Dry ice

Dry ice is the solid state of carbon dioxide and is widely recognised for its distinctive physical properties and versatile industrial applications. At normal atmospheric pressure, carbon dioxide does not possess a liquid phase and transitions directly from solid to gas through sublimation. This behaviour, combined with its extremely low temperature, has resulted in its extensive use as a refrigerant, in theatrical fog production, and in numerous commercial and scientific settings.

Physical and Chemical Properties

Dry ice consists of carbon dioxide molecules, each composed of a single carbon atom covalently bonded to two oxygen atoms. The solid is colourless, odourless, and non-flammable, exhibiting a non-polar molecular structure. As a result, only van der Waals forces act between the molecules, leading to low thermal conductivity and high electrical resistivity. When dissolved in water, carbon dioxide forms carbonic acid, thereby lowering the pH of the solution.
A defining characteristic of dry ice is that sublimation occurs at atmospheric pressure, enabling the direct transition to gaseous carbon dioxide without forming a liquid. This process takes place at approximately –78.5 °C, a temperature that accounts for its effectiveness as a cooling agent but also its potential to cause frostbite if handled without proper protection. Density increases as temperature decreases, while the enthalpy of sublimation is approximately 571 kJ per kilogram. These combined properties make dry ice a highly efficient refrigerant, leaving no liquid residue apart from incidental frost formed from atmospheric moisture.
The phase behaviour of carbon dioxide underlines its inability to exist as a liquid at pressures below the triple-point threshold. At such pressures and low temperatures, any warming leads directly to sublimation, and cooling produces deposition, the reverse transition from gas to solid.

Historical Development and Discovery

Dry ice was first identified in the early nineteenth century. In 1835, the French inventor Adrien-Jean-Pierre Thilorier provided the earliest known published description of solid carbon dioxide after observing its formation during experiments involving liquefied carbon dioxide in a pressurised cylinder. His work on gas compression machinery, recognised by the French Academy of Sciences, provided the foundations for later industrial production.
Commercialisation began in the United States in the 1920s. In 1924, Thomas B. Slate applied for a patent to market solid carbon dioxide for practical use. By 1925, the DryIce Corporation of America had trademarked the term Dry ice, facilitating its widespread adoption, particularly in refrigeration applications before the widespread availability of mechanical cooling.

Industrial Manufacture

Dry ice production today relies on carbon dioxide-rich gases often generated as by-products of industrial processes such as ammonia synthesis, petroleum refining, or large-scale fermentation. The manufacturing procedure generally follows several steps:

• Collection of gas with a high concentration of carbon dioxide.
• Pressurisation and cooling until the gas liquefies.
• Rapid depressurisation, causing partial vaporisation of the liquid and consequent cooling.
• Solidification of the remaining liquid into a snow-like solid.
• Compression of this material into blocks or pellets.

Dry ice is commonly sold in three standard forms: large blocks, cylindrical pellets, and very small high–surface-area pellets. Blocks sublimate more slowly owing to their low surface-to-volume ratio, making them suitable for shipping and long-distance transport. Pellets, typically a few millimetres in diameter, are preferred for laboratory use and small-scale commercial settings. Specialised micro-pellets are employed in dry ice blasting, rapid cooling, fire suppression, and other technical roles.
An alternative source of dry ice arises as a by-product of cryogenic air-separation plants, where carbon dioxide must be removed to prevent clogging of equipment cooled to temperatures required for liquefying nitrogen and oxygen. This captured carbon dioxide can then be processed into commercial dry ice.

Commercial and Scientific Applications

Dry ice plays a crucial role in refrigeration, particularly in environments where mechanical cooling is not feasible. It is extensively used to preserve frozen foods, including ice cream, and to ship biological samples or materials requiring cold-chain handling. Some vaccines that necessitate ultra-low storage temperatures rely on dry ice during transport and temporary storage.
In laboratory and industrial contexts, dry ice supports a range of processes:

• Flash freezing of foodstuffs and biological material.
• Carbonation of beverages via controlled dissolution of carbon dioxide.
• Oil spill solidification, allowing easier removal of contaminants.
• Preservation of grains, inhibiting insect activity by displacing oxygen.
• Prevention of rancidity in oils and fats during storage.

When placed in water, rapid sublimation produces dense, low-lying fog. This fog originates from chilled water droplets formed within the bulk water rather than from atmospheric moisture, creating a dramatic effect widely utilised in theatres, nightclubs, and amusement attractions.
Dry ice is also applied in niche medical and domestic contexts. It may be used to freeze and remove warts, although liquid nitrogen is generally preferred due to its lower temperature. In plumbing, dry ice can freeze water in pipes to form temporary plugs, allowing repairs without shutting down entire water systems.

Safety Considerations and Handling

The extreme cold of dry ice makes protective equipment such as insulated gloves essential to prevent frostbite. Although gaseous carbon dioxide is not highly toxic, its accumulation in confined spaces can lead to hypercapnia, a condition marked by elevated carbon dioxide levels in the bloodstream. For this reason, adequate ventilation is critical when using dry ice in enclosed environments. The sublimation process continually releases gas, making the storage of dry ice in sealed containers dangerous owing to the risk of pressure build-up and potential rupture.
Dry ice must therefore be handled in ventilated areas and stored in insulated containers that allow gas to escape safely. The absence of residual liquid and its rapid cooling capacity have nonetheless made it indispensable in situations requiring portable, efficient refrigeration and controlled atmospheric modification.