When placing water in a freezer, the expectation is that cooler water will freeze first. The process of freezing involves water molecules giving up energy to the environment until they slow down enough to lock into the crystalline structure of ice. Since water’s freezing point is precisely 32 degrees Fahrenheit or 0 degrees Celsius, any water above this temperature must first cool down to this threshold before solidification can begin. The common-sense assumption is that the starting temperature difference determines the time it takes, meaning colder water should always win the race.
The Mpemba Effect: When Hot Water Freezes First
Despite the logic of basic thermodynamics, centuries of observation suggest that under certain specific conditions, hotter water can, in fact, freeze faster than identical cold water. This strange phenomenon is known as the Mpemba Effect, named for a Tanzanian student, Erasto Mpemba, who brought the observation to modern scientific attention in the 1960s. While making ice cream in school, Mpemba noticed that his mixture, which he had rushed to place into the freezer while still hot, solidified before the mixtures that had been allowed to cool first.
Similar accounts date back to ancient times, with thinkers like Aristotle and Francis Bacon noting the peculiar behavior of pre-warmed water. Mpemba’s persistent questioning and subsequent collaboration with physicist Denis Osborne cemented the observation in the scientific literature, defining the effect as when a hot liquid freezes more quickly than an initially colder one under similar conditions. The Mpemba Effect remains a debated topic among physicists.
Standard Thermodynamics of Freezing
The standard scientific explanation for why cold water usually freezes faster relies on the principles of heat transfer and the need for nucleation. Cooling water from 100°F down to 32°F requires the removal of significantly more energy than cooling water from 40°F to 32°F. This heat loss occurs through a temperature gradient; the greater the difference between the water temperature and the surrounding environment, the faster the heat energy is transferred away.
Once water reaches 32°F, it must begin the process of crystallization. This process requires a nucleus, or a tiny seed particle, around which the ice structure can form. Impurities, dust, or microscopic irregularities on the container walls serve as these nucleation sites, allowing the freezing process to start.
If the water is exceptionally pure and lacks these sites, it can exhibit a state called supercooling, remaining a liquid even when its temperature drops well below the standard freezing point. The time it takes for a liquid to freeze is dependent on the cooling rate and the unpredictable timing of this nucleation event.
Scientific Mechanisms Driving the Anomaly
The various proposed explanations for the Mpemba Effect suggest that the initial heating alters the physical properties of the water, giving it an advantage during the cooling phase.
Evaporation and Mass Loss
One straightforward theory involves mass loss through evaporation, which occurs much faster in hot water. Rapid evaporation removes a small amount of mass, leaving less water to freeze, and carries away significant heat energy through evaporative cooling. This volume reduction means the hot water has a smaller amount of liquid to cool down and solidify.
Dissolved Gases
Another factor is the concentration of dissolved gases, such as oxygen and carbon dioxide, which are expelled when water is heated. Colder water retains a higher concentration of these gases, which can interfere with the molecular arrangement necessary for ice crystal formation. By pre-heating the water, these gases are driven out, creating a purer sample that may freeze more readily once it reaches the supercooled state.
Convection Currents
The internal movement of the water, known as convection, also plays a role in heat transfer. Hot water exhibits more vigorous convection currents, which efficiently circulate the warmest water to the surface where it can lose heat more quickly to the environment. This rapid movement facilitates a faster overall cooling rate by preventing a warm layer from insulating the rest of the liquid.
Hydrogen Bonds
More complex theories focus on the unique structure of water molecules, specifically the hydrogen bonds that link them together. Some researchers propose that heating water changes the configuration of these bonds. This structural change may allow the hot water to release energy more efficiently as it cools, essentially jump-starting the freezing process once it reaches the critical temperature.