Water (H₂O) — Properties, Uses & Safety

Overview

Water (H2O) is the most abundant compound on Earth's surface and is essential for all known forms of life. With a molar mass of just 18.02 g/mol, this deceptively simple molecule consists of two hydrogen atoms covalently bonded to one oxygen atom in a bent molecular geometry with a bond angle of approximately 104.5 degrees. Water is a colorless, odorless liquid at standard temperature and pressure, with a melting point of 0 °C and a boiling point of 100 °C — values so fundamental they define the Celsius temperature scale. One of water's most remarkable properties is its polarity. The oxygen atom is far more electronegative than hydrogen, creating a significant dipole moment that allows water molecules to form hydrogen bonds with each other. This intermolecular hydrogen bonding explains many of water's unusual properties: its high specific heat capacity (4.184 J/g·°C), high heat of vaporization, surface tension, and the fact that ice is less dense than liquid water — a rare property that allows aquatic life to survive under frozen lakes. Often called the universal solvent, water dissolves more substances than any other common liquid, making it indispensable in biological systems, industrial processes, and laboratory chemistry. In nature, water participates in countless chemical reactions including photosynthesis, hydrolysis, and acid-base neutralization. The electrolysis of water (2 H2O → 2 H2 + O2) is a classic demonstration of decomposition reactions and is studied as a potential source of clean hydrogen fuel. Water is central to nearly every topic in a high school chemistry curriculum — from understanding molecular geometry and VSEPR theory, to exploring intermolecular forces, colligative properties, solution chemistry, and thermodynamics. Its role as both a reactant and product in neutralization reactions (HCl + NaOH → NaCl + H2O) makes it a staple of stoichiometry lessons. For students learning chemistry, water is the perfect starting point because its familiar, everyday presence belies a rich and complex chemistry that connects to virtually every branch of the science.

IUPAC Name

oxidane

CAS Number

7732-18-5

PubChem CID

962

Molecular Formula

H₂O

Molar Mass

18.02 g/mol

State at STP

liquid

Appearance

Colorless, odorless liquid

Physical Properties

Melting Point

0 °C

Boiling Point

100 °C

Density

0.997 g/cm³

Specific Heat

4.184 J/(g·K)

ΔH°f

-285.83 kJ/mol

ΔG°f

-237.13 kJ/mol

S°

69.91 J/(mol·K)

Composition

Bonding

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