The Molecular Structure of Water, Its Unique Properties and Water Sustainability

• Heat relation: Q = m·c·ΔT with c_water = 4184 J/(kg·°C) = 1 cal/(g·°C) — the highest specific heat of any common liquid.
• Latent heat of vaporisation: 2,260 kJ/kg at 100 °C (≈ 540 cal/g); evaporation of 1 g of sweat removes about 580 cal from the skin.
• Latent heat of fusion: 334 kJ/kg (80 cal/g) of ice.
• Density: ρ = 1.000 g/cm³ at 4 °C (maximum), 0.917 g/cm³ for ice at 0 °C — ice is therefore about 9% lighter and floats.
• Capillary rise: h = 2γcosθ/(ρgr) — the narrower the tube, the higher the rise.
• Water footprint: consumption per capita per year; below 1,000 m³ a country is in water poverty, below 500 m³ under absolute scarcity (Egypt is under this line and meets the gap by virtual-water imports).

Oxygen's electron configuration is 1s² 2s² 2p⁴; it needs two electrons to complete its shell, so it shares them with two hydrogen atoms. The four electron pairs around oxygen are sp³ hybridised, but two of them are lone pairs, so the molecule is bent (V-shaped) with an H–O–H angle of 104.5°, not tetrahedral 109.5°.

• Oxygen (EN 3.5) attracts the shared electrons more strongly than hydrogen (EN 2.1), so oxygen carries a partial negative charge δ− and each hydrogen a partial positive charge δ+ — the molecule is a dipole.
• Each water molecule can form up to four hydrogen bonds with its neighbours: two through its hydrogen atoms and two through the lone pairs of oxygen.
• In the liquid the bonds break and reform about 10¹² times a second; in ice the network is fixed and open, which is why ice is less dense.
• Hydrogen bonding also explains the anomalously high boiling points of the hydrides of group 16: H₂O (100 °C) far above H₂S (−60 °C), H₂Se and H₂Te.

• Universal solvent for polar and ionic substances. Water molecules surround ions with their oppositely charged ends (hydration shells) and pull them apart; NaCl dissolves, while non-polar oils and fats do not — "like dissolves like". Ions are therefore electrolytes and conduct the current.
• Cohesion and adhesion. Hydrogen bonds hold water molecules together (cohesion) and to the walls of a xylem vessel or a glass tube (adhesion). Together they give high tensile strength to the water column.
• Surface tension lets a needle float, makes water striders walk on ponds, and produces the beading of droplets on a waxed surface. Detergents lower surface tension and allow water to wet grease.
• Capillarity raises water in the fine tubes of plant xylem and in soil, counter part of transpiration pull.
• Thermal buffering. High specific heat moderates coastal climates and keeps the body temperature constant; high latent heat of vaporisation makes sweating an effective cooling mechanism.
• Transparency to visible light lets algae and aquatic plants photosynthesise, while water absorbs infrared and ultraviolet — the protection of organisms below the surface.

Most substances contract continuously on cooling. Water contracts until 4 °C and then expands on further cooling because the open hydrogen-bonded lattice of ice holds the molecules farther apart than the liquid does.

• Consequence: at 0 °C ice floats and forms an insulating lid on lakes and seas, so the water below stays near 4 °C and aquatic life survives the winter.
• In deep lakes in cold regions, water turns over twice a year because of this density maximum — spring and autumn overturn — which brings oxygen down and nutrients up.
• Without the anomaly, ice would sink, lakes would freeze from the bottom up and freshwater ecosystems would not exist in cold climates.
• Same principle applied: water pipes burst, and rocks crack by frost shattering — a case of mechanical weathering.

A lake or a river is an ecosystem in which the food chain runs from phytoplankton (producers) to zooplankton (primary consumers), small fish, large fish and top predators. Energy moves one way and is lost at each trophic level as heat, giving the classic 10% rule: only about one tenth of the energy of a level reaches the next.

1. How much heat raises 2 kg of water from 20 °C to 100 °C? — Q = 2 × 4184 × 80 = 669,440 J ≈ 669 kJ.
2. How much steam is needed to cool a 70 kg athlete by 2 °C by sweating alone (assume no other loss)? — Q = 70 × 3500 × 2 ≈ 490 kJ (body tissue c ≈ 3500 J/kg·°C), mass of water = 490,000 / 2,400,000 ≈ 0.2 kg = 200 mL.
3. A water sample gives an effervescence with dilute HCl and a white precipitate on boiling. Identify the hardness. — Temporary hardness due to calcium bicarbonate.
4. A capillary of radius 0.2 mm is dipped in water (γ = 0.073 N/m, θ = 0°). Find the rise. — h = 2γ/(ρgr) = 2(0.073)/(1000 × 9.8 × 2×10⁻⁴) ≈ 7.4 cm.
5. A family uses 0.25 m³ of water a day. Its annual volume, and how it compares with the water footprint of 1 kg of beef (about 15,000 L)? — 0.25 × 365 = 91 m³/yr, so one kilogram of beef costs about 5 months of the family's domestic water.