Reaching zero degrees, forming the first ice crystal, and completely solidifying a container are three different milestones. Confusing them can cause two seemingly identical experiments to give different answers.
This paradox is known as the Mpemba effect, a name associated with the observation of a Tanzanian student while preparing ice cream. Since then it has posed a challenge to experimental physics. A 2016 study, for example, did not find that initially hot water reached zero degrees sooner than cold water under the conditions it examined. That result, however, does not by itself respond to all possible ways of measuring freezing.
The starting conditions and what happens during the test also matter. If a hot sample loses more water through evaporation, eventually the containers no longer contain the same amount. The type of container, the volume, and the way each sample loses heat can modify the comparison. Furthermore, water can remain liquid below zero degrees: it is subcooling, another factor that complicates the appearance of the first crystal.
Santiago Campillo (@Scruzcampillo) is a biologist and scientific communicator specialized in health, nutrition and nature.
