Question

### The Mpemba Effect

The Mpemba effect is the observation that, under certain conditions, initially warm water freezes faster than initially cold water. Two scientists present conflicting hypotheses to explain this phenomenon.

Scientist 1

The Mpemba effect is primarily caused by evaporation. When warm water is placed in a cooling environment, it evaporates much more rapidly than cold water. This rapid evaporation has two key consequences. First, it reduces the total mass of the water sample, meaning there is less mass that must undergo the phase change to ice. Second, evaporation is an endothermic process that removes significant latent heat from the remaining liquid, which accelerates the cooling rate. Therefore, the higher initial temperature leads to a faster reduction in temperature and mass, allowing the water to freeze first.

Scientist 2

The Mpemba effect is primarily caused by differences in dissolved gas concentration. Heating water decreases the solubility of gases, driving out dissolved carbon dioxide ($CO_2$) and oxygen ($O_2$). Water with lower concentrations of dissolved gases has a higher thermal conductivity and a slightly higher freezing point than water saturated with gases. Cold water, which retains a high concentration of dissolved gases, has its freezing point depressed and experiences reduced heat transfer. Consequently, pre-heated water freezes faster because it contains fewer dissolved gases, which enhances heat transfer and prevents freezing point depression.

According to the explanation provided by Scientist 2, pre-heated water is able to freeze faster than cold water primarily because heating the water: