Does hot water freeze faster than cold?
Sometimes, under some conditions, in some setups — and after decades of experiments there is still no agreed explanation, which makes this a better example of how a scientific question stays open than of a settled fact.
The claim
The Mpemba effect is the observation that a container of hot water can, under some circumstances, reach freezing before an otherwise identical container of cold water. It is named after Erasto Mpemba, a Tanzanian schoolboy who noticed it making ice cream in 1963 and was told he was mistaken.
Why it is hard to test
Freezing is not a single event. Do you mean when ice first appears, when the whole container is solid, or when the water reaches 0 °C? Different definitions give different answers, and the 2016 attempt to reproduce it carefully found no effect once the measurement was pinned down.
The proposed explanations, and their problems
Evaporation reduces the mass of hot water, so there is less to freeze — but the effect is reported in sealed containers. Dissolved gases differ — but degassed samples still show it in some setups. Convection currents and supercooling both plausibly matter. None of these has survived as a general explanation, which is the honest state of it.
What to take from it
The interesting thing here is not the answer. It is that a question this simple to state, this cheap to test, and this widely repeated has resisted a clean explanation for sixty years — usually because the experiment is much harder to control than it looks.
Why it refuses to go away
Because the conditions under which it is reported are exactly the conditions hardest to control: an open container, a domestic freezer with a defrost cycle, a shelf that conducts heat unevenly, and a definition of frozen the observer picks afterwards. Any one of those can produce the effect without anything unusual happening in the water, and ruling out all four at once is a far harder experiment than the story suggests.
What Mpemba actually observed
He was making ice cream at school, put a hot mix into the freezer rather than waiting for it to cool, and found it froze before his classmates cold mixes. His physics teacher told him he was confusing himself. Denis Osborne, a visiting professor, took the question seriously enough to try it, and the two published together in 1969 — which is the part of the story worth keeping regardless of what the effect turns out to be.
Sources
Burridge & Linden, Questioning the Mpemba effect (Scientific Reports, 2016); Royal Society of Chemistry — Mpemba competition. Every claim above is drawn from these; where a claim is contested or unsettled the article says so rather than picking a side.
How this article was made
Written by a person. Reviewed by Editorial review, Leumas on 2026-08-30. 416 words.
Questions
Does hot water freeze faster than cold?
Sometimes, under some conditions, in some setups — and after decades of experiments there is still no agreed explanation, which makes this a better example of how a scientific question stays open than of a settled fact.
How do I know this is right?
The sources are listed on the page and a person reviewed it on 2026-08-30. Where a question is genuinely open, the article says so instead of resolving it.