Water and Ice

Like most substances, water exists in three distinct forms or phases: solid ice, liquid water, and gaseous steam. These phases differ in how easilytheir shapes and volumes can be changed. Solid ice is rigid and incompressible,preventing you from altering an ice cube’s shape or volume. Liquid water is anincompressible fluid, so that you can reshape a water drop but can’t change its volume. Gaseous steam is a compressible fluid, so that you can vary both theshape and volume of the steam in a teakettle.

These different characteristics reflect the different microscopic structures ofsteam, water, and ice. Steam is a gas, a collection of independent molecules that is also called water vapor. These water molecules bounce around their container, periodically colliding with one another or with the walls. The water molecules fill the container uniformly and easily accommodate any changes in its shape or size. Enlarging the container simply decreases the steam’s density and lowers its pres-sure.

To remain independent of one another, the water molecules in steam need a certain amount of thermal energy. Without this energy, they stick together to form water. Water is a liquid, a disorderly, fluid collection of molecules that is held together by chemical bonds. These bonds pack the molecules tightly to-gether and give water its fixed volume. However, water still has enough thermal energy for its individual molecules to separate briefly and form new bonds with different partners. Its evolving microscopic structure makes water fluid. While its volume is fixed, its shape is not.

When the water molecules have even less thermal energy, they are unableto rearrange and cling together stiffly as ice. Ice is a solid, a rigid collection ofchemically bound molecules. Like most solids, ice is crystalline, with orderly ar-rangements of molecules that extend over large distances. This order producesthe beautiful crystalline facets of snowflakes and frost.

Just as an orderly arrangement of cannon balls takes up less volume than a disorderly one, a crystalline solid usually occupies less volume than its corre-sponding liquid. That’s why the solid phase of a typical substance sinks in theliquid phase of that same substance.
But there is an exception to this rule: water. Ice’s crystalline structure is un-usually open and its density is surprisingly low. Almost unique in nature, solidice is slightly less dense than liquid water so that ice floats on water. That’s why icebergs float on the open ocean and ice cubes float in your drink. In fact, water reaches its greatest density at about 5 °C (40 °F). Heated above that temperature, water behaves normally and expands. However water also expands as you cool it below that temperature, a very unusual effect.

When you heat ice, it remains solid until its rising temperature reaches 0°C. Atthat point, the ice stops getting warmer and begins to melt. Melting is a phase transition, a transformation from the ordered solid phase to the disordered liq-uid phase. This transition occurs when heat breaks some of the chemical bondsbetween water molecules and permits the molecules to move about. The icetransforms into water, losing its rigid shape and crystalline structure.

0°C is ice’s melting temperature, the temperature at which heat goes into breaking bonds and converting ice into water, rather than making the ice hotter. The ice-water mixture remains at 0 °C until all of the ice has melted. When only water remains, heating it can again cause its temperature to rise.

The heat used to transform a certain mass of solid into liquid, without changing its temperature, is called the latent heat of melting. The bonds betweenwater molecules are relatively strong, so that water has an enormous latent heatof melting: it takes about 333,000 J of heat to convert 1 kg of ice at 0 °C into 1 kg of water at 0 °C. That same amount of heat would raise the temperature of 1 kgof liquid water by about 80 °C so that it takes almost as much heat to melt an ice cube as it does to warm the resulting water all the way to boiling.

The latent heat of melting reappears when you cool the water back to its melting temperature and it begins to solidify. As you remove heat from water at 0 °C, the water freezes into ice rather than becoming colder. Because the water molecules release energy as they bind together to form ice crystals, the water re-leases heat as it freezes. The heat released when transforming a certain mass ofliquid into solid, without changing its temperature, is again the latent heat ofmelting. You must add a certain amount of heat to ice to melt it and you mustremove that same amount of heat from water to solidify it.

Ice’s huge latent heat of melting is what keeps a mixture of water and ice at 0 °C. As long as both water and ice are present together in your glass, they are inthe process of either melting or freezing. Any heat you add to the glass goes intomelting more ice, not into raising its temperature. Any heat you remove from theglass comes from freezing more water, not from lowering its temperature. With ice floating in your drink, it will remain at 0 °C, even in the hottest or coldest weather.