IGCSE, Edexcel, IB and AP Science Courses | Dr Dabour Hub Academy

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

│ ├── O is sp³
two lone pairs → bent shape
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

├── Renewability of the water cycle vs aquifer mining
└── Rational use, treatment, re-use and desalination

The Molecular Structure of Water, Its Unique Properties and Water Sustainability · 1 / 10

Scientific terms

Comparison aspect	Main idea	Keywords
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
Capillarity	the rise or fall of a liquid in a narrow tube caused by adhesion to the walls together with cohesion within the liquid	risefallliquidnarrow
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

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Laws and quantitative relations

Comparison aspect	Main idea	Keywords
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.	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 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	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

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

Comparison aspect	Main idea	Keywords
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 column	Hydrogenbondsholdwater
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

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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

Comparison aspect	Main idea	Keywords
•	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

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Comparison tables

Comparison aspect	Main idea	Keywords
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

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Worked examples

How much heat raises 2 kg of water from 20 °C to 100 °C? — Q = 2 × 4184 × 80 = 669,440 J ≈ 669 kJ.
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.
A water sample gives an effervescence with dilute HCl and a white precipitate on boiling. Identify the hardness. — Temporary hardness due to calcium bicarbonate.
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.
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

Comparison aspect	Main idea	Keywords
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 temperatures	Questionsexchangetemperatures
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 excluded	dissolvespolarionicsubstances
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 error	Reversingclassicobjectiveerror
Eutrophication kills by oxygen depletion	not because the algae are poisonous — the damage is done by the decomposing bacteria	becausealgaepoisonousdamage

The Molecular Structure of Water, Its Unique Properties and Water Sustainability · 10 / 10

Curriculum

Integrated Science

Unit 1

Unit 2

Unit 3

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