Ice-nine is a fictional material that appears in Kurt Vonnegut's novel Cat's Cradle. Ice-nine is described as a polymorph of water which instead of melting at 0 °C (32 °F), melts at 45.8 °C (114.4 °F). When ice-nine comes into contact with liquid water below 45.8 °C, it acts as a seed crystal and causes the solidification of the entire body of water, which quickly crystallizes as more ice-nine. As people are mostly water, ice-nine kills nearly instantly when ingested or brought into contact with soft tissues exposed to the bloodstream, such as the eyes or tongue.
In the story, it is invented by Dr. Felix Hoenikker and developed by the Manhattan Project in order for the Marines to no longer need to deal with mud. The project is abandoned when it becomes clear that any quantity of it would have the power to destroy all life on earth. A global catastrophe involving freezing the world's oceans with ice-nine is used as a plot device in Vonnegut's novel.
Vonnegut encountered the idea of ice-nine while working at General Electric. He attributes the idea of ice-nine to his brother Bernard, who was researching the formation of ice crystals in the atmosphere . A later account of the events attributes the idea to the chemist Irving Langmuir, who devised the concept while helping H.G. Wells conceive ideas for stories. Vonnegut decided to adapt the idea into a story after Langmuir's death in 1957.
- While multiple polymorphs of ice do exist (they can be created under pressure), none have the properties described in this book, and none are stable at standard temperature and pressure, that is above the ordinary melting point of ice. The real Ice IX has none of the properties of Vonnegut's creation, and can exist only at extremely low temperatures and high pressures.
- The ice-nine phenomenon has occurred with a few other kinds of crystals, called "disappearing polymorphs". In these cases, a new variant of a crystal has been introduced into an environment, replacing many of the older form crystals with its own form. One example is the anti-AIDS medicine ritonavir, where the newer polymorph destroyed the effectiveness of the drug until improved manufacturing and distribution was developed.
- Ice-nine has been used as a model to explain the infective mechanism of mis-folded proteins called prions which are thought to catalyze the mis-folding of the corresponding normal protein leading to a variety of spongiform encephalopathies such as kuru, scrapie and Creutzfeldt–Jakob disease.
- Ice VII, stable at room temperature but only under very high pressures. 
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