14.8 Properties of Water Related to Pollution | Environmental Chemistry Water | Chemistry 11

Water is a unique substance whose chemical and physical properties make it an excellent solvent, earning it the title of "universal solvent." These same properties, however, also make water susceptible to pollution. Understanding these characteristics is crucial for assessing water quality and developing strategies to combat pollution.

Water as a Universal Solvent

Water's ability to dissolve a wide range of substances is due to several key properties:

Polarity: The water molecule ( $H_{2} O$ ) has a bent shape with an uneven distribution of charge, creating a positive end (hydrogen atoms) and a negative end (oxygen atom).

High Dielectric Constant: This property allows water to weaken the electrostatic forces between ions in a crystal lattice, enabling ionic compounds to dissolve.

Hydrogen Bonding: Water molecules form strong intermolecular hydrogen bonds with each other and with other polar molecules (such as alcohols, amines, and carboxylic acids), facilitating their dissolution.

Ion-Dipole Forces: The polar nature of water allows it to surround and stabilize ions, pulling them into the solution.

This solvent ability is why rainwater can dissolve atmospheric carbon dioxide ( $C O_{2}$ ), forming a weak acid (carbonic acid, $H_{2} C O_{3}$ ). This acidic rainwater can then dissolve carbonate rocks (such as limestone) by converting insoluble carbonates into soluble bicarbonates.

$H_{2} O + C O_{2} ⇌ H_{2} C O_{3}$

$H_{2} C O_{3} ⇌ H_{(a q)}^{+} + H C O_{3 (a q)}^{-}$

Properties and Their Relation to Pollution

1. High Surface Tension

Property: Water molecules cohere strongly at the surface due to hydrogen bonding, creating high surface tension.

Pollution Impact: This allows many microorganisms, including disease-causing microbes, to live and move on the water's surface, making water a habitat for pathogens. It also affects how oil and detergents spread over water bodies — non-polar oil spreads as a thin film, blocking atmospheric $O_{2}$ from dissolving into the water below.

2. High Heat Capacity

Property: Water can absorb a large amount of heat with only a small change in its own temperature.

Pollution Impact: Industries often use water as a coolant, discharging heated water back into rivers and lakes. This thermal pollution raises the water temperature, reducing dissolved oxygen levels and making the water uninhabitable for many aquatic species.

3. High Dielectric Constant and Solubility of Heavy Metals

Property: Water's high dielectric constant (~80) greatly reduces electrostatic attraction between ions, enabling ionic compounds — including toxic heavy metal salts — to dissolve readily.

Pollution Impact: Toxic heavy metal ions such as $P b^{2 +}$ , $H g^{2 +}$ , and $C d^{2 +}$ from industrial effluents dissolve in water and are transported through the environment, posing serious health risks including neurological damage, organ failure, and cancer.

4. Solubility of Surfactants

Property: Water's solvent ability extends to soaps and detergents (surfactants).

Pollution Impact: When surfactants enter waterways, they can leach out toxic heavy metal ions from soil and minerals, introducing them into the water supply. They also lower surface tension, disrupting the protective lipid coatings of aquatic organisms.

5. Capillary Action and Osmosis

Property: Water moves through narrow spaces (capillary action) and across semi-permeable membranes (osmosis).

Pollution Impact: This property is responsible for the swelling of organic matter and dead bodies in water due to the movement of water into the tissues by osmosis and imbibition.

Summary Table: Water Properties and Pollution

Property	Pollution Impact
Polarity / Universal Solvent	Dissolves both nutrients and toxic pollutants (nitrates, pesticides, heavy metals)
High Dielectric Constant	Enables ionic pollutants ( $P b^{2 +}$ , $H g^{2 +}$ ) to dissolve and spread
High Surface Tension	Supports pathogens on surface; oil films block $O_{2}$ dissolution
High Heat Capacity	Thermal pollution from industrial cooling water discharge
Solubility of Surfactants	Detergents leach heavy metals; disrupt aquatic organisms
Osmosis / Capillary Action	Swelling of organic matter and dead bodies in water

Sources and Impacts of Water Pollution

Sources of Pollution:

Point Sources: Pollutants discharged from a single, identifiable location (such as a factory pipe).
Non-Point Sources: Pollutants from diffuse sources over a large area (such as agricultural runoff).

For more details, refer to Sources of Water Pollution→.

Major Pollutants and Their Effects:

Agricultural Runoff: Fertilizers (nitrates, phosphates), pesticides, and herbicides can cause eutrophication — an overgrowth of algae that depletes oxygen and kills aquatic life.
Industrial Effluents: Heavy metals (such as lead, $P b^{2 +}$ , and mercury, $H g^{2 +}$ ) and toxic chemicals are highly dangerous to human health, potentially causing neurological damage, organ failure, and cancer.
Health Problems: Contaminated water can lead to epidemics of diseases such as cholera and typhoid, as well as chronic stomach and liver problems.

