Why does hot water freeze faster than cold water?

The above question may be absurd as it is somehow pretty apparent that the answer should be a straight no. We know that ice is cold, and hot water will take a longer period to cool down, so why would it freeze faster than cold water?

However, through thorough observations across many thousands of years, plus the numerous researches and experiments that were conducted regarding this subject, you might be surprised that the answer is actually a yes. When under specific and well-regulated condition, hot water could turn to ice quicker than cold water.

Such quirk of nature stems down to the work of Erasto Mpemba, a Tanzanian high school student, who argued that ice cream could actually freeze faster if it is heated first at a certain extent. He first observed the strange phenomenon in 1963, which showed that when two bodies of water at varying temperatures are bared under the similar subzero condition, the hotter water will freeze quicker. Mpemba’s observation proves the same though of other well-known thinkers, such as Francis Bacon, Rene Descartes, and Aristotle, that hot water freeze first versus cold water. Thus, the occurrence was regarded as the “Mpemba effect.”

But how does the Mpemba effect actually happens? Experts say the phenomenon revolves on evaporation, dissolved gases, and convection.

First is evaporation. It is believed that as hot water starts to cool down, it loses some of its mass through the process of evaporation. Since there is less mass, the water technically has less water to freeze, making the process a whole a lot quicker. However, this won’t work if the hot water is enclosed in a container, as the water vapor will just revert to the water.

The second explanation talks about dissolved water. Experts believe that the dissolved has that water usually has is eliminated in hot water. With that, it leads to changes in water’s properties, causing the Mpemba effect. Since there is no dissolved has, the water’s ability to conduct heat, the freezing point, and the amount of heat required to freeze the water may dramatically change, all causing hot water to freeze faster than cold water.

Lastly, convection also plays an essential role in the Mpemba effect. As the hot water cools down, it begins to develop both temperature gradients and convection current, accelerating the process of freezing. The warmer fluid rises to the surface and releases heat, then pulls down cooler fluid on the bottom. Meanwhile, cold water remains stagnant on its initial temperature, as it is unable to release any heat.

For instance, you have hot water at around 70° Celsius and cold water at 25° Celsius. Placing them on the freezer will force the hot water to cold down rapidly and create convection currents. Meanwhile, the temperature of the cold water remains stagnant at 25° Celsius on average.

By the time, the hot water reaches the same average temperature of 25°, it will still have part of its surface water warmer than 25° that is continuously releasing heat. Since hot water still loses heat at the given average temperature, it freezes faster than of the cold water at the same condition.

Simply, a glass of hot water rapidly cooling down, release heat quicker from the surface, compared to a glass of water, that has a uniform temperature throughout, lacing the convection current to speed up the freezing process.

Now, that you know about the Mpemba effect, try to use warmer water the next time you refill that ice cube tray. Through that, you can enjoy your favorite beverages a little sooner.

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