Dry Ice and Sublimation. It's sorta like magic.

'Tis the season for spooky sights and sounds, as October nears its end, and the Halloween season is upon us. Many of you will be donning your costumes and attending parties and festivities that embrace the spirit of All Hallow's Eve. As you venture out, you're likely to encounter a familiar and eerie sight – the mysterious fog and mist that seems to cling to the ground or rise like the steam from a witch's cauldron. What you might not realize is that the source of this otherworldly spectacle is none other than frozen carbon dioxide, commonly known as "dry ice."

Dry ice is the solid form of carbon dioxide (CO2). It's created through a process that involves pressurization and refrigeration, resulting in the transformation of gaseous CO2 into a solid. This process involves cooling the gas to the point where it becomes a liquid, and then allowing some of it to vaporize, akin to the evaporation of water. The remaining liquid experiences a significant drop in temperature, causing it to solidify into a snow-like substance. This solid CO2 can then be shaped into various forms, such as bars or pellets. Just how cold is dry ice? It's chillingly cold, with temperatures plummeting to as low as -109 degrees Fahrenheit (-78.5 degrees Celsius). The real magic of dry ice occurs when it returns to higher temperatures.

Under normal conditions of temperature and pressure, carbon dioxide doesn't exist in a liquid state. That's why when you exhale, you're not releasing liquid CO2 into the air, a scenario that would undoubtedly be detrimental. In regular conditions, the frozen CO2 undergoes a fascinating process known as sublimation.  Sublimation is the process of transitioning directly from a solid state

to a gaseous state. Essentially the liquid state is skipped altogether.  This is the reason why a solid block of dry ice appears to be smoking like smoldering ember (see picture to the right, courtesy of Karolina Halatek). When exposed to warmer environments, sublimation occurs at an accelerated rate.  For example, dropping dry ice into some warm water.  The effect is amplified greatly. It's this phenomenon that makes dry ice the perfect tool for creating spooky scenes and dramatic effects commonly associated with Halloween.

Moving beyond its theatrical applications, dry ice is an incredibly versatile substance with a wide range of practical uses in various industries. One of its most straightforward uses is in freezing and cooling. Thanks to its frigid temperature, it's often employed in freezing foods or as part of freezing mixtures in laboratory settings. In plumbing, dry ice can be used to freeze water inside a pipe, providing a temporary solution for repairs when shutting off the water source is not feasible. Once the repair is complete, the ice plug melts, and water can flow freely again.

A less well-known but widely used application of dry ice is in industrial cleaning. Dry ice's primary residue is the sublimated CO2, making it an ideal choice for cleaning situations where using water is impractical due to sensitive electronics or the risk of moisture-induced corrosion. Traditional solvents can be abrasive and lead to unintended damage. Enter dry ice – when compressed air propels dry ice pellets through a nozzle, they become highly effective at removing residues. This cleaning method has been utilized to remove inks, glues, oils, paints, molds, and even rubber. More recently, it has been employed to restore sensitive structures damaged by smoke after fires.

Speaking of fire, dry ice can be used to degas flammable environments. Consider large storage tanks where the removal of flammable vapors is crucial. Carbon dioxide is heavier than regular air, which means that by introducing dry ice pellets into such a tank with appropriate venting, sublimation carries the CO2 and the accompanying flammable vapors out, creating a safe working environment once the CO2 has completely vented. After all, nobody wants to be breathing in carbon dioxide. That would be very bad indeed.

This brings us to an essential point – while dry ice is incredibly versatile and aids in creating captivating effects, it should be handled with caution. The extreme cold of dry ice can cause severe frostbite burns if touched without proper protection, like gloves.  At -109 degrees, frostbit can occur in seconds. There have also been instances where dry ice pellets were dropped into drinks to produce the classic smoke effects, but not enough time was allowed for the pellets to dissolve, leading to dry ice to be ingested and internal burns (commonly referred to as cold burns). In certain environments, dry ice can even cause suffocation. Carbon dioxide is heavier than air and displaces oxygen, making proper ventilation a necessity when using it.  Without proper ventilation, CO2 buildup can occur leading to suffocation, asphyxiation, and even death.

So, as Halloween approaches, revel in the eerie sublimation of CO2. It's an excellent addition to your haunted house or spooky gatherings. Just remember to adhere to the necessary safety precautions when handling and using dry ice. It's a fantastic tool, but like any powerful magic, it must be wielded with care and respect for its potential dangers.