An Experimental Investigation of the Mpemba Effect

There are reports of hot water freezing faster than cold water, a phenomenon known as the Mpemba effect. Whether the Mpemba effect describes initially hotter water reaching 0 ℃ before initially colder water or describes initially hotter water beginning its liquid to ice transition before colder water that supercools is not decided. We explore the two definitions by measuring the cooling curves of water samples of deionized water in lidded tins on a styrofoam tray; deionized water in lidded tins on a baking sheet; deionized water in unlidded plastic beakers in a styrofoam tray; and tap water in lidded tins on a styrofoam tray. Our temperature measurements fit well with Newton's law of cooling and extract the exponential decay rate of a sample's temperature curve. From 136 measurements, we observe 52 and 63 instances of the Mpemba effect as defined in the two aforementioned ways. We attribute the Mpemba effect to a spread in the exponential decay rates of different sample cooling curves and posit that uncontrolled convective currents in the freezer cause the spread in decay rate.