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Hydrogen (Water)

Water

Water ($H_2O$) is a remarkable substance, essential for all known forms of life. Its unique properties arise from its molecular structure and the presence of hydrogen bonds.

Physical Properties Of Water

Appearance: Pure water is a colorless, odorless, and tasteless liquid.

Boiling Point: 373.15 K (100°C) at standard atmospheric pressure (1 atm).

Freezing Point: 273.15 K (0°C) at standard atmospheric pressure.

Density: The maximum density of water occurs at 4°C ($3.98^\circ C$). At this temperature, its density is approximately $1.000 \text{ g cm}^{-3}$. However, water's density decreases as it freezes into ice, which is unusual. Ice has a density of about $0.917 \text{ g cm}^{-3}$ at 0°C.

High Specific Heat Capacity: Water has a very high specific heat capacity ($4.184 \text{ J g}^{-1} \text{ K}^{-1}$). This means it takes a significant amount of heat to raise the temperature of water, allowing it to moderate climate and body temperature.

High Heat of Vaporization: Water has a high heat of vaporization (2260 kJ/kg at 100°C). This property is crucial for evaporative cooling, such as in transpiration in plants and sweating in animals.

High Surface Tension: Due to strong hydrogen bonding, water exhibits high surface tension, which is responsible for capillary action.

High Dielectric Constant: Water has a very high dielectric constant, which allows it to dissolve ionic compounds by separating and stabilizing the ions.

Anomalous Expansion: Unlike most substances, water expands upon freezing, causing ice to be less dense than liquid water. This prevents lakes and rivers from freezing solid from the bottom up, thus protecting aquatic life.

Structure Of Water

Molecular Geometry: A water molecule ($H_2O$) consists of one oxygen atom covalently bonded to two hydrogen atoms. The molecule is bent or V-shaped, with a bond angle of approximately 104.5°. This is due to the presence of two lone pairs of electrons on the oxygen atom, which repel the bonding pairs more strongly.

Polarity: Oxygen is significantly more electronegative than hydrogen. This unequal sharing of electrons creates polar covalent bonds ($O^{\delta-}-H^{\delta+}$). The bent geometry of the molecule prevents the bond dipoles from canceling each other out, resulting in a net molecular dipole moment. Therefore, water is a polar molecule.

Hydrogen Bonding: The polarity of water molecules leads to strong intermolecular forces called hydrogen bonds. The partially positive hydrogen atom of one water molecule is attracted to the lone pair of electrons on the partially negative oxygen atom of another water molecule.

Properties due to Hydrogen Bonding: Hydrogen bonding is responsible for many of water's unique physical properties:

Structure Of Ice

Crystal Structure: Ice forms a crystalline solid structure where water molecules are arranged in a hexagonal lattice.

Hydrogen Bonding in Ice: Each water molecule in ice participates in four hydrogen bonds:

Open Cage Structure: This extensive hydrogen bonding network creates an open, cage-like structure with significant empty space between the molecules. This open structure is why ice is less dense than liquid water, where the molecules are closer together and hydrogen bonds are constantly breaking and reforming, allowing molecules to pack more closely.

Dynamic Nature in Liquid Water: In liquid water, hydrogen bonds are constantly breaking and reforming, allowing for more fluid movement and closer packing than in ice.

Chemical Properties Of Water

Water is a relatively stable compound but exhibits various chemical behaviors:

1. Amphoteric Nature: Water can act as both an acid and a base (amphiprotic).

2. Reaction with Active Metals: Water reacts with highly electropositive metals (alkali and some alkaline earth metals) to liberate hydrogen gas.

Examples:

3. Reaction with Non-metallic Oxides: Water reacts with acidic non-metallic oxides to form acids.

Examples:

4. Reaction with Metallic Oxides: Water reacts with basic metallic oxides to form bases (hydroxides).

Examples:

5. Hydrolysis: Water can hydrolyze salts of weak acids or weak bases, affecting the pH of the solution.

6. Hydration: Water molecules can hydrate ions, surrounding them with coordinated water molecules, which is important for the solubility of ionic compounds.

7. Redox Reactions: Water can act as an oxidizing agent (e.g., with active metals) or a reducing agent (less common, but can happen with very strong oxidizing agents).

Hard And Soft Water

Hard Water: Water that contains dissolved mineral cations, particularly calcium ($Ca^{2+}$) and magnesium ($Mg^{2+}$) ions, is called hard water.

Soft Water: Water that has a low concentration of these dissolved mineral cations is called soft water.

Problems with Hard Water:

Temporary Hardness

Definition: Temporary hardness is due to the presence of dissolved bicarbonates (hydrogencarbonates) of calcium and magnesium.

Cause: Rainwater percolating through deposits of limestone ($CaCO_3$) and magnesium carbonate ($MgCO_3$), dissolving them by forming soluble hydrogencarbonates:

$$CaCO_3(s) + H_2O(l) + CO_2(aq) \rightarrow Ca(HCO_3)_2(aq)$$ $$MgCO_3(s) + H_2O(l) + CO_2(aq) \rightarrow Mg(HCO_3)_2(aq)$$

Removal of Temporary Hardness: Temporary hardness can be removed by:

Permanent Hardness

Definition: Permanent hardness is due to the presence of dissolved soluble salts of calcium and magnesium, such as chlorides and sulfates.

Cause: These salts do not precipitate out upon boiling.

Removal of Permanent Hardness: Permanent hardness can be removed by: