Anomalous Behaviour of Water: MDCAT Chemistry notes
Anomalous Behaviour of Water MDCAT notes: open cage structure of ice, why ice floats, 9% expansion on freezing, maximum density at 4 °C and its effects.
What is anomalous about water?
Most liquids contract when they freeze, so their solid is denser than the liquid and sinks. Water does the opposite: ice is less dense than liquid water and floats on it. This and several other unusual properties are due to hydrogen bonding.
Structure of ice
- In ice, each water molecule is hydrogen bonded to four other water molecules arranged tetrahedrally around it. Two of these bonds use its own hydrogens and two use its lone pairs.
- This fixed tetrahedral arrangement produces an open cage-like (hexagonal) structure with empty spaces in it.
- Because of these empty spaces, a given mass of ice occupies more volume than the same mass of liquid water.
- When ice melts, some hydrogen bonds break and the open structure partly collapses. Water molecules move into the empty spaces, so the volume decreases and the density increases.
Density and temperature
| Change | What happens to density |
|---|---|
| Ice at 0 °C melts to water at 0 °C | Density increases (volume decreases) |
| Water warmed from 0 °C to 4 °C | Density continues to increase |
| At 4 °C | Maximum density of water, about $1.00\ \mathrm{g\,cm^{-3}}$ |
| Above 4 °C | Normal thermal expansion, density decreases |
Water expands by about 9% on freezing. That is, ice occupies about 9% more volume than the same mass of liquid water. The density of ice is about $0.92\ \mathrm{g\,cm^{-3}}$.
Consequences
- Ice floats on water, so lakes and ponds freeze from the top downward. The ice layer insulates the water below, which stays liquid (near 4 °C at the bottom). This lets fish and other aquatic life survive winter.
- Water pipes burst in very cold weather because water expands on freezing.
- Weathering of rocks: water entering cracks expands on freezing and splits the rock.
Other hydrogen-bond effects in water
- Much higher boiling point than $\mathrm{H_2S}$ or other group 16 hydrides.
- High surface tension, high specific heat ($4.18\ \mathrm{J\,g^{-1}\,K^{-1}}$) and high heat of vaporization.
Common MDCAT traps
- Ice floats because of empty spaces in its hydrogen-bonded structure, not because of weak intermolecular forces or a cubic structure.
- The expansion on freezing is about 9 percent, not "9 times". Read options carefully.
- Maximum density of water is at 4 °C, not at 0 °C.
- Ice at 0 °C occupies more volume than water at 0 °C; the cause is empty spaces, not ionic bonds or Debye forces.
Quick revision
- Each molecule in ice is hydrogen bonded to four others tetrahedrally.
- Ice has an open cage structure with empty spaces.
- Water expands by about 9% on freezing.
- Water is densest at 4 °C.
- Lakes freeze from the top, protecting aquatic life.