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

Integrated Science

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

Overview

Overview

  • Explain the bent structure of the water molecule and how hydrogen bonding produces its anomalous properties.
  • Relate cohesion, adhesion, surface tension and capillarity to living systems.
  • Interpret the density anomaly at 4 °C and its biological consequence in lakes and seas.
  • Judge water quality from dissolved oxygen, pH and hardness, and discuss the sustainability of water resources.

🌳 Concept tree

Water→Structure, Properties and Sustainability

├── The molecule

  1. │ ├── O is sp3
  2. two lone pairs → bent shape
  3. bond angle 104.5°

│ ├── Polar covalent O–H bonds (ΔEN) → dipole

│ └── Hydrogen bonds→2 donated + 2 accepted per molecule

├── Bulk properties

  • │ ├── Cohesion / adhesion → surface tension, capillarity
  • │ ├── High specific heat capacity (4184 J/kg·°C) → climate buffering
  • │ ├── High latent heat of vaporisation → evaporative cooling
  • │ └── Universal solvent polar and ionic substances dissolve

├── The density anomaly

│ ├── Maximum density at 4 °C

  • │ └── Ice is less dense → it floats → aquatic life survives winter

├── Water quality

  • │ ├── pH 6.5–8.5 · dissolved oxygen · BOD · turbidity
  • │ └── Hardness temporary (Ca(HCO₃)₂) vs permanent (CaSO₄, MgSO₄)

└── Sustainability

├── Virtual water and water footprint

  1. ├── Renewability of the water cycle vs aquifer mining
  2. └── Rational use, treatment, re-use and desalination
The Molecular Structure of Water, Its Unique Properties and Water Sustainability · 1 / 10
Scientific terms

Scientific terms

Comparison aspectMain ideaKeywords
Polar molecule
  • a molecule with a partial negative charge on one side and a partial positive charge on the other
  • as in H₂O
partialnegativechargeside
Hydrogen bond
  • the weak electrostatic attraction between a hydrogen atom covalently bonded to O
  • N or F and a lone pair on another electronegative atom
weakelectrostaticattractionbetween
Cohesion
  • attraction between like molecules (water–water)
  • adhesion is attraction between unlike surfaces (water–glass)
attractionbetweenlikemolecules
Surface tension
  • the energy needed to increase the surface area of a liquid by one unit
  • it is the result of cohesion at the surface
energyneededincreasearea
Capillaritythe rise or fall of a liquid in a narrow tube caused by adhesion to the walls together with cohesion within the liquidrisefallliquidnarrow
Hardness
  • the dissolved calcium and magnesium salts in water
  • temporary hardness is due to bicarbonates
  • permanent hardness to sulfates and chlorides
dissolvedcalciummagnesiumsalts
Dissolved oxygen (DO)
  • the amount of O₂ free in water
  • typically 4–14 mg/L
  • it indicates the health of an aquatic ecosystem
amountfreewatertypically
Biological oxygen demand (BOD)
  • the oxygen needed by micro-organisms to decompose the organic matter in a volume of water
  • high BOD means pollution
neededmicroorganismsdecompose
Virtual water
  • the water embedded in a produced commodity — 1 kg of wheat needs about 1
  • 000–1
  • 500 L of water
embeddedproducedcommoditywheat
The Molecular Structure of Water, Its Unique Properties and Water Sustainability · 2 / 10
Laws and quantitative relations

Laws and quantitative relations

Comparison aspectMain ideaKeywords
Heat relationQ = m·c·ΔT with cwater = 4184 J/(kg·°C) = 1 cal/(g·°C) — the highest specific heat of any common liquid.cwater4184highestspecific
Latent heat of vaporisation
  • 260 kJ/kg at 100 °C (≈ 540 cal/g)
  • evaporation of 1 g of sweat removes about 580 cal from the skin
evaporationsweatremovesskin
Latent heat of fusion334 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
maximumthereforelighterfloats
Capillary rise
  • h = 2γcosθ/(ρgr) — the narrower the tube
  • the higher the rise
2γcosθnarrowertubehigher
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)
consumptioncapitayearbelow
The Molecular Structure of Water, Its Unique Properties and Water Sustainability · 3 / 10
Molecular structure and hydrogen bonding

Molecular structure and hydrogen bonding

  • 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.
Give the reason→water has a high boiling point and a high specific heat. Because energy is first consumed in breaking the network of hydrogen bonds between the molecules before they can separate or move faster.
The Molecular Structure of Water, Its Unique Properties and Water Sustainability · 4 / 10
Unique properties: solubility, surface tension and capillarity

