Chemistry - Water (Physical/Chemical Properties, Hard and Soft Water)

Water as a Universal Solvent: Water dissolves more substances than any other liquid due to its high dielectric constant and polar nature ($H_2O$ molecules have a partial positive charge on $H$ and partial negative on $O$).

Anomalous Expansion: Water contracts when heated from $0^\circ C$ to $4^\circ C$ and expands when cooled from $4^\circ C$ to $0^\circ C$. It has maximum density at $4^\circ C$ ($1 \, g/cm^3$).

Physical Constants: Freezing point is $0^\circ C$ at $1 \, atm$, and Boiling point is $100^\circ C$ at $760 \, mm \, Hg$. Specific heat capacity is high at $4.2 \, J/g^\circ C$.

Chemical Properties: Water reacts with active metals (like $Na$, $K$, $Ca$) to form hydroxides and hydrogen gas. Example: $2Na + 2H_2O \rightarrow 2NaOH + H_2 \uparrow$.

Hard Water vs Soft Water: Soft water produces lather with soap easily. Hard water contains dissolved salts of $Ca^{2+}$ and $Mg^{2+}$ which react with soap to form insoluble scum (stearates).

Temporary Hardness: Caused by dissolved bicarbonates ($Ca(HCO_3)_2$ and $Mg(HCO_3)_2$). It can be removed by boiling or Clark's process.

Permanent Hardness: Caused by dissolved chlorides and sulphates ($CaCl_2, MgCl_2, CaSO_4, MgSO_4$). It cannot be removed by boiling; requires chemical treatment like the Addition of Washing Soda or Ion-exchange process.

Water of Crystallisation: A fixed number of water molecules chemically combined with a salt in its crystalline form, e.g., $CuSO_4 \cdot 5H_2O$ (Blue Vitriol).

Efflorescence, Deliquescence, and Hygroscopy: Efflorescent substances lose water of crystallization to the atmosphere (e.g., $Na_2CO_3 \cdot 10H_2O$). Deliquescent substances absorb moisture and dissolve in it (e.g., $MgCl_2$). Hygroscopic substances absorb moisture without changing state (e.g., $conc. H_2SO_4$).