Water (H2O)
Also known as: dihydrogen monoxide, oxidane
Water is the most familiar compound on Earth and the standard against which chemistry measures much else — the litre, the calorie and the pH scale all lean on it. Each molecule is two hydrogen atoms covalently bonded to one oxygen at a bent angle of about 104.5°, making water strongly polar. That polarity is why water dissolves salts and sugars so well, why ice floats, and why life is possible in it.
Molar mass breakdown
18.015 g/mol| Element | Atoms | Mass (g/mol) | % by mass |
|---|---|---|---|
| Oxygen (O) | 1 | 15.999 | 88.81% |
| Hydrogen (H) | 2 | 2.016 | 11.19% |
What water is used for
- Universal solvent for biological and lab chemistry
- Temperature regulation (high specific heat capacity)
- Reactant in photosynthesis and hydrolysis reactions
- Standard for density (1 g/mL at 4 °C) and the Celsius scale
Key facts
- Molar mass ≈ 18.02 g/mol — one of the smallest molecules, yet liquid at room temperature thanks to hydrogen bonding.
- Pure water self-ionises slightly, giving pH 7 at 25 °C.
- Ice is less dense than liquid water — almost unique among substances.
Frequently asked questions
What is the molar mass of H2O?
About 18.02 g/mol: 2 × 1.008 (H) + 15.999 (O).
Is water an element or a compound?
A compound — it contains two elements, hydrogen and oxygen, chemically bonded in a fixed 2:1 ratio.
Why is water polar?
Oxygen attracts the shared electrons more strongly than hydrogen and the molecule is bent, so charge is distributed unevenly — a negative side (O) and a positive side (H).