Does hot water freeze faster than cold water? It seems obvious that the answer should be no, because all things being equal, hot water takes longer to cool down than cold water, and so it couldn't possible freeze faster.
But observations over thousands of years, as well as countless modern experiments, have shown that the opposite is true - under carefully controlled conditions, hot water at times seems to freeze faster than cold water.
How is this possible? Well, that's something that still has scientists baffled - in fact, they're still struggling to prove the effect exists in the first place, as the first episode of Derek Muller's new science channel, Sciencium, explains.
It turns out that freezing water is a lot more complicated than you might think.
As the video above explains, the phenomenon of hot water freezing faster than cold water is known as the Mpemba effect, named after Erasto Mpemba, a Tanzanian student who in 1963 was making ice cream as part of a school project.
The students were meant to boil a mixture of cream and sugar, let it cool down, and then put it in the freezer.
Worried about getting a spot in the freezer, Mpemba instead put his mixture in while it was still scorching hot. But after 1.5 hours, his mixture had frozen, while his classmates' mixtures had not.
Intrigued by this phenomenon, he went on to work with physics professor Denis Osborne, and together they were able to replicate the findings and publish a paper in 1969 showing that warm water freezes faster than cold water.
It was the first peer-reviewed study on the effect, but as Derek notes in the video above, observations date all the way to Aristotle in the 4th century BCE, who observed that hot water cooled sooner than cold during his experiments.
Sir Frances Bacon and Descartes also noted the phenomenon in their studies.
But what's the physics behind this strange phenomenon?
As the video above explains, there are five proposed mechanisms for what's going on here:
- Frost melting: Frost is an insulator, and so frosty cold water might keep its heat better than a warm beaker that melts the frost off its sides.
- Dissolved gasses: There are more dissolved gasses in cold water than warm water, and researchers have predicted that this could play a role in cooling rates, although it's not clear how.
- Supercooling: We all know that water freezes at zero degrees Celsius, but sometimes it gets a lot colder before it freezes - a phenomenon known as supercooling. This occurs because ice needs a nucleation site, such as an air bubble or impurity in the water in order to form. So maybe warm water experiences less supercooling than cold water.
- Evaporation: The hot water beaker loses more water molecules through evaporation, so there's less of it to freeze.
- Convection: Finally, there's the idea that warm water might cool faster due to increased convection currents. These currents occur because water cools primarily from its surface and the sides of the beaker, causing cold water to sink and warm water to rise up and take its place. The currents are greater in warm beakers, and could affect cooling rates.
There's merit in all those ideas, but the problem is that experiments over the years have controlled for all these effects, and the results have been frustratingly inconsistent.
Some labs have failed to show the Mpemba effect happening at all, while others show it happening even under varying conditions.
So what's the answer? Well a new study published this year suggests that maybe the Mpemba effect is being caused by something else entirely - and it has nothing to do with how quickly hot water cools.
It's pretty crazy to think that after millennia of observations, we still have so much to learn about something as basic as freezing water. Science is the best.