By definition, a coolant is simply a liquid that's circulated to cool down systems that both generate a lot of heat, and need to remain relatively cool to function properly. Usually, all you need is a liquid that can soak up a lot of heat and get circulated elsewhere, so it can be cooled back down; it's usually not particularly cold when it enters the system. It all depends on the process: Things like superconductors will need liquid helium coolant close to absolute zero, while certain nuclear reactors need to use molten metal (sodium to be exact) due to their absurd heat and high resistance.
Fiction, however, has taken the idea that coolant is for cooling things to its natural conclusion, and thus the bigger, more complex devices need substances far below the freezing point to stay functional. Any breaches in a coolant pipe will instantly flash-freeze anything in the vicinity, turning the whole section fragile, and purposefully venting the coolant acts like an Emergency Weapon that hits like a tidal wave of liquid nitrogen and quickly turns everyone present into easily-shattered statues.
This is particularly common in Science Fiction, where it could be excused as future engines, vessels, and other machinery running so hot it needs icy Phlebotinum to stay functional in the first place, but it's by no means absent from other settings, even otherwise realistic ones (which don't always have the aforementioned superconductor excuse).
A Sub-Trope of Kill It with Ice. Contrast Convection, Schmonvection, for when people can approach hot substances like lava up close without getting cooked (unless they touch it directly). For more intentional, energy-based chillers, see Freeze Ray.
- Pacific Rim: Gypsy Danger's solution to being grappled by a particularly hardy Kaiju is to vent its coolant on it, freezing its whole tail, shattering it, and softening the creature up for more attacks.
- Sunshine: The ship's mainframe panel needs this kind of coolant at all times, as the ship warns; Mace gets freeze-burns just dipping his hand in retrieving a dropped spanner. Things get much worse when he needs to repair the panel, and thus has to outright dive into the stuff to work.
- Bats: The plan to get rid of the titular bats is to use an industrial refrigeration unit to disperse coolant above and into their cave while they sleep, so they go into hibernation and eventually freeze to death.
- Demolition Man: John Spartan and Simon Phoenix duke it out in the cryo-prison at the climax. During the fight, a vial of coolant for the chambers shatters, flash freezing the area. Spartan is able to jump free in time; Phoenix isn't so lucky, and Spartan delivers the coup de grace by kicking Phoenix's frozen head off his shoulders.
- James Bond:
- In GoldenEye, Boris survives the destruction of the Goldeneye base in Cuba. But just as he stands up and declares "Yes! I AM INVINCIBLE!", the liquid nitrogen coolant tanks rupture, dousing him in liquid nitrogen and flash-freezing him into a statue.
- The Man with the Golden Gun: The technician monitoring Scaramanga's solar power facility falls into the vat of liquid helium that cools the machinery, freezing him to death.
- The Devil's Messenger: In "The Girl in the Glacier", Dr. Seastrom knocks Dr. Olsen to the floor and possibly kills him during an argument in the cold room. He then makes sure the doctor's dead by opening a pipe and dousing Olsen in the freezing solution they are using to keep the titular Human Popsicle frozen.
- Men in Black 3: In the Final Battle, K blasts a tube delivering coolant to a rocket, causing his opponent Boris the Animal's arm to freeze solid. K then shatters the arm with a blast from his laser gun.
- In the gamebook Rebel Planet, if you mishandle the final task of destroying the Arcadian's computer, its liquid helium coolant floods out over the floor freezing your feet solid, you then snap them off at the ankles when you try to move.
- Aversion via having Shown Their Work in BattleTech (2018): in urban maps, if a generator's cooling system is damaged, it will explode and splash everyone in a hundred-meter radius with coolant... but since this is coolant was up until recently doing work cooling a generator, it's boiling hot and will dump an uncomfortable 30 heat points (of a theoretical maximum of 100) onto anyone unlucky enough to be within the explosion radius. Interestingly, once the coolant has been spread around, venting heat to the atmosphere, anyone who walks through the puddle receives a trickle of passive heat reduction instead. As the coolant is still designed to absorb heat, it will therefore wick away a few points of heat from the surface of any Humongous Mecha that steps in it.
- Hardspace: Shipbreaker: Coolant tanks and pipes are one of the many hazards you can face while disassembling ships for salvage. They're fragile, and damage will cause leaking that'll utterly freeze anything nearby, turning it frail and sometimes outright shattering it; it can be just as damaging as the fire-starting fuel when it explodes, too, precisely because of the brittleness it induces.
- Horizon Zero Dawn and Horizon Forbidden West feature Chillwater, coolant used by machines that can be harvested to make frost ammunition. The Frost Bellowback carries large amounts of Chillwater in the liquid container on its back, which it can fire at you as a ranged attack.
- Warframe: One option when sabotaging an enemy spaceship is to insert a coolant cell into the reactor core, which will cause ship-wide leaks of coolant that turns into freezingly-cold patches of ice.
- Metal Gear Rising: Revengeance: Raiden defeats Mistral by tricking her into stabbing a coolant tank and slicing it further open while she's stuck, instantly freezing Mistral and allowing Raiden to chop her to pieces.
- Combat levels in the original Mass Effect occasionally feature barrels of coolant that, when shot, snap-freeze any enemies in their vicinity.
- Resident Evil 4 has a fight against Verdugo in the basement. The best way to damage it is by knocking over liquid nitrogen canisters - while it's not completely frozen, it's still slowed for about 10 seconds and is vulnerable to damage.
- Discussed in Quake IV, near the end of the "Nexus Hub Tunnels" level. After reaching the Airlock, Lieutenant Voss gives the order to open it, to which Technician Strauss replies "negative, stay where you are". When asked why, Strauss responds that the soldiers' lungs would freeze in seconds if they opened it in its current state; the structure they're on requires such obscene amounts of cooling to function, the spot they need to reach would chill them to death in seconds. Then he suggests first bringing up the temperature of the area. Cue an Escort Mission to the controls to divert the coolant.
- SWAT Kats has Razor discover that Doctor Viper's Plant Monster mooks are susceptible to cold, and radios this info to Chance. Chance then "borrows" a tank of rocket coolant, and flies it to the office building where Doc Viper is poised to unleash the spores of his Blob Monster upon Megakat City. The coolant flash-freezes the spore creature and foils Viper's Evil Plan.
- The Tom and Jerry cartoon "Mice Follies" has Jerry and Nibbles turn on every tap in the kitchen until the overflow leaves an inch or more of water on the floor. They then reroute the freon pump of the refrigerator into the water, which freezes the water all the way back to the spouts. Tom dons a pair of ice skates in order to pursue the mischievous rodents, but they thwart him by switching the freon pump to "thaw," which reverts all the ice back into water. The mice conclude by switching the pump to "quick freeze," which instantly converts all the water back into ice, and turns poor Tom into a feline popsicle.