Water

Water is the chemical compound H₂O, made of two hydrogen atoms and one oxygen atom. At ordinary temperatures and pressure it is liquid, and it is essential to known life.

Water molecule: A molecule consisting of two hydrogen atoms bonded to one oxygen atom. Its bent shape and uneven charge distribution give bulk water many distinctive properties.
Chemical bond: An attraction that holds atoms or ions together in a substance. The O–H bonds within each molecule are distinct from attractions between water molecules.
Solvent: A substance that dissolves another substance to form a solution. Water dissolves many ionic and polar compounds, making it a common solvent in nature and technology.
Hydrogen sulfide: A compound of hydrogen and sulfur with the formula H₂S, gaseous at ordinary temperatures. Its similarity to water’s molecular formula makes their sharply different boiling points revealing.
Liquid water: Water in the fluid state, with a definite volume but no fixed shape. Its persistence across much of Earth’s surface provides habitats and a medium for life’s chemistry.
Hydrogen bond: An attraction between a hydrogen atom bonded to an electronegative atom and another electronegative atom. Hydrogen bonds link neighboring water molecules and help explain cohesion and water’s high boiling point.
Oxygen: A chemical element with atomic number 8 that forms a major component of Earth’s atmosphere and many compounds. Oxygen supplies the central atom in every water molecule.
Osmosis: The net movement of water across a selectively permeable membrane toward a region of higher solute concentration. Water movement by osmosis helps regulate cell volume and transport across membranes.
Ammonia: A nitrogen hydride with the formula NH₃, usually encountered as a colorless gas. Like water, ammonia is polar and hydrogen-bonding, but its molecular structure and bulk properties differ.
Habitability: The capacity of an environment to support life as understood under specified conditions. The availability of liquid water is a major criterion in assessing environments for known life.

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