explain how hydrogen bonding occurs between water molecules and relate the properties of water to its roles in living organisms, limited to solvent action, high specific heat capacity and latent heat of vaporisation

Water: Molecular Structure, Hydrogen Bonding & Core Biological Properties

1. Molecular structure of H₂O

  • Formula: H₂O – one oxygen atom covalently bonded to two hydrogen atoms.

  • Geometry: Bent (V‑shaped) with an H–O–H bond angle ≈ 104.5° (≈ 105°) (Cambridge 9700, p. 2.4).

  • Polarity: Oxygen (χ = 3.44) is far more electronegative than hydrogen (χ = 2.20); the molecule carries a partial negative charge (δ⁻) on O and partial positive charges (δ⁺) on each H.

  • Dipole moment: ≈ 1.85 D (Cambridge 9700, p. 2.4).

2. Hydrogen bonding between water molecules

When two water molecules approach each other, the δ⁺ hydrogen of one is attracted to the δ⁻ oxygen of another. This electrostatic attraction is a hydrogen bond (≈ 20 kJ mol⁻¹). Each molecule can form up to four hydrogen bonds – two as a donor (via its H atoms) and two as an acceptor (via the two lone pairs on O).

O

O

H‑bond

Water molecule (blue) forming hydrogen bonds (red arrows) with neighbours in a tetrahedral arrangement.

3. Core properties required by the syllabus

PropertyTypical value (SI)Biological significance (role in living organisms)
Solvent action (polarity)Dipole ≈ 1.85 D

• Dissolves ions and polar molecules by forming hydration shells.

• Allows transport of nutrients, metabolites, gases and waste in blood plasma and cytosol.

• Provides the medium for enzyme‑catalysed reactions and cellular signalling.

Specific heat capacity (c)4.18 J g⁻¹ °C⁻¹ (Cambridge 9700, p. 2.4)

• Large amount of heat must be absorbed/released before temperature changes – buffers internal temperature of cells, blood and whole organisms.

• Stabilises temperature of aquatic habitats, protecting ectothermic animals and preventing rapid thermal shock.

Latent heat of vaporisation (Lᵥ)2260 J g⁻¹ at 100 °C (Cambridge 9700, p. 2.4)

• Energy required to break the extensive hydrogen‑bond network during evaporation.

• Enables efficient evaporative cooling: sweating in mammals, panting in birds, and transpiration in plants.

3.1 Solvent action – example

When solid sodium chloride dissolves:

NaCl(s) → Na⁺(aq) + Cl⁻(aq)

The polar water molecules surround each ion, orienting the δ⁻ oxygen towards Na⁺ and the δ⁺ hydrogens towards Cl⁻, forming a hydration shell that stabilises the ions in solution.

3.2 High specific heat – quantitative illustration

Energy to raise 10 g of water by 5 °C:

q = m c ΔT = 10 g × 4.18 J g⁻¹ °C⁻¹ × 5 °C ≈ 209 J

This relatively large energy requirement means that body fluids can absorb metabolic heat without large temperature fluctuations.

3.3 Latent heat of vaporisation – biological use

During sweating, 1 g of water evaporated from the skin removes ≈ 2260 J of heat, a substantial cooling effect that helps maintain a stable core temperature during exercise or in hot environments.

A‑Level extensions (beyond the AS requirement)

4. Cohesion, surface tension & capillary action

  • Cohesion: hydrogen bonds between water molecules give rise to a high surface tension (≈ 0.072 N m⁻¹ at 20 °C).

    – Enables capillary rise in plant xylem (cohesion‑tension theory).

    – Contributes to alveolar stability in lungs.

5. Density anomaly

  • Maximum density at 4 °C (1.00 g cm⁻³).

    – Ice is less dense than liquid water, so it floats, insulating aquatic life during winter.

6. Viscosity

  • Viscosity ≈ 1.0 mPa·s at 20 °C – low enough for rapid circulation of blood and sap, yet provides sufficient drag for controlled cytoplasmic streaming.

7. Colligative properties (boiling‑point elevation & freezing‑point depression)

  • Solutes lower the freezing point (e.g., seawater freezes at ≈ ‑1.9 °C).

    – Important for organisms in cold habitats and for the production of antifreeze proteins.

  • Boiling‑point elevation is modest for physiological solute concentrations but illustrates the effect of solute particles on phase change.

8. Water potential (A‑Level only)

Water potential (Ψ) quantifies the tendency of water to move:

Ψ = Ψₛ + Ψₚ + Ψg + Ψm

  • Ψₛ (solute potential) = ‑RT C (negative, proportional to solute concentration).

    Example: 0.1 M NaCl → Ψₛ ≈ ‑2.5 MPa at 25 °C.

  • Ψₚ (pressure potential) is positive in turgid cells, negative in xylem under tension.

  • Ψg (gravitational) and Ψm (matrix) become relevant in tall plants and soils.

Summary

  • The bent, polar structure of H₂O leads to extensive hydrogen bonding (up to four bonds per molecule).

  • Hydrogen bonds give water three core properties required by the Cambridge AS & A‑Level syllabus:

    • Solvent action – dissolves ionic and polar substances, enabling transport and biochemical reactions.

    • High specific heat capacity – buffers temperature changes, crucial for homeostasis and aquatic ecosystems.

    • High latent heat of vaporisation – provides powerful evaporative cooling mechanisms.

  • Additional phenomena such as cohesion, surface tension, density anomaly, viscosity, colligative properties and water potential are valuable A‑Level extensions that illustrate the wider impact of hydrogen bonding on plant transport, respiration, and environmental adaptation.

  • These properties collectively make water indispensable for the structure, function, and survival of living organisms.