Pure water (H₂O) is composed of two elements: hydrogen (H) and oxygen (O). In each water molecule, there are 2 hydrogen atoms and 1 oxygen atom, giving an atom ratio of H:O = 2:1. By mass, water contains 11.11% hydrogen and 88.89% oxygen. This composition is always fixed — obeying the law of constant composition. Water is the most abundant compound on Earth's surface.
Pure water formula: H₂O — 2 hydrogen atoms and 1 oxygen atom per molecule.
Mass percentage: 11.11% H and 88.89% O.
Mass ratio H:O = 1:8 (2g H : 16g O in one mole of water).
Molar mass of water = 18 g/mol.
Electrolysis produces H₂:O₂ = 2:1 by volume, confirming water composition.
Water's composition is always fixed — obeys the law of constant composition.
Pure water has melting point 0°C, boiling point 100°C, density 1 g/mL at 4°C.
Water is the universal solvent — dissolves more substances than any other liquid.
Molecular formula: H₂O
Atoms per molecule: • 2 hydrogen atoms • 1 oxygen atom • Total: 3 atoms per molecule
Atom ratio: H : O = 2 : 1
Mass composition: • Molar mass of H₂O = 2(1) + 16 = 18 g/mol • Mass of H in H₂O = 2 g/mol • Mass of O in H₂O = 16 g/mol
Percentage by mass: • % H = (2/18) × 100 = 11.11% • % O = (16/18) × 100 = 88.89%
Mass ratio H : O = 2 : 16 = 1 : 8
Therefore: • In 9 g of water: 1 g of H and 8 g of O • In 18 g of water: 2 g of H and 16 g of O
The composition of water can be verified by electrolysis:
2H₂O → 2H₂ + O₂ (electrolysis)
Volume ratio: • H₂ : O₂ = 2 : 1 (by volume, at the same temperature and pressure) • This matches the atom ratio in H₂O (2H : 1O)
In electrolysis experiment: • Twice as much hydrogen is collected at the cathode as oxygen at the anode • Confirming H₂O has H and O in 2:1 ratio by volume/moles
This experiment verifies:
Note: Pure water is required for electrolysis. A few drops of H₂SO₄ or NaOH are added to increase conductivity (ionic conductors), but only water is electrolysed.
Physical properties: • Molecular formula: H₂O • Molar mass: 18 g/mol • Melting point: 0°C (273.15 K) • Boiling point: 100°C (373.15 K) at 1 atm • Density: 1 g/mL (1 g/cm³) at 4°C (maximum density) • Colourless, odourless, tasteless • pH: 7 (neutral) • Kw = 1.0 × 10⁻¹⁴ at 25°C
Chemical properties: • Amphoteric — acts as acid or base • Universal solvent — dissolves many ionic and polar compounds • Reacts with reactive metals: 2Na + 2H₂O → 2NaOH + H₂↑ • Reacts with basic oxides: CaO + H₂O → Ca(OH)₂ • Reacts with acidic oxides: CO₂ + H₂O → H₂CO₃
Pure water vs tap water: Pure water contains only H₂O; tap water contains dissolved minerals, salts, and gases.
Pure water (H₂O) is composed of hydrogen and oxygen. It contains 2 hydrogen atoms and 1 oxygen atom per molecule (atom ratio H:O = 2:1). By mass, water is 11.11% hydrogen and 88.89% oxygen (mass ratio H:O = 1:8).
Water contains 11.11% hydrogen and 88.89% oxygen by mass. This is calculated from the molar masses: H₂O = 18 g/mol. Hydrogen contributes 2 g (11.11%) and oxygen contributes 16 g (88.89%) per mole of water.
The mass ratio of hydrogen to oxygen in water is 1:8. In every 9 g of water, 1 g is hydrogen and 8 g is oxygen. This ratio is always fixed regardless of the source of water.
Electrolysis of water (2H₂O → 2H₂ + O₂) produces hydrogen and oxygen in a 2:1 volume ratio. This confirms that water contains H and O in a 2:1 mole ratio (as in H₂O), verifying its chemical composition.
The molar mass of water (H₂O) is 18 g/mol. It is calculated as: 2 × mass of H + mass of O = 2(1) + 16 = 18 g/mol.
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