For more information, see Health Effects of Water Pollutants→ and Environmental Problems Caused by Water Pollution→.

Water Treatment

To ensure water is safe for consumption and to protect ecosystems, treatment is essential.

Raw Water Treatment: Cleaning water from reservoirs (lakes and dams) before it is supplied to municipalities. This involves steps such as coagulation, sedimentation, filtration, and disinfection.
Wastewater Treatment: Cleaning used water (sewage and industrial discharge) before it is released back into the environment to prevent pollution of larger water bodies.

For detailed procedures, refer to Water Treatment Methods→.

Key Questions

Q: Why is water called a "universal solvent"?

A: Due to its polarity, high dielectric constant, and ability to form hydrogen bonds, water can dissolve a wide range of ionic and polar covalent substances. This makes it both essential for life and susceptible to carrying dissolved pollutants.

Q: How does water's high heat capacity lead to thermal pollution?

A: Because water absorbs large amounts of heat with minimal temperature change, industries use it as a coolant. The heated water discharged back into natural water bodies raises their temperature, reducing dissolved oxygen and harming aquatic life.

Q: What property of water causes dead bodies to swell when submerged?

A: Osmosis and imbibition — water moves across semi-permeable organic membranes into the tissues from the surrounding water (lower solute concentration to higher), causing swelling.

Water as a Universal Solvent

Water's ability to dissolve a wide range of substances is due to several key properties:

Polarity: The water molecule ( $H_{2} O$ ) has a bent shape with an uneven distribution of charge, creating a positive end (hydrogen atoms) and a negative end (oxygen atom).

High Dielectric Constant: This property allows water to weaken the electrostatic forces between ions in a crystal lattice, enabling ionic compounds to dissolve.

Ion-Dipole Forces: The polar nature of water allows it to surround and stabilize ions, pulling them into the solution.

$H_{2} O + C O_{2} ⇌ H_{2} C O_{3}$

$H_{2} C O_{3} ⇌ H_{(a q)}^{+} + H C O_{3 (a q)}^{-}$

Property	Pollution Impact
Polarity / Universal Solvent	Dissolves both nutrients and toxic pollutants (nitrates, pesticides, heavy metals)
High Dielectric Constant	Enables ionic pollutants ( $P b^{2 +}$ , $H g^{2 +}$ ) to dissolve and spread
High Surface Tension	Supports pathogens on surface; oil films block $O_{2}$ dissolution
High Heat Capacity	Thermal pollution from industrial cooling water discharge
Solubility of Surfactants	Detergents leach heavy metals; disrupt aquatic organisms
Osmosis / Capillary Action	Swelling of organic matter and dead bodies in water

Sources and Impacts of Water Pollution

Sources of Pollution:

Point Sources: Pollutants discharged from a single, identifiable location (such as a factory pipe).
Non-Point Sources: Pollutants from diffuse sources over a large area (such as agricultural runoff).

For more details, refer to Sources of Water Pollution→.

Major Pollutants and Their Effects:

Agricultural Runoff: Fertilizers (nitrates, phosphates), pesticides, and herbicides can cause eutrophication — an overgrowth of algae that depletes oxygen and kills aquatic life.
Industrial Effluents: Heavy metals (such as lead, $P b^{2 +}$ , and mercury, $H g^{2 +}$ ) and toxic chemicals are highly dangerous to human health, potentially causing neurological damage, organ failure, and cancer.
Health Problems: Contaminated water can lead to epidemics of diseases such as cholera and typhoid, as well as chronic stomach and liver problems.

For more information, see Health Effects of Water Pollutants→ and Environmental Problems Caused by Water Pollution→.

Water Treatment

To ensure water is safe for consumption and to protect ecosystems, treatment is essential.

Raw Water Treatment: Cleaning water from reservoirs (lakes and dams) before it is supplied to municipalities. This involves steps such as coagulation, sedimentation, filtration, and disinfection.
Wastewater Treatment: Cleaning used water (sewage and industrial discharge) before it is released back into the environment to prevent pollution of larger water bodies.

For detailed procedures, refer to Water Treatment Methods→.

Key Questions

Q: Why is water called a "universal solvent"?

Q: How does water's high heat capacity lead to thermal pollution?

Q: What property of water causes dead bodies to swell when submerged?

A: Osmosis and imbibition — water moves across semi-permeable organic membranes into the tissues from the surrounding water (lower solute concentration to higher), causing swelling.