Have you ever wondered why on occasion hot water freezes faster then cold water? We all know it happens but the reasoning is a bit more challenging to grasp. There are many theories to explain this conundrum labeled the Mpemba Effect and they go as follows:
- Hot water will evaporate faster which means there is less volume for freezing. Or the faster evaporation an endothermic process cools the water faster.
- Cold water forms a frost layer which acts as an insulator.
- Solutes (like carbon dioxide) quantities alter the hot water from the cold water since when the hot water is heated these quantities change.
- The freezing point is affected by the impurities inside the water (like dust, or salts), which have nucleation temperatures.
- Warm containers enable better thermal conduction with the freezer, which means that heat is conducted better.
- The bonds in the actual water have unique properties that cause this weird effect. Specifically the hydrogen bonds, which lead to many of water’s weird properties, like high boiling points. The theory is that the water molecules are pulled close together by the hydrogen bonds leading to the repulsion in the covalent bonds to stretch out the bonds and store lots of energy, the heated water stretches the hydrogen bonds which allows the covalent bonds to lose some of their stretch and energy which leads to faster cooling, this along with normal old freezing makes the hot water freeze faster then the cold water.
- Hot water cools quicker due to the larger difference in temperature between the hot water and the cold freezer, which allows it to reach its freezing point before the cold water has time to reach its own freezing temperature.
- Supercooling can be described as when water freezes at a temperature lower than the known freezing point of zero degrees Celsius if the water reaches a temperature lower than zero then it may want to be in the solid lattice formation but may not know how to actually form this pattern (requiring a nucleation site to provide the instructions which on occasion takes time for the molecules to find below their freezing point). Cold water is shown in some experiments to actually supercool more than hot water and this time and effort preformed by the cold water means a slower freezing.
All right so we've go the theories but we do not understand the basics of water and how it freezes. Temperature is all based on energy in the molecules; heat ergo can be directly related to the amount of energy. More molecules are in more water and more molecules means more potential energy storage which goes back to temperature. As thermal energy in the water decreases so does temperature and once zero degrees Celsius is reached the water will start to freeze, this temperature is known as the freezing point. Conduction helps with freezing in order for water (a liquid) to freeze it must be in some type of container the container sits on the freezer and as it gets cold the water gets cold, so using conductive materials like metals can speed up cooling since the conductive material can easily transfer the cool temperature to the water. In the freezer there is also cold air, which will exchange the hot air from the water with the cold air in the freezer quickening the freezing. Evaporation leaves the lower energy molecules behind; less energy as we've established means less heat this lowers the temperature. The process of convection also aids in water freezing, one example being that the cold water is denser then hot water so the cold sinks to the bottom of the container this leads to convection currents. The problem is that if the water moves too much then the process of freezing actually slows down. All these factored are important to note when looking at the Mpemba Effect and are what led to the theories looked at in the list above.
In the end conclusions are hard to draw, the only answer is that there are many possible answer and one of them or maybe a bunch of them are correct or maybe a combination of these theories answers the weird, wonderful question left by the Mpemba Effect.
Watch this video for a summary of the Mpemba Effect:
Websites Consulted
http://phys.org/news/2010-03-mpemba-effect-hot-faster-cold.htmlhttps://medium.com/the-physics-arxiv-blog/why-hot-water-freezes-faster-than-cold-physicists-solve-the-mpemba-effect-d8a2f611e853#.qme2mhyeg
http://www.school-for-champions.com/science/mpemba.htm#.Vn8dwpMrJEI
http://math.ucr.edu/home/baez/physics/General/hot_water.html
