Question

What is dry ice?

Answer 1

Dry ice is carbon dioxide in its solid state. It's usually used as a cooling agent, but it's also employed in theatre fog machines for dramatic effects. Its benefits include a lower temperature than water ice and the absence of residue (other than incidental frost from moisture in the atmosphere). When mechanical cooling is not available, it is effective for preserving frozen items.

Complete answer:
Carbon dioxide $\left( {C{O_2}} \right)$ is a molecule made up of a single carbon atom linked to two oxygen atoms. Dry ice is the solid form of this molecule. Dry ice is colourless, odourless, and non-flammable, and when dissolved in water, it can drop the pH of a solution, generating carbonic acid $({H_2}C{O_3}).$
At Earth's atmospheric pressure, Dry ice sublimates at 194.7 K because of the intense temperature, handling the solid without protection from frostbite is perilous. While it is generally non-toxic, the outgassing from it can cause hypercapnia (excess carbon dioxide in the blood) if it builds up in confined spaces.
Sublimation occurs when carbon dioxide transforms from a solid to a gas with no intervening liquid form at pressures below 5.13 atm and temperatures below $- {56.4^o}\,C$ (the triple point). Deposition is the process through which carbon dioxide transforms from a gas to a solid state (dry ice). Sublimation and deposition happens at 194.7 K at atmospheric pressure.

Note:
Dry ice was originally detected in 1835, according to popular belief, by French inventor Adrien-Jean-Pierre Thilorier (1790–1844), who published the first account of the substance. When he opened the lid of a big cylinder containing liquid carbon dioxide, he noticed that most of the liquid carbon dioxide soon vanished. The container was left with solely solid dry ice.