Why Does Hot Water Freeze Faster than Cold Water?
If you’ve ever chucked some hot and cold water into your freezer at the same time, you may have ended up noticing that the hot water froze before the cold water. Or maybe you didn’t (which is what makes this whole topic so finicky). Whether you personally have observed this phenomenon or not, intuitively; it doesn’t make sense. Hot water is farther away from water’s freezing temperature, and given that things don’t change temperature and states of matter at the same time; the cold should freeze first. It’s got less temperature change to go before it can start freezing. So what’s up with that? Why does hot water freeze faster than cold water?
The Mpemba Effect
If you thought we were pulling your leg with all this there’s actually a name for this whole ordeal. It is, as the subtitle implies, called the Mpemba effect.
It’s origins aren’t as grandiose as doing a bunch of math or slamming particles together; they’re actually quite mundane. Perhaps not as mundane as an apple falling on someone’s head, but we digress. The Mpemba effect’s origins actually lie in high school ice cream making.
That’s right, Erasto Mpemba, a then Tanzanian high school student, made the discovery that hot water can freeze faster than cold water. But just so we’re clear, yes, Francis Bacon, Rene Descartes, and others had made similar claims previously. But it wasn’t until Mpemba’s accidental discovery in the 1960s, where he was able to describe the phenomena in greater detail than his predecessors, that the idea gained traction.
The story goes Mpemba was making ice cream as part of a high school lab; which at some point required milk to be boiled. Instead of letting his mixture cool down before putting it in the icebox, Mpemba put it in the freezer still boiling. Turns out his mix set faster than the other students, and in 1969, he helped publish a paper explaining this ordeal. Thus, the Mpemba effect.
Something’s Amiss with Hot and Cold Water
Right off the bat, it’s not actually possible to replicate the Mpemba effect with 100% certainty or accuracy. Which means we can’t consistently get hot water to freeze faster than cold water. So it’s easy, right? Toss out the notion that hot water freezes faster, go with our intuition, move on? No, we can’t do that either. Turns out there’s enough times where hot water does freeze before cold water for us to keep the Mpemba effect around.
There are a lot of theories circulating regarding how exactly the Mpemba effect works. But even the most commonly toted theories seem to collapse.
Hot Water Evaporates Faster
One of the original, common theories for justifying the Mpemba effect was how quickly water evaporates. Given that hot water is closer to boiling than cold water is, it evaporates faster. You can thank vapor pressure for that.
So because hot water evaporates faster, some of the liquid becomes gaseous as the water freezes. Therefore, by the time freezing occurs in the hot/cold water samples, the hot sample will have less liquid mass. With less liquid mass, it’ll freeze faster.
Here’s the problem, you can prove this is inconsistent yourself. We’ve put hot and cold water in airtight containers to ensure that evaporation will not occur, but we still get mixed results. Logically, if this hypothesis holds true, the cold water should freeze first. Turns out, we’ve gotten the hot water to freeze first even under these conditions, so this theory doesn’t hold water.
Convection Cooling
We know that hot water is less dense than cold water–you observe this with heat rising. As such, the other hypothesis says hot water has a greater temperature gradient as it freezes. Since it’s hotter, and hot stuff rises, the temperature of the hot water sample will be less uniform than the cold one.
Because of the temperature gradient you get a convection current (hot stuff rises, cools, sinks to the bottom, pushing the new relatively warmer stuff up, which cools, and perpetuates the cycle). Therefore, the hot water sample loses heat a lot faster than the cold one, therefore it freezes first.
But alas, it turns out we haven’t fully pinned that theory down either.
Hot Containers; Cold Water
There’s another theory out there regarding containers. The hot water heats up its container, and then the container makes better thermal contact with the freezer than the cold container. Therefore, faster freezing.
This didn’t sound as convincing to us either.
Hydrogen Bonds
We’re not going to go super in depth with what hydrogen bonds are; intermolecular forces are a time. But you should know that water can make them, and it can form hydrogen bonds with itself. We mean water molecules form hydrogen bonds with each other, not one molecule bonds with itself. Hydrogen bonds are also one of the stronger types of intermolecular forces.
Xi Zhang at the Nanyang Technological University in Singapore (and you know, her team) posit a theory related to hydrogen bonds.
We can visualize a water molecule kind of like this thing below (Picasso would be proud).
So Xi Zhang’s theory: Hydrogen bonds between multiple water molecules bring them close together. However, these molecules also repel each other (because if molecules and atoms really touched for real we’d get nuclear fusion and that’s bad). Due to this repulsion, the bonds within the water molecule stretch a bit. This means they contain more energy than if the bonds were not stretched.
When we heat up water, it lets the hydrogen bonds between molecules stretch. As a result, the bonds within the water molecule can contract a bit. When they contract, they can release the energy they had while stretched. Losing energy is basically cooling down a far as thermodynamics is concerned. Under this theory, hot water has access to a cooling method that cold water simply does not have on a molecular and atomic level.
After some math, Xi Zhang’s team was able to actually determine a quantifiable difference in cooling rate attributed to this bond fiasco. While this probably isn’t the end of research into the Mpemba effect, this where we’re at right now.
Think you’re a water expert now? See how well you know all the wet words here.
