Water exists in any one of the three classical states: solid (ice), liquid (water), and gas (steam or water vapor). The state of water depends on i) the intermolecular forces that draw molecules together and ii) the kinetic energy that leads to movements that pull them apart.
Water freezes when the intermolecular forces are greater than the kinetic energy. Unlike most other substances, water is less dense in its solid state than in its liquid state. This is because each water molecule can form hydrogen bonds with four molecules, forming a tetrahedral structure. This property allows ice to float on liquid water. Without the floating ice, bodies of water would freeze from the bottom up, killing aquatic life.
When kinetic energy is applied to ice in the form of heat, ice melts into liquid water. In this state, hydrogen bonds between water molecules constantly break and form again. When ice melts, the temperature of the water remains at the melting point until all the water is liquid. Only then will the water temperature increase.
As kinetic energy exceeds intermolecular forces, water becomes vapor. The process of generating steam from liquid water is called vaporization. Increases in kinetic energy can occur within the sample, as with boiling, or at the surface when water evaporates. Sublimation is the process by which a gas is generated directly from a solid without first going through a liquid phase. This transition depends on low atmospheric pressure, augmenting the effects of kinetic energy.
When kinetic energy decreases, steam can transition into a liquid in a process called condensation or directly into a solid in a process called deposition. Condensation leads to for rain and deposition for snow.
In the search for other biocompatible planets, the presence of water is a crucial feature, especially in liquid form, because life on Earth began in water. Enceladus is an ice-covered moon of Saturn that has water plumes or geysers on its southern pole. This initially generated much debate over whether Enceladus had liquid water under the ice, as the plumes were found to carry both steam and ice. However, the moon's orbit around Saturn and other clues suggest the presence of a vast liquid ocean in Enceladus, making it potentially hospitable for life.
