similarly, dry ice sublimation creates carbon dioxide gas, which is heavier than air and can accumulate in low areas.
It is also important to never store either substance in a tightly sealed container. The dramatic expansion that occurs as the material changes from a solid or liquid to a gas will rapidly build pressure, potentially causing the container to rupture or explode. Using these materials safely requires proper ventilation and containers specifically designed to vent the accumulating gas.