Big picture: Water's unique properties arise from its molecular structure — a bent molecule with polar covalent bonds creating a dipole that enables hydrogen bonding.
- Polarity
- The uneven distribution of charge in a molecule. In water, oxygen attracts electrons more strongly than hydrogen, creating a partial negative charge on oxygen and partial positive charges on hydrogen atoms.
- Hydrogen bond
- A weak electrostatic attraction between the partial positive charge on hydrogen and the partial negative charge on oxygen in adjacent water molecules.
- Cohesion
- The attraction between water molecules due to hydrogen bonding, creating surface tension.
- Adhesion
- The attraction between water molecules and other surfaces (e.g., glass, soil particles), enabling capillary action.
Environmental significance of water's polarity
- Excellent solvent for ionic and polar substances — enables nutrient transport
- Cohesion creates surface tension — supports small organisms and enables capillary action in plants
- Adhesion enables water movement through soil and plant tissues
- Dissolves gases (O2, CO2) — essential for aquatic life and carbon cycling
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- Specific heat capacity
- The amount of energy required to raise the temperature of 1 kg of a substance by 1°C. Water has a very high specific heat capacity (4,186 J/kg/°C).
- Latent heat
- The energy absorbed or released during phase changes (melting, evaporation) without temperature change. Water has high latent heat values.
How water regulates climate
- High specific heat capacity means oceans absorb and release heat slowly — moderating coastal climates
- Ocean currents redistribute heat from equator to poles
- Evaporation absorbs large amounts of energy — cooling effect
- Water vapour is a greenhouse gas — amplifies warming through positive feedback
- Lakes and rivers moderate local temperatures
IB exam tip: Link water's thermal properties directly to climate regulation — explain the mechanism, not just state the fact.
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Key concept: Water is one of few substances that is less dense as a solid than as a liquid. Ice floats. This property is critical for aquatic ecosystems.
Why ice floating matters
- Insulates water below, preventing lakes and oceans from freezing solid
- Aquatic organisms survive winter beneath ice cover
- Prevents destruction of benthic (bottom-dwelling) ecosystems
- Maintains habitat continuity through cold seasons
Water's transparency
- Light penetrates water, enabling photosynthesis by aquatic plants and phytoplankton
- Photic zone depth depends on water clarity
- Turbidity (suspended particles) reduces light penetration and productivity
- Clear water supports higher biodiversity in the photic zone