Ice is the solid form of water. Water is most dense at 4 °C, but it can stay liquid below 0 °C if the pressure is high enough. Above 0 °C, increasing pressure has little effect on melting point until about 50-80 km depth in the oceans (Pressure and Melting Point of Ice).

As long as some liquid water is present, chunks of ice will continue to coalesce until the solid form is less dense than the liquid. This forms big crystals at the bottom, which rise toward the surface as they grow larger. They can break apart on their way up due to pressure changes or collisions with other pieces.

If somehow all the liquid water below 0 °C was kept away from the chunks of ice, it would continue to crystallize until reaching 0 °C and remain solid up to the surface.

During winter, water at or below 0 °C can freeze on the surface of bodies of water due to a drop in air temperature. This is known as “black ice” and is extremely dangerous to drive over as it appears identical to the surrounding liquid.

To prevent ice buildup on roads, people use salt or calcium chloride to increase the melting point. This is known as brine and has a much lower freezing point, melting ice on contact. This prevents large chunks of ice from forming and lessens the chance of sticking to the road, making driving safer.

Ice also goes in various stages in the cold and hot months. This is due to how air temperature relates to water temperature. In winter, atmospheric pressure is high, so it becomes hard for ice to melt or stay liquid at 0 °C. In summer, atmospheric pressure is low, and ice will melt at higher temperatures than in winter.

In terms of state changes, ice will melt in heat until it reaches 4 °C while water vaporizes into the air at 100 °C. This is because heat raises the energy of particles, while cold does the opposite.

As for solid to liquid, the change is known as fusion. Solid to gas is called sublimation, and liquid to solid is called deposition. Solid to gas requires heat while liquid to solid requires cold.