Water is a substance with which we have much familiarity. It is present on our planet in the three typical states: solid, liquid, and gas. The liquid form is present within the oceans, the lakes, the rivers and in the aquifers. In its solid form it appears as ice or snow in the ice-caps as well as in mountain glaciers. In its gaseous form (or better: as water vapour) it is found in clouds and steam. The passage from one state to the other occurs under the influence of temperature changes, between 0° and 100°C.
A water molecule consists of one atom of oxygen (O) and two atoms of hydrogen (H). Water molecules are bound to one another with strong hydrogen bonds between oxygen and hydrogen atoms belonging to different molecules, but close to each other.
Evaporation and condensation
Evaporation is the passage of water from the liquid state to the gaseous state. Water changes into vapour when the thermal movement breaks the intermolecular bonds so that molecules are free to move independently: they depart and evaporate upward. Such transformation is influenced by several factors like temperature, humidity, evaporation surface and wind. While evaporation occurs only on the water surface, boiling is different in that it involves all of the liquid mass and takes place only at the temperature of 100°C (if the pressure is 1 atmosphere).
Since a fairly high temperature is necessary to boil water, most of the water on earth remains in a liquid state throughout most seasons and in most regions.
Condensation finally is the process whereby water changes from a gaseous to a liquid state. As solar heating causes the evaporation of the surface water of oceans, rivers and lakes, the newly formed vapour travels higher, transported by upward currents, in the atmosphere and is in turn cooled and transformed into liquid that then returns to the ground in the form of precipitation.
The process of solidification or freezing represents the transformation of water from its liquid state to a solid state (ice) and occurs at 0°C. At this temperature water molecules create a crystalline grid, made up of hydrogen bonds, granting stability and shape to the ice cube. But water presents a truly unique characteristic which distinguishes it from all other existing substances: in fact, at temperatures close to the freezing-point, and in particular around 4°C, water reaches its maximum density. Over 4°C water behaves like all other substances: its molecules draw closer, thus reducing inter-molecular spaces and causing the volume of the mass of water to decrease whilst its density increases. Below this temperature level, however, molecules can not approach each other any more. On the contrary, they begin to draw further apart because of the hydrogen bonds keeping them in a rigid structure. This characteristic has repercussions that we can witness in nature, for example in the case of icebergs. Icebergs are true and proper mountains of ice which, owing to their lower density, are able to float on water. Hence, water density levels are at their highest, not when the element becomes solid, but at its liquid state.
Figure 6: Structure of hydrogen bonds (source: Wikipedia)
An additional aspect of water that bears great relevance on living-beings is its ability to absorb, and give off heat without causing abrupt changes in its own temperature (specific heat). This means that our body, which consists largely of water, is capable of moving from hot to cold environments, and vice-versa, while maintaining a constant body-temperature. For this same reason, water plays a key-role in the climate of coastal zones and around lakes. That explains how seas and lakes absorb heat during the warmer months with the water temperature remaining cooler than the surface temperature of the surrounding land masses. Conversely, during the winter months, water bodies mitigate the low temperatures by giving off accumulated heat.