Unique properties: solubility, surface tension and capillarity

Comparison aspectMain ideaKeywords
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
Watermoleculessurroundions
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 columnHydrogenbondsholdwater
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
letsneedlefloatmakes
Capillarity
  • raises water in the fine tubes of plant xylem and in soil
  • counter part of transpiration pull
raiseswaterfinetubes
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
Highspecificheatmoderates
Transparency
  • to visible light lets algae and aquatic plants photosynthesise
  • while water absorbs infrared and ultraviolet — the protection of organisms below the surface
visiblelightletsalgae
The Molecular Structure of Water, Its Unique Properties and Water Sustainability · 5 / 10
The density anomaly and its biological meaning

The density anomaly and its biological meaning

  • 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.
The Molecular Structure of Water, Its Unique Properties and Water Sustainability · 6 / 10
Aquatic ecosystems, water quality and sustainability

Aquatic ecosystems, water quality and sustainability

Comparison aspectMain ideaKeywords
•
  • A lake or a river is an ecosystem in
  • the food chain runs from phytoplankton (producers) to zooplankton (primary consumers)
  • small fish
  • large fish and top predators
lakeriverecosystemfood
•
  • 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
Energymoveslosttrophic
Good-quality fresh water
  • pH 6.5–8.5
  • DO above 5 mg/L
  • low BOD
  • low turbidity and no pathogenic coliforms
aboveturbiditypathogeniccoliforms
Eutrophication
  • excess nitrates and phosphates from fertilisers and detergents cause an algal bloom
  • the algae die
  • bacteria decompose them
  • BOD rises
  • DO falls and fish die — the water becomes septic and anaerobic
excessnitratesphosphatesfertilisers
Thermal pollution
  • from power stations lowers the solubility of oxygen and speeds metabolism
  • worsening the oxygen deficit
powerstationslowerssolubility
Sustainable use
  • lining canals and pipes to stop seepage
  • drip and sprinkler irrigation instead of flooding
  • treated sewage re-use for agriculture
  • industrial cooling loops
  • rainwater harvesting and desalination of seawater for drinking
liningcanalspipesstop
The water–energy–food nexus
  • pumping
  • desalinating and treating water all consume energy
  • so saving water saves energy and cuts carbon emissions at the same time
pumpingdesalinatingtreatingconsume
The Molecular Structure of Water, Its Unique Properties and Water Sustainability · 7 / 10
Comparison tables

Comparison tables

Comparison aspectMain ideaKeywords
Cohesion vs adhesion
  • like vs unlike molecules
  • responsible for surface tension and the unbroken water column vs responsible for wetting
  • capillary rise and meniscus shape
likeunlikemoleculesresponsible
Temporary vs permanent hardness
  • Ca(HCO₃)₂ removable by boiling (it decomposes to CaCO₃↓) vs CaSO₄/MgSO₄ not removed by boiling
  • removed by washing soda Na₂CO₃ or by ion-exchange resin
HCO₃removableboilingdecomposes
Soft vs hard water
  • lathers easily with soap
  • dangerous to boilers if used directly vs forms scum with soap (calcium stearate)
  • wastes detergent
  • and is better for drinking
latherseasilysoapdangerous
Surface water vs groundwater
  • rivers
  • lakes and reservoirs exposed to pollution and evaporation vs aquifers that are filtered and protected but refill extremely slowly
  • so over-pumping is mining
  • not use
riverslakesreservoirsexposed
Distilled vs mineral water
  • pure H₂O
  • no minerals
  • flat taste
  • used in laboratories and batteries vs contains dissolved salts needed by the body
puremineralsflattaste
The Molecular Structure of Water, Its Unique Properties and Water Sustainability · 8 / 10
Worked examples

Worked examples

  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.
The Molecular Structure of Water, Its Unique Properties and Water Sustainability · 9 / 10
Ministry exam traps

Ministry exam traps

Comparison aspectMain ideaKeywords
104.5°, not 109.5°.
  • The lone pairs push the bonding pairs closer
  • so the angle is smaller than a perfect tetrahedron
lonepairspushbonding
Maximum density is at 4 °C, not at 0 °C.Questions exchange the two temperaturesQuestionsexchangetemperatures
Ice floating is a density argument
  • not a temperature one
  • state it as “because the density of ice is less than that of water”
temperaturestatebecauseless
Water is not a universal solvent.It dissolves polar and ionic substances only — non-polar fats are excludeddissolvespolarionicsubstances
Hydrogen bonds are intermolecular
  • (between molecules)
  • not the covalent O–H bond inside the molecule
betweenmoleculescovalentinside
Temporary hardness is removed by boiling; permanent is not.Reversing this is a classic objective errorReversingclassicobjectiveerror
Eutrophication kills by oxygen depletionnot because the algae are poisonous — the damage is done by the decomposing bacteriabecausealgaepoisonousdamage
The Molecular Structure of Water, Its Unique Properties and Water Sustainability · 10 / 10