Water exists in three main states: solid (ice), liquid, and gas (steam). The state water is in depends on the intermolecular forces that draw water molecules together and the kinetic energy that pulls 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 it is in its liquid state. This is because each water molecule is capable of bonding with four molecules, forming a spaced, tetrahedral organization. This characteristic of water allows ice to float. Without 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, bonds between water molecules constantly break and form again. When ice melts, the temperature of the water remains at the melting point until the whole volume is liquid. Only then will the water temperature increase beyond the melting point.

As kinetic energy overpowers intermolecular forces, liquid water (or even ice) turns into a gas. The process of generating steam from liquid water is called vaporization. Increases in kinetic energy can occur within the sample, as with boiling, or can happen at the surface when water evaporates. The process by which a gas is generated directly from a solid, without first going through a liquid phase, is called sublimation. This transition is dependent on low atmospheric pressure, augmenting the effects of kinetic energy.

When kinetic energy drops, steam can transition into a liquid, a process called condensation, or directly into a solid, a process called deposition or desublimation. Condensation accounts for rain, and deposition accounts 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 has 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.