Question

### The Mpemba Effect

Under certain conditions, initially warm water has been observed to freeze faster than initially cold water. This phenomenon is known as the Mpemba effect. Three scientists discuss the physical mechanisms responsible for this effect.

Scientist 1
The Mpemba effect is primarily caused by evaporation. As warm water cools, it loses mass through evaporation at a much higher rate than cold water. Because a smaller mass of water requires less heat removal to undergo a phase change, the initially warm water completes freezing first. Additionally, the rapid evaporation increases the solute concentration in the remaining warm water, which lowers its freezing point only slightly, but this is outweighed by the rapid decrease in volume. Convection currents do play a role, but only in maintaining a high temperature at the evaporating surface, rather than directly accelerating heat transfer to the surrounding air.

Scientist 2
Evaporation is negligible; instead, the primary driver is the temperature-induced difference in convection. Warm water has a lower density at its surface relative to its base, establishing strong convection currents that rapidly transport heat to the container's surface, where it is lost to the environment. This rapid heat loss continues even as the water cools, because the established flow momentum persists. In contrast, cold water has much weaker convection currents, leading to a slow, conduction-dominated heat transfer. Convection speeds up cooling so significantly that the warm water reaches $0^\circ\text{C}$ and freezes before the cold water. Solutes play no role in this process because the water used is highly purified.

Scientist 3
Neither evaporation nor convection is the primary cause. The effect is chemical and relates to hydrogen bonding. In warm water, the covalent $O-H$ bonds within water molecules are shorter and stronger because the intermolecular hydrogen bonds are stretched and weaker due to high thermal motion. As warm water cools, the hydrogen bonds reform and release covalent energy, which accelerates the cooling rate in a non-linear fashion. This chemical energy release allows warm water to reach $0^\circ\text{C}$ and freeze faster than cold water, where hydrogen bonds are already fully formed and covalent bonds are in a lower-energy state. Solutes do not affect this molecular mechanism.

Matching Task
Match each statement regarding the proposed primary mechanism of the Mpemba effect to the scientist who would support it.