You can find striking videos circulating on social media: A person tosses a cup of boiling water overhead in freezing weather, and the liquid instantly turns into a thick white cloud. While this appears like magic, simple laws of physics underpin the trick. Let’s explore why this phenomenon doesn’t occur with cold water.
Why Hot Water Turns to “Snow” When Splashed in the Cold
© Dmitrii Melnikov/www.legion-media.com
This maneuver is frequently referred to as the Mpemba effect, as intuitively, one might expect cold water to freeze faster than hot water when chilled. The reasoning seems to be that hot water must first cool down to $0^{\circ}\text{C}$ before it can begin to crystallize. However, the secret to this effect isn’t about freezing speed; it lies in accelerated evaporation. The hotter the water, the faster its molecules move. This increased kinetic energy causes a greater frequency of molecules escaping the surface as vapor. When you rapidly splash the water, you significantly increase its surface area exposed to the air, causing a dramatic surge in the amount of moisture that evaporates.
Concurrently, another physical principle comes into play: the colder the air, the less moisture it is capable of retaining. Thus, if the air temperature is sufficiently low—around $-20^{\circ}\text{C}$ or colder—the evaporated moisture condenses immediately into minute droplets that freeze instantly, creating ice dust and a mist effect.
If you attempt the same action using cold water, the trick will fail. Evaporation proceeds much more sluggishly, and the air cannot become saturated enough with steam to trigger condensation before the splashed water hits the ground. Most likely, you will simply end up with a puddle on the snow or ice.
Should you decide to replicate this experiment yourself, remember essential safety precautions. Pay close attention to wind direction to prevent the hot liquid from blowing back onto your face, and confirm that the outdoor temperature is indeed frigid. Also, bear in mind that for the trick to succeed, the water temperature should be near $100^{\circ}\text{C}$, while the air temperature must be extremely low. Under mild frost conditions or if the water is not hot enough, the droplets may not freeze entirely before they descend.
About the Author
