By Ann Johnson, Updated Mar 24, 2022
When water shifts from liquid to solid, its molecules go from a constant, chaotic dance to a highly ordered lattice. In the liquid state, molecules collide, slide past one another, and never stay in the same spot for long. As the temperature drops to 32 °F (0 °C), the motion slows, and the molecules lock into a repeating, crystalline arrangement that we see as ice.
Every molecule experiences forces that pull it toward its neighbors. Strong attractions, like those in carbon, can keep atoms together even at thousands of degrees, while weak forces in helium mean it remains gaseous until extreme cold. Water’s intermolecular forces are moderate—strong enough to keep molecules together at 32 °F, yet weak enough that it melts again at 32 °F (0 °C). This balance gives water its unique freezing temperature.
Adding substances such as sugar, salt, or alcohol lowers the temperature at which water freezes. The more solute you add, the greater the drop. That’s why road crews spread salt on icy pavements—salt lowers the freezing point, turning ice into liquid water even when temperatures hover just below 32 °F. Similarly, vodka, which contains roughly 40 % ethanol, remains liquid in a standard freezer because alcohol’s presence depresses the freezing point by several degrees.
Most materials contract when cooled, but water behaves differently. It reaches maximum density at about 39 °F (4 °C). Below that, water expands as its molecules adopt a hexagonal lattice, creating gaps that make ice less dense than liquid water. This expansion is the reason snowflakes form, as the hexagonal arrangement continues to grow into delicate crystals.
Because ice expands, a sealed container of water can rupture when frozen. The pressure exerted by expanding ice can reach up to 40,000 psi at –7.6 °F (–22 °C). Even robust containers, such as steel or iron, can fail under this stress, which is why you’ll often see bottles burst in the freezer.
