Water doesn't instantly freeze when placed in a freezer due to a combination of factors, including temperature variations, impurities, and the presence of nucleation sites. Let's break it down:
Temperature variations: Freezing occurs when the temperature of water reaches or falls below its freezing point, which is 0 degrees Celsius (32 degrees Fahrenheit). However, freezers are not uniformly cold throughout. They have different temperature zones, and the cooling process takes time. The water closest to the freezer's coldest regions will freeze first, while the rest of the water will gradually cool down.
Impurities: Pure water, under ideal conditions, can freeze at 0 degrees Celsius. However, most water contains impurities such as dissolved minerals, gases, or other substances. These impurities lower the freezing point of water, causing it to remain in a liquid state even at temperatures slightly below 0 degrees Celsius. The degree to which impurities affect the freezing point depends on their concentration and composition.
Nucleation sites: Freezing usually begins around nucleation sites, which are points where ice crystals can start to form. In the absence of nucleation sites, water can be cooled below its freezing point and still remain in a liquid state. In everyday situations, nucleation sites can be irregularities on the surface of the container or impurities present in the water. Once ice crystals begin to form at these nucleation sites, they can propagate and spread throughout the water, eventually leading to complete freezing.
It's important to note that while these factors explain why water doesn't freeze instantly in a freezer, they don't guarantee that freezing will occur in all cases. In some circumstances, supercooling can happen, which means water remains in a liquid state even below its freezing point. Supercooling can occur when the water is extremely pure, and there are no nucleation sites to initiate freezing. Sudden agitation or the introduction of a nucleation site, such as tapping the container or adding an ice cube, can trigger rapid crystallization and freezing of the supercooled water.