The question: at what temperature does water freeze? Today the outside temperature is in the mid-90s (degrees F) and so naturally one might begin to think about an ice-cooled drink. Which leads one to think about ice. If you don’t have an ice maker in your refrigerator and someone forgot to refill the ice cube tray (or even worse, returned it empty to the freezer), perhaps you wonder how long it will take until ice cubes are ready. Perhaps you are overly anxious about it and when you check the cubes are part solid-ice, part water, and part a thin layer of almost-ice crusting the top of the tray. What can you do to speed up the process…. apart from shut the freezer door and be patient? This is where it pays to be curious.

Since elementary school we have known that water freezes at thirty-two degrees Fahrenheit. Later on we came to know other equivalent answers: zero degrees Celsius and 273 degrees Kelvin. But at this point in life we know that temperature is only one possible variable. Does changing atmospheric pressure affect the point at which water freezes? Technically yes, but hardly worth the effort. It is also a bit counterintuitive. You’d think increasing the pressure would push molecules closer together and somehow make it easier to form a solid, aka ice. Normally, yes, but not in the case of water. Water’s unique crystalline structure causes the opposite effect. But if you could convert your freezer into a working vacuum chamber you can raise the temperature at which water freezes all the way up to 0.1 degrees Celsius.

But in case you are thinking “that’s all very interesting…sort of….for all practical purposes the answer remains water freezes at zero degrees Celsius. For home ice cube purposes, true, but in clouds, scientists have found supercooled water droplets as chilly as minus 40 C, and in a lab they cooled water to minus 46 C before it froze. Should your refrigerator have a “supercooling” feature, you can supercool water at home. Just place a tray of distilled water in your supercooling freezer, …actually don’t try this at home… but if you insist, while wearing Antarctica-rated gloves, open the freezer, nudge the supercooled distilled water and it will flash into ice cubes.

What is equally interesting is that if you simply place an open container of distilled water in the regular freezer, once it reaches zero degrees Celsius, if you shake the container almost the entire content of the container will crystallize.  (If it already crystallized, someone was probably looking for ice cubes and ruined the experiment)

Freezing usually doesn’t happen right at zero degrees because ice needs a nucleus — a seed of ice around which more and more water molecules arrange themselves into a crystal structure. The formation of these seeds is called ice nucleation. Nucleation is so slow for pure water at zero degrees that it might as well not happen at all. But in nature, impurities provide surfaces for nucleation, and these impurities can change how quickly and at what temperature ice forms. Impurities are practical – not for ice cubes – but for seeding clouds for rain and snow-making at ski resorts. Snow guns used at ski resorts spray water into the air mixed with an ice-nucleating agent, often proteins from the bacterium Pseudomonas syringae – whose name alone might qualify as an impurity.

You might not want to add bacterium to your ice cube tray. But salt, sugar, or alcohol technically raises the temperature at which those ice cubes will form, but not so much that you’d notice the difference.

For all practical purposes, the elementary school answer remains the correct answer. But if you are curious enough to spend time researching and writing, by the time you’re done, the correct answer remains correct, but the ice cubes are ready.