LAB Physics - file 01
Hot water ice? - Mpemba Effect
When you want ice in a hurry, hot water may be a cleverer choice to put in a freezer than cold water…
This phenomenon is called the Mpemba Effect. In 1963, a Tanzanian student, Erasto Mpemba, noticed that his hot ice cream mix froze faster than his classmates' cold mix.
But according to the laws of Thermodynamics, hot water must first pass through the state of being cold water in principle. Why does this paradox occur? Modern physics has explained several reasons.
Why does this weird phenomenon happen?
Freezing is actually a complex process, depending on more than just the temperature reading on a thermometer. In this process, several physical phenomena occur simultaneously.
1. Evaporation
This can testify to the most common reason. Hot water evaporates into the air more rapidly than cold water. As it evaporates, the total mass of the water decreases. Since there is less water left to freeze, it can reach the freezing point faster. It makes sense, doesn't it?
In addition, evaporation itself is a cooling process, which carries heat away from the surface into the air.
Hot water creates stronger circulations of the internal currents, as the warmer water rises up and the cooler water sinks down simultaneously. This movement of the molecules by heat is called Convection. These convection currents help distribute heat to the surface and the container walls more actively than in still, cold water.
3. Dissolved gases
Boiling water removes dissolved oxygen and nitrogen. In principle, such a degassed state of water is to have different thermal properties, or a higher freezing point. This chemical property leads boiled water to crystallize more easily, so more rapidly than gas-rich cold water.
4. Supercooling & Nucleation
The temperature of cold water often drops below 0°C (the freezing point of water in theory) without actually turning into ice. This phenomenon is called Supercooling.
When the process to form a new thermodynamic phase is starting, so for example, a liquid is going to crystallize to a solid, the common mechanism Nucleation (the initial process in crystal formation), works. This process is explained by the Classical Nucleation Theory (CNT). In general, cold water is pure and lacks nuclei (nucleuses) or seed crystals (nucleation sites) to start the crystal. So by the Supercooling, cold water is inclined to start freezing a little later at a lower temperature than the general theory.
On the other hand, due to the history of the heating process, hot water may have different impurities or interactions with the container material to form the nucleation seeds. So as a result, such impurities allow the liquid to trigger ice formation earlier at a higher temperature than cold water.
Summing up for a rough sketch, we can also imagine hot water holding higher energy than cold water, so that it releases the energy more rapidly than the colder state.
Can we replicate the Mpemba Effect by experimentation?
In reality, the experimentation for the Mpemba Effect is one of the most controversial and difficult to replicate in physics. While many people have seen the phenomenon happen in reality, to observe the typical phenomenon on command in a laboratory is surprisingly delicate and tricky by condition…
Why scientists struggle so much to demonstrate the Mpemba Effect is that water is NOT just pure H²O in reality. For example, as you know, tap water contains very small amounts of chlorine for public health purposes. In real-world experiments, several hidden factors change the subtle condition to occur.
1. Freezer factor
If you place a hot cup of water on a layer of frost in your freezer, it melts the frost by heat, creating better contact with the cold metal shelf. On the other hand, the cold cup just sits on top of the frost insulation. This definitely makes the hot cup freeze faster due to the more efficient Thermal Conduction.
2. Dissolved gases
When you boil water before the experiment, you drive out dissolved gases at the same time. This chemical change of the water can lead to a change in its freezing point.
3. Thermometer bias
If you place a thermometer in the top of a cup of hot water in a freezer, it might read as frozen earlier. Because the surface of the water freezes first, while the bottom still remains in the liquid state. But in a cup of cold water, the cooling process is observed to be more uniform in every part. This might result in different readings of the temperature.
Would you like to try to observe the Mpemba Effect at home? If you could find certain ingenious conditions to replicate, you might win an award in your own name in the history of physics?!
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