Water — Revision Notes

⚡ 30-Second Revision

Structure: — Bent, $sp^3$ hybridized oxygen, $104.5^circ$ bond angle.
Polarity: — Highly polar due to electronegativity difference and bent shape.
Hydrogen Bonding: — Extensive H-bonding, responsible for unique properties.
Anomalous Properties: — High BP ($100^circ C$), MP ($0^circ C$), specific heat ($4.184,\text{J/g}^circ C$), latent heats. Max density at $4^circ C$ (ice floats).
Hardness: — Due to $Ca^{2+}$, $Mg^{2+}$ salts.

- Temporary: Bicarbonates ($Ca(HCO_3)_2$, $Mg(HCO_3)_2$). Removed by boiling ($Ca(HCO_3)_2 xrightarrow{\text{heat}} CaCO_3 downarrow + H_2O + CO_2$) or Clark's method ($Ca(HCO_3)_2 + Ca(OH)_2 \rightarrow 2CaCO_3 downarrow + 2H_2O$).

- Permanent: Chlorides, sulfates ($CaCl_2$, $MgCl_2$, $CaSO_4$, $MgSO_4$). - Removal (Permanent): - Washing Soda: $CaCl_2 + Na_2CO_3 \rightarrow CaCO_3 downarrow + 2NaCl$. - Calgon: $Na_6P_6O_{18}$ forms soluble complexes with $Ca^{2+}$, $Mg^{2+}$.

- Ion-exchange (Zeolite/Resins): $2NaZ + Ca^{2+} \rightarrow CaZ_2 + 2Na^+$; $2RSO_3H + Ca^{2+} \rightarrow (RSO_3)_2Ca + 2H^+$; $RNH_3OH + Cl^- \rightarrow RNH_3Cl + OH^-$.

Heavy Water ($D_2O$): — Deuterium oxide. Higher density, MP, BP. Used as nuclear moderator (slows neutrons, low absorption).

2-Minute Revision

Water ($H_2O$) is a polar molecule with a bent geometry, forming extensive hydrogen bonds. These bonds are responsible for its anomalous properties: high boiling point, high specific heat capacity, and the unique phenomenon of ice floating (maximum density at $4^circ C$).

Water is an excellent 'universal solvent' due to its polarity and high dielectric constant. Natural water often contains dissolved calcium and magnesium salts, leading to 'hardness'. Temporary hardness, caused by bicarbonates, can be removed by boiling or Clark's method (adding lime).

Permanent hardness, due to chlorides and sulfates, requires more advanced methods like the washing soda method (precipitating carbonates), Calgon method (sequestering ions in soluble complexes), or ion-exchange (exchanging hardness ions for $Na^+$ or $H^+/OH^-$).

Heavy water ($D_2O$), containing deuterium, has higher physical constants than $H_2O$ and is primarily used as a moderator in nuclear reactors to slow down neutrons without significant absorption. Water also exhibits amphoteric behavior, acting as both an acid and a base, and participates in various hydrolysis and redox reactions.

5-Minute Revision

Water ($H_2O$) is a crucial compound, characterized by its bent molecular structure ($104.5^circ$ bond angle) and $sp^3$ hybridized oxygen. Its high polarity, stemming from the electronegativity difference between oxygen and hydrogen, enables extensive hydrogen bonding.

This network of hydrogen bonds dictates many of water's unique properties: an unusually high boiling point ($100^circ C$) and melting point ($0^circ C$), high specific heat capacity ($4.184,\text{J/g}^circ C$), and high latent heats of fusion and vaporization.

A key anomaly is its maximum density at $4^circ C$, meaning ice is less dense than liquid water and floats, which is vital for aquatic ecosystems. Water's high dielectric constant (approx. 80) makes it an exceptional solvent for ionic and polar compounds.

Water hardness is caused by dissolved $Ca^{2+}$ and $Mg^{2+}$ ions. Temporary hardness, due to bicarbonates like $Ca(HCO_3)_2$, can be removed by simple boiling, which precipitates $CaCO_3$: $Ca(HCO_3)_2(aq) xrightarrow{\text{heat}} CaCO_3(s) downarrow + H_2O(l) + CO_2(g)$.

Clark's method also removes temporary hardness by adding lime ($Ca(OH)_2$): $Ca(HCO_3)_2(aq) + Ca(OH)_2(s) \rightarrow 2CaCO_3(s) downarrow + 2H_2O(l)$. Permanent hardness, caused by chlorides and sulfates (e.

g., $CaCl_2$, $MgSO_4$), requires more robust methods. The washing soda method uses $Na_2CO_3$ to precipitate the ions: $CaCl_2(aq) + Na_2CO_3(aq) \rightarrow CaCO_3(s) downarrow + 2NaCl(aq)$. The Calgon method employs sodium hexametaphosphate ($Na_6P_6O_{18}$) to form soluble complexes with $Ca^{2+}$ and $Mg^{2+}$ ions, effectively sequestering them.

