Mpemba Effect
Mpemba effect
One of the most noticeable and significant effects of heat transfer is when it causes a phase change in a substance. The mpemba effect is an effect that occurs during freezing when water freezes faster from a higher temperature than it would from a lower temperature. The concept was first recorded by Aristotle and has been the cause of various experiments since.
Despite there being many experiments to test if the mpemba effect is real, they tend to face the issue that many factors affect the freezing point of a liquid and therefore it can be hard to reliably measure. For
example, factors that affect the time taken to freeze include, but are not limited to, the volume of liquid, whether the fluid is moving, the shape of the container the fluid is in, how rough the container’s edges are, and any dissolved gasses or other impurities that may be present. As such getting reliable results regarding the mpemba effect can be challenging but the various experiments have done what they can to control and vary these factors to see what could cause the effect.
There are various ways that the mpemba effect could be explained with the 2 most referenced in the journal articles being that dissolved air is released or that heating the water causes a reaction in some of the impurities commonly in water which form nucleation sites that make freezing easier. The loss of dissolved water would cause the freezing temperature to increase as the gasses would have a lower melting temperature and so would lower the mixture melting temperature when present. This is considered to not have a large enough effect to be noticeable and doesn’t have much evidence to support it. The chemical reaction changing the impurities (the only one specifically mentioned is the production of calcium bicarbonate), to add nucleation sites that are active at higher temperatures and therefore enabling the water to freeze sooner when undisturbed that it would otherwise.
This second explanation would explain the results in both articles, despite only one of them referencing it as the other finds that the effect is only noticeable within a small range of temperatures (between -5 and -11˚C), implying that this contains the nucleation temperature of the solutes created by heating.