Unit 4: Water chemistry

Chemical and physical properties of water

Water (H₂O) is a unique substance because of its polar covalent bonds and the resulting hydrogen bonding between its molecules.

Key Physical Properties:

• High Boiling Point (100°C) & Freezing Point (0°C): Hydrogen bonds require a lot of energy to break, keeping water liquid over a wide range of temperatures.
• High Heat Capacity: Water can absorb and store large amounts of heat without its temperature changing dramatically. This stabilizes climates and body temperatures.
• High Surface Tension: Hydrogen bonds cause water molecules to "stick" together tightly at the surface.
• Density Anomaly: Unlike most substances, ice (solid) is less dense than liquid water (at 4°C). This is why ice floats, insulating lakes in winter and allowing life to survive underneath.

Key Chemical Properties:

• Universal Solvent: Its polarity allows it to dissolve many ionic and polar substances (salts, acids, sugars).
• Amphoteric: It can act as both an acid (donating H⁺) and a base (accepting H⁺).

Alkalinity and acidity of water

These two properties measure water's ability to resist pH changes.

Acidity

Acidity is the water's capacity to neutralize a strong base. It's a measure of all the H⁺-donating substances in the water.
Sources: Strong acids (like acid rain, H₂SO₄), weak acids (like dissolved CO₂ forming carbonic acid, H₂CO₃), and dissolved metal ions (like Fe³⁺) that release H⁺.

Alkalinity

Alkalinity is the water's capacity to neutralize a strong acid. It is the "buffering capacity" of water against pH changes.

Sources: It is primarily caused by dissolved carbonate rocks (limestone). The main "buffers" are:

• Bicarbonate (HCO₃⁻)
• Carbonate (CO₃²⁻)
• Hydroxide (OH⁻)

Hardness of water

Hardness is the total concentration of dissolved divalent metal cations (ions with a 2+ charge).

• Primarily caused by: Calcium (Ca²⁺) and Magnesium (Mg²⁺).
• Other ions like Iron (Fe²⁺) and Manganese (Mn²⁺) also contribute.
• Problems: Hard water prevents soap from lathering and causes "scale" (CaCO₃ deposits) in pipes and boilers.

Types of Hardness:

• Temporary Hardness (Carbonate Hardness): Caused by calcium/magnesium bicarbonates (e.g., Ca(HCO₃)₂). It is "temporary" because it can be removed by boiling, which causes the calcium carbonate (CaCO₃) to precipitate out as scale.
• Permanent Hardness (Non-Carbonate Hardness): Caused by calcium/magnesium sulfates or chlorides (e.g., CaSO₄). It is "permanent" because it is not removed by boiling.

Calculation of total hardness

Total hardness is the sum of temporary and permanent hardness. It is always expressed in a common unit: milligrams per liter (mg/L) as calcium carbonate (CaCO₃), regardless of what ions are actually present.

Step-by-Step Calculation:

1. Find the Molar Mass of CaCO₃ = 100.1 g/mol (approx.)
2. Find the Molar Mass of Ca²⁺ = 40.1 g/mol
3. Find the Molar Mass of Mg²⁺ = 24.3 g/mol
4. Convert the concentration of each ion (in mg/L) to its equivalent concentration as CaCO₃.

   Hardness (as CaCO₃) = [Ion Conc. (mg/L)] × (Molar Mass CaCO₃ / Molar Mass of Ion)

Solubility of metals

The solubility of metals in water (whether they are dissolved or solid) is highly dependent on the "master variables" pH and pE (redox potential).

• Effect of pH: Most toxic heavy metals (like Lead, Pb²⁺; Cadmium, Cd²⁺) become more soluble in acidic (low pH) water. This is why acid rain is dangerous: it not only acidifies the lake but also dissolves toxic metals from the surrounding soil into the water.
• Effect of pE: The redox state (oxidizing or reducing) can change the metal's charge, which changes its solubility. For example, Iron(II) (Fe²⁺) is soluble, but in an oxidizing environment, it becomes Iron(III) (Fe³⁺), which forms insoluble iron hydroxide (Fe(OH)₃), the "rust" you see at the bottom of streams.

Complex formation and chelation

Dissolved metal ions (M⁺) rarely exist by themselves. They are "sticky" and attract other molecules or ions (called ligands, L) to form metal complexes.

Complex Formation

A central metal ion bonded to one or more ligands. Ligands can be simple ions (like Cl⁻) or molecules (like H₂O or NH₃).
Example: Cu²⁺(aq) + 4NH₃(aq) ⇌ [Cu(NH₃)₄]²⁺ (a dark blue complex)

Chelation

Chelation (from the Greek word *chele* for "claw") is a special, stronger type of complex formation. It occurs when a single ligand (a chelating agent) can "grab" the metal ion with two or more bonds, like a claw.

Example: EDTA (Ethylenediaminetetraacetic acid) is a famous chelating agent that can form 6 bonds with a single metal ion, "wrapping it up" completely.

Colloidal particles

Colloids are extremely small particles (1 nm to 1 µm in size) that are larger than dissolved molecules but too small to settle out by gravity. They remain suspended in the water, making it look turbid or "cloudy".

Examples: Clay, silt, fragments of bacteria/algae, proteins.

Key Property: Colloids have a very large surface area for their size and are almost always negatively charged.

• This negative charge causes them to repel each other, so they never clump together and settle.
• This large, charged surface is "sticky" and can adsorb (stick to) pollutants like heavy metals or pesticides, transporting them through water.

Application (Water Treatment): To remove colloids, chemicals like Alum (Aluminum Sulfate) are added. The Al³⁺ ions neutralize the negative charge, allowing the colloids to clump together (flocculation) and settle out (sedimentation).

Heavy metals in water

Heavy metals are metals with high density that are toxic or poisonous, even at low concentrations. They are a major class of water pollutants because they are persistent (don't break down) and can bioaccumulate (build up in an organism's tissues).