General Chemistry
Water Molecule
H₂O — bent, polar, life's universal solvent with extraordinary properties
Water is H₂O — two hydrogen atoms covalently bonded to one oxygen. Bent geometry (104.5° angle from VSEPR with two lone pairs on O). Highly polar (large dipole moment 1.85 D) due to electronegativity difference (O = 3.44, H = 2.20) and bent shape. Strong hydrogen bonding network gives water unique properties: high BP (100°C), high specific heat, ice less dense than liquid, universal solvent for ionic/polar substances. Essential for life — covers 71% of Earth, comprises 60% of human body. Most-studied molecule in science.
- FormulaH₂O (2 H atoms, 1 O atom)
- GeometryBent (104.5° H-O-H angle)
- PolarityDipole moment 1.85 D (very polar)
- Boiling point100°C (anomalously high due to H-bonds)
- Specific heat4.18 J/g·K (very high)
- Density anomalyMax density at 4°C; ice less dense than water
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Why water matters
- Life. Universal solvent for biochemistry.
- Climate. Heat capacity moderates temperatures.
- Biology. Drives protein folding, DNA structure.
- Geology. Erosion, weathering.
- Industry. Most reactions in aqueous solution.
- Energy. Hydropower, steam.
- Astrobiology. Life-supporting solvent on other worlds?
Common misconceptions
- Water is straight. Bent at 104.5°.
- Water doesn't bond strongly. H-bonds extensive.
- Ice is denser than water. Less dense — that's why it floats.
- Pure water conducts electricity. Very poorly; needs ions.
- Water has fixed properties. Many properties unusual due to H-bonds.
- Water is simple. One of most-studied molecules in science.
Frequently asked questions
Why is water bent, not linear?
VSEPR. O has 4 electron domains: 2 bonds to H + 2 lone pairs. Tetrahedral electron geometry. Lone pairs make angle smaller than 109.5° → 104.5°. Linear shape would require sp hybridization (no lone pairs). Bent shape from tetrahedral electron arrangement plus lone pair compression.
Why is water polar?
Two reasons. (1) Each O-H bond is polar (ΔEN = 3.44 - 2.20 = 1.24). Oxygen pulls electrons toward itself. (2) Bent geometry — bond dipoles don't cancel. Result: net dipole moment 1.85 D (one of highest for small molecules). If water were linear (like CO₂), bond dipoles would cancel — nonpolar.
Why do water's properties seem anomalous?
Hydrogen bonding. Without H-bonds, water would be a gas at room temperature. (1) BP 100°C (vs predicted -90°C). (2) Surface tension 72.8 mN/m (very high). (3) High specific heat — energy goes into breaking H-bonds. (4) Density anomaly — H-bond network creates open structure when frozen → ice floats. (5) Universal solvent — H-bonds with many polar molecules.
What's water's role in biology?
(1) Solvent for biochemistry — most reactions in aqueous solution. (2) Temperature regulation — high specific heat moderates climate, body temperature. (3) Hydrogen bonding — drives protein folding, DNA structure. (4) Hydrophobic effect — creates cell membranes, protein cores. (5) Ice floats — protects aquatic life in winter. (6) Universal medium for life as we know it.
How does water dissolve salt?
Ion-dipole interaction. NaCl crystal: Na⁺ and Cl⁻ ions held by ionic bonds. Water molecules surround ions: O end (slightly negative) faces Na⁺; H end (slightly positive) faces Cl⁻. Hydration energy of ions exceeds lattice energy of crystal. Crystal dissolves. Each ion surrounded by hydration shell of water.
How does water freeze?
At 0°C, molecules slow enough for H-bonds to hold them in fixed positions. Hexagonal lattice forms (think snowflake). Each water bonds to 4 neighbors via H-bonds. Open structure → ice less dense than liquid water. Liquid water has more compact arrangement (4°C is densest). Below 4°C: structure starts opening up; freezes into hexagonal ice at 0°C.
What about heavy water?
D₂O (deuterium oxide) — H replaced with D (deuterium, ²H). Slightly different properties: BP 101.4°C, max density at 11.6°C, denser than regular water. Used in nuclear reactors as moderator (slows neutrons). Some toxicity to organisms at high concentration. Biochemistry uses deuterium labeling for tracing reactions.