Convection, Conduction, Evaporation: the Mpemba effect?– Anousha Athreya

Updated: Sep 5, 2020

For the past 5 months, social distancing policies have been implemented to slow the spread of the COVID-19 pandemic and reduce the amount of people in contact with the virus. However, in doing so, hundreds of people across the world have been stuck inside their homes while they wait out the virus. With many people wondering what they can do during the pandemic, one of the easy answers is at-home experiments! With so much time on their hands, students, adults, and teachers can learn new science topics by performing easy, at-home, chemistry experiments and learn more about the basic phenomenons that drive their everyday lives.

One basic phenomenon that is quite intriguing and can be experimented with is the Mpemba effect. The Mpemba effect is the concept that hot water will freeze faster than cooler water. It began with a simple at-school science experiment where a student theorized that a hot ice cream mixture would freeze faster than a cooler one. While counterintuitive, this method is actually extremely true! Performing a few simple experiments can help you determine the reasons behind the Mpemba effect and how it changes when other elements are added to the water that affect its freezing point. Here’s one way to think about it. You have three main ways that heat transfers through objects: conduction, convection, and evaporation. For each of these types of heat transfer, you can perform an experiment to determine why the Mpemba effect is true. For convection, one experiment is to simply place a control cup with room temperature water in the freezer, a cup with warm water, and a cup with cold water. One explanation as to why this methodology works is that as the water cools, convection currents form, causing the cooler water on the sides of the cup to sink and warm water to circulate, thus cooling the water faster. However, in a cooler cup, since there is no drastic temperature difference or drop to form these currents, the cooler water reduces in temperature at a slower rate. For the next type of heat transfer, conduction, you can perform the same experiment as before; yet, there is a different explanation as to why perhaps the hot water would freeze faster. Once the cold water starts freezing, it forms frost on its surfaces; however, since frost is an insulator, it maintains the minimal heat within the cup and prevents the cold water from completely freezing. In comparison, when hot water freezes, the heat from the water melts the frost, essentially removing the insulator and allowing it to form a direct connection with the freezer shelf. With a direct source that draws out the rest of the heat from the cup, the temperature of the water experiences a massive decrease in temperature, leading it to freeze significantly faster.

The final experiment tests evaporation with the Mpemba effect. First, perform the same experiment as before but add lids on top of the cups to prevent evaporation. Since evaporation occurs faster with hot water(boiling water removes impurities), the lessened water volume could cause it to freeze faster. However, adding solutes such as salt or other impurities could cause the water to lower its freezing point and take longer to evaporate and freeze. Finally, the Mpemba effect is still rather unexplored in its applications and leads to many questions in its methodology, especially since the placement of two cups within a freezer could trigger numerous explanations and processes at the same time that contribute to hot water freezing faster. So, the next time you’re bored on a weekend, try out this experiment and one application to remember is that it lets you make ice cream faster!

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