Water

Water (H₂O) has a bent shape and uneven electron distribution. Oxygen is more electronegative than hydrogen → pulls bonding electrons closer → O has partial negative charge (δ−), H atoms have partial positive charge (δ+).

This makes water a polar molecule.

Hydrogen bonds. The δ+ H of one water molecule attracts the δ− O of another. Each water molecule can form up to 4 hydrogen bonds simultaneously. Individually weak, but collectively they give water its remarkable properties.

Cohesion — water molecules stick to other water molecules. Enables continuous columns of water in xylem (transpiration pull) and surface tension (e.g. pond skaters walking on water).

Adhesion — water sticks to other polar surfaces. Enables capillary rise in narrow xylem vessels; root hairs absorb water.

Solvent properties — polar water dissolves polar and ionic substances (glucose, salts, amino acids). Non-polar substances (lipids) are hydrophobic — basis for membrane structure. Essential for transport in blood and tissue fluid.

Thermal properties.

• High specific heat capacity — a lot of energy needed to raise water's temperature (breaks H-bonds first). Stabilises cell and body temperature; moderates ocean and lake temperatures.
• High heat of vaporisation — converting liquid to vapour requires breaking many H-bonds. Sweating and panting in animals, transpiration in plants, all use water's evaporation to cool effectively.

Ice less dense than water. In ice, water molecules lock into a rigid lattice with more space between them than in liquid water. Ice floats — insulating water below in winter, allowing aquatic life to survive frozen surfaces.

Comparison: water (polar) vs methane (non-polar).

Property	Water (H₂O)	Methane (CH₄)
Polarity	Polar	Non-polar
Hydrogen bonds	Yes	No
Boiling point	100 °C	−162 °C
State at 25 °C	Liquid	Gas
Solvent ability	Polar/ionic	Non-polar

Without hydrogen bonding water would be a gas at biological temperatures — no liquid medium for life.