Water (H2O)

Summary: Water (H₂O) is a simple covalent molecule in which oxygen shares one electron pair with each of two hydrogen atoms, leaving two lone pairs on oxygen and giving a bent shape; it is tested with anhydrous copper(II) sulfate (white → blue) or cobalt(II) chloride (blue → pink) and is the most important solvent in chemistry. Tags: igcse chemistry Created: 2026-07-18

Water (H₂O) is a Compound of hydrogen and oxygen and one of the key examples of Covalent Bonding in the IGCSE syllabus. An oxygen atom (electronic configuration 2,6) needs two more electrons to reach the octet of neon (see Group 0 Noble Gases), while each hydrogen atom needs one more electron to reach the helium duplet; these needs are met when oxygen shares one pair of electrons with each of two hydrogen atoms, forming two single O—H covalent bonds. The two lone pairs left on the oxygen atom push the bonding pairs together, so the Molecule is bent (V-shaped) rather than linear. Water is a simple molecular substance, yet its boiling point (100 °C) is unusually high for such a small molecule because the attractions between water molecules are relatively strong for an Intermolecular Force. Water is an exceptionally good solvent, dissolving many ionic compounds and some covalent substances, which makes it the medium for most aqueous chemistry, aqueous electrolysis and reactions of acids and bases.

Bonding: Dot-and-Cross Description

In the Dot-and-Cross Diagram of H₂O, oxygen’s 6 outer electrons are drawn as dots and each hydrogen’s single electron as a cross:

• Oxygen contributes 1 electron to each of two shared pairs; each hydrogen contributes its 1 electron (Valence Electron) to one of them.
• The molecule therefore has 2 bonding pairs (each shown as one dot + one cross between O and an H).
• Oxygen’s remaining 4 electrons form 2 lone pairs on the oxygen atom; the hydrogens have no lone pairs.
• Counting shared electrons, oxygen has 8 outer electrons (octet) and each hydrogen has 2 (duplet).

Displayed formula: H—O—H, with the molecule bent at the oxygen because the two lone pairs repel the bonding pairs.

Key Data

Chemical Tests for Water

Two standard tests (see Chemical Tests):

These tests show that water is present, not that it is pure. Purity is checked by physical constants: pure water melts at exactly 0 °C and boils at exactly 100 °C; impurities lower the melting point and raise the boiling point (see Melting, Boiling).

Properties and Solvent Role

• Simple molecular liquid: colourless, with melting point 0 °C and boiling point 100 °C at atmospheric pressure.
• Excellent solvent: dissolves most Ionic Compounds (salts), and gases such as Ammonia and hydrogen chloride — the latter dissolving to give hydrochloric acid.
• Aqueous solutions are essential for Acids and Bases chemistry, Making Salts, Qualitative Analysis and Electrolysis of Aqueous Solutions.
• Pure water is a very poor conductor of electricity; it conducts when ions from dissolved substances are present.
• Formed in many reactions, e.g. combustion of hydrogen and methane, and in neutralisation (acid + base → salt + water).
• Domestic water supplies are purified by sedimentation, filtration and chlorination — see Water Treatment.

Sources

• Cambridge IGCSE Chemistry 0620 Syllabus (2023-2025) — Section 2.5, Cambridge Assessment International Education

Common Misconceptions