The ion-exchange method, using zeolite or synthetic resins, exchanges hardness ions for $Na^+$ or $H^+/OH^-$ ions, producing demineralized water. For example, with a cation resin: $2RSO_3H(s) + Ca^{2+}(aq) \rightarrow (RSO_3)_2Ca(s) + 2H^+(aq)$.

Heavy water ($D_2O$), containing deuterium, has higher physical constants (density, MP, BP) than normal water. Its primary use is as a moderator in nuclear reactors, slowing down neutrons without significant absorption, crucial for sustaining chain reactions.

Water also exhibits amphoteric behavior, acting as both a Bronsted-Lowry acid (donating $H^+$ to form $OH^-$) and a base (accepting $H^+$ to form $H_3O^+$), as seen in its autoionization: $2H_2O(l) \rightleftharpoons H_3O^+(aq) + OH^-(aq)$.

It also participates in various redox and hydrolysis reactions.

Prelims Revision Notes

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Water Structure & Bonding: — $H_2O$ is bent, $sp^3$ hybridized oxygen, bond angle $104.5^circ$. Highly polar molecule. Forms extensive hydrogen bonds (H-bonds).
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Consequences of H-bonding:

* High melting point ($0^circ C$) and boiling point ($100^circ C$). * High specific heat capacity ($4.184,\text{J/g}^circ C$). * High latent heats of fusion and vaporization. * High surface tension and viscosity. * Anomalous Expansion: Maximum density at $4^circ C$. Ice is less dense than water and floats, due to open cage-like structure formed by H-bonds in solid state. Ecological significance: insulates aquatic life.

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Solvent Properties: — 'Universal solvent' due to high polarity and high dielectric constant (approx. 80). Dissolves ionic and polar covalent compounds via hydration.
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Hardness of Water: — Presence of dissolved $Ca^{2+}$ and $Mg^{2+}$ salts.

* Temporary Hardness: Caused by bicarbonates ($Ca(HCO_3)_2$, $Mg(HCO_3)_2$). * Removal: * Boiling: $Ca(HCO_3)_2(aq) xrightarrow{\text{heat}} CaCO_3(s) downarrow + H_2O(l) + CO_2(g)$. * Clark's Method: Add lime ($Ca(OH)_2$).

$Ca(HCO_3)_2(aq) + Ca(OH)_2(s) \rightarrow 2CaCO_3(s) downarrow + 2H_2O(l)$. * Permanent Hardness: Caused by chlorides and sulfates ($CaCl_2$, $MgCl_2$, $CaSO_4$, $MgSO_4$). * Removal: * Washing Soda Method: Add $Na_2CO_3$.

$CaCl_2(aq) + Na_2CO_3(aq) \rightarrow CaCO_3(s) downarrow + 2NaCl(aq)$. * Calgon Method: Sodium hexametaphosphate ($Na_6P_6O_{18}$) forms soluble complexes with $Ca^{2+}$, $Mg^{2+}$ ions. * Ion-Exchange Method (Zeolite/Permutit): Hydrated sodium aluminosilicate ($NaZ$).

$2NaZ(s) + Ca^{2+}(aq) \rightarrow CaZ_2(s) + 2Na^+(aq)$. Regeneration: $CaZ_2(s) + 2NaCl(aq) \rightarrow 2NaZ(s) + CaCl_2(aq)$. * Synthetic Resins (Demineralization): Cation exchange resin ($RSO_3H$) exchanges $H^+$ for $Ca^{2+}/Mg^{2+}$.

Anion exchange resin ($RNH_3OH$) exchanges $OH^-$ for $Cl^-/SO_4^{2-}$. $H^+ + OH^- \rightarrow H_2O$. * Reverse Osmosis (RO): Physical separation through semi-permeable membrane. * Disadvantages of Hard Water: Soap wastage (scum formation), boiler scaling, corrosion.

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**Heavy Water ($D_2O$):**

* Preparation: Prolonged electrolysis of ordinary water. * Properties: Higher density, MP, BP, viscosity than $H_2O$. Slower reactions (kinetic isotope effect). * Uses: Nuclear moderator (slows neutrons, low absorption), tracer in mechanisms, NMR solvent.

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Chemical Reactions of Water:

* Amphoteric Nature: Acts as both acid ($H_2O + NH_3 \rightleftharpoons OH^- + NH_4^+$) and base ($H_2O + HCl \rightleftharpoons H_3O^+ + Cl^-$). Autoionization: $2H_2O \rightleftharpoons H_3O^+ + OH^-$. * Redox Reactions: Reduced by active metals ($2Na + 2H_2O \rightarrow 2NaOH + H_2$). Oxidized by strong oxidizers ($2F_2 + 2H_2O \rightarrow 4HF + O_2$). * Hydrolysis: Reacts with many compounds (salts, carbides, nitrides, esters) to break them down.

Vyyuha Quick Recall

To remember the methods for removing Permanent Hardness: We Can Initiate Removal.

Washing soda method
Calgon method
Ion-exchange method
Reverse Osmosis