There are 17 known forms of ice. Many of them form under extreme pressure, such as in the interiors of frozen planets; 2 take on a quartz-like crystalline structure.
Whereas most solids sink into their liquid forms because the solids are denser than the liquids, ice floats on top of water. Ice, like water, has unusual structural properties.
Water attains its maximum density at 4 °C. Liquids typically condense as they cool but water expands upon freezing.
Whereas molecules of water vapor behave rather independently, ice forms a strong structural lattice. Liquid water molecules have weaker interactions with each other than they do in ice. Water also exists in a liquid-crystal state when adjacent to hydrophilic surfaces, which tend to interact with water or be dissolved by it.
Ice possesses quantum properties opposite those of other crystals. Typically, crystalline molecules shrink as they cool, but the shrinking stops before reaching absolute zero (0 K), owing to zero-point energy.
In theoretical physics, zero-point energy is the lowest possible energy that a particulate matter may have: the energy of its ground state. Zero-point energy is inherently tied to the uncertainty principle.
Typically, less massive atoms have more zero-point energy. Lighter nuclei need more room to move than heavier elements, which translates into larger crystals for lighter elements as they cool.
At higher temperatures, this quantum effect becomes less pronounced. Hence volume differences decrease as temperature rises.
The opposite occurs with ice. D2O (heavy water) occupies more volume than H2O ice. This volume difference increases as temperature rises.
In order to access and measure quantum mechanical effects in matter, we usually need to go to very low temperatures, but in water ice some zero-point effects actually become more relevant as the temperature increases. ~ physicist Marivi Fernández-Serra
The structural integrity of ice is illustrated in its melting. Ice melt is not continuous. Instead, ice discontinuously liquefies layer by layer.
Ice’s structure varies at different scales. Macroscopically, ice crystals form a hexagonal lattice, which is why all snowflakes are 6-sided. In contrast, in crystallites of up to 100,000 molecules, ice crystals form from water in layers of hexagonal and cubic arrays.
Ice formation in clouds – nucleation – is a critical aspect of precipitation development. The semi-disordered cubic-hex stacks of ice that form from cloud droplets facilitate faster development of rain than if ice nucleation occurred solely like snowflakes (hexagonally). Earth would be much different if ice formation physics was otherwise, as the water cycle would be hindered.