Hot water DOES freeze faster than cold water.
The Mpemba Effect
The concept of hot water freezing faster than cold water was reintroduced to modern science by a Tanzanian high school student named Erasto B. Mpemba in the year 1969. Erasto B. Mpemba narrates how he stumbled upon his discovery while making ice cream as a young boy in Tanzania. He says that after buying milk from the local women in his town, he started to boil this milk and mixed it with sugar. Because he was in a hurry to be the first to freeze his mixture for lack of abundant freezer space, he immediately placed his ice cream mix in the freezer without cooling it, as was the normal process. Thereafter, he discovered that his mixture was the first to freeze into ice cream.
Unlike in Mpemba’s time, where scientists were not able to answer why hot water freezes faster than cold water, there are now several theories offered by scientists to explain this phenomenon.
The Evaporation Theory
One theory is based on evaporation. The evaporation theory says that hot water has a higher rapid vaporization rate. It is observed that hot water loses mass as it cools during evaporation, and because there is less mass and less heat, the hot water cools faster than cold water would. The dissolving of gases is also attributed to the reason for why hot water freezes faster than cold water. This theory says that because dissolved gases are expelled from hot water, as can be observed from boiling water that expels a lot of dissolved gas, it changes the ability of water to conduct heat.
The Convection Theory
The convection theory meanwhile states that as water cools, the temperature gradients and convection currents develop, and over time, the water develops a “hot top”, wherein the surface of the water is hotter than the bottom. According to the convection theory, water loses heat faster because of the hot surface.
Jeng, Monwhea. “Can Hot Water Freeze Faster Than Cold Water?” Physics@NCKU. National Cheng Kung University, Nov. 1998. Web. 24 Feb. 2011. http://www.phys.ncku.edu.tw/mirrors/physicsfaq/General/hot_water.html.
Mpemba, E. B., and D. G. Osborne. “Cool?” Physics Education 14.7 (1979): 410-13. Print.