Anomalous Water Expansion
Water density peaks at 4°C — unique behavior.
Key Notes
Water has ANOMALOUS thermal expansion: density INCREASES as it COOLS from 4°C down to 0°C (volume increases).
Density MAXIMUM at 4°C: ρ = 1000.0 kg/m³.
Below 4°C: water EXPANDS as it cools further (negative γ).
When water freezes at 0°C: volume EXPANDS by ~9% (ice less dense than water).
Why ice FLOATS on water: ice (ρ ≈ 917 kg/m³) < water (ρ = 1000 kg/m³).
Lakes freeze TOP-DOWN: dense 4°C water sinks; cold (less dense) water and ice float on top.
Result: aquatic life survives under ice — bottom water stays at 4°C through winter.
Origin: hydrogen bonding in water creates open hexagonal structure in ice, denser packing in liquid.
Formulas
Maximum density of water
Critical reference point.
Density of ice (0°C)
About 8% less than water.
Volume of ice vs water
Volume increases ~9% on freezing.
Fraction of iceberg above water
About 9% above water (90% below).
Important Points
Most substances: density decreases monotonically with T. Water has a unique maximum at 4°C.
Why lakes don't freeze SOLID: dense 4°C water sinks, ice floats. Marine life survives winter.
Pipe bursts in winter: water expands ~9% on freezing — destructive force inside containers.
Iceberg: only ~10% sticks above water (Titanic was struck by submerged portion).
Anomaly is due to hydrogen bonding — at low T, water molecules form an open, less-dense lattice.
Few other substances (Si, Ge, Ga, Bi) also expand on freezing — but water's effect is most dramatic.
Anomalous Water Expansion notes from sciphylab (also known as SciPhy, SciPhy Lab, SciPhy Labs, Physics Lab). Class 11 physics revision for JEE Mains, JEE Advanced, NEET UG, AP Physics 1/2/C, SAT, and CUET-UG.