Natural water bodies are essential for sustaining life, ecosystems, and human activities. Understanding the various substances present in natural water, such as oxygen, metals, plastics, sewage, microbes, and fertilizers, is crucial for assessing water quality and environmental health. This article delves into these substances, providing comprehensive insights tailored for the Cambridge IGCSE Chemistry syllabus (0620 - Core).

Key Concepts

Oxygen in Natural Water

Oxygen is a vital component of natural water, playing a critical role in sustaining aquatic life and maintaining healthy ecosystems. Dissolved oxygen (DO) refers to the amount of oxygen present in water, which is essential for the respiration of fish and other aquatic organisms.

Sources of Dissolved Oxygen:

Atmospheric Diffusion: Oxygen from the air dissolves into water surfaces.
Aquatic Plant Photosynthesis: Plants produce oxygen during photosynthesis, increasing DO levels.
Water Movement: Flowing water, such as in rivers and streams, enhances oxygenation.

Factors Affecting Dissolved Oxygen Levels:

Temperature: Cooler water can hold more oxygen than warmer water.
Biological Activity: High levels of decomposition consume oxygen, reducing DO.
Pollutants: Substances like sewage and fertilizers can decrease oxygen levels by increasing biochemical oxygen demand (BOD).

Measurement of Dissolved Oxygen: DO is typically measured in milligrams per liter (mg/L) using methods such as the Winkler titration or electronic DO sensors.

Metals in Natural Water

Metals can enter natural water bodies through both natural processes and anthropogenic activities. While some metals are essential for biological functions in trace amounts, others can be toxic to aquatic life and humans.

Common Metals in Water:

Iron (Fe): Often found in groundwater, it can cause staining and affect taste.
Aluminum (Al): Can be present due to natural weathering and industrial runoff, harmful to aquatic organisms at high concentrations.
Lead (Pb): Introduced through industrial discharges and lead pipes, toxic even at low levels.
Mercury (Hg): Released from industrial processes, highly toxic and bioaccumulative.

Sources of Metal Pollution:

Industrial Effluents: Factories may discharge metals during manufacturing processes.
Mining Activities: Extraction of metals can lead to runoff containing various metal ions.
Agricultural Runoff: Use of metal-containing pesticides and fertilizers.
Urban Runoff: Vehicles and infrastructure contribute metals like lead and zinc.

Impact of Metallic Contaminants:

Toxicity to Aquatic Life: Metals can interfere with metabolic processes and cause mortality.
Human Health Risks: Consumption of contaminated water can lead to serious health issues, including neurological and developmental disorders.
Environmental Degradation: Metals can disrupt ecosystem balance and reduce biodiversity.

Remediation Techniques:

Precipitation: Adding chemicals to form insoluble metal compounds that can be filtered out.
Ion Exchange: Using resins to replace metal ions with harmless ions.
Membrane Filtration: Employing technologies like reverse osmosis to remove metals.

Plastics in Natural Water

Plastics are ubiquitous pollutants in natural water systems, posing significant environmental and health risks. They originate from various sources, including littering, industrial discharges, and microplastics from larger debris.

Types of Plastic Pollutants:

Macroplastics: Larger plastic items such as bottles, bags, and fishing nets.
Microplastics: Particles smaller than 5 mm, resulting from the breakdown of larger plastics or manufactured for specific uses.

Sources of Plastic Pollution:

Urban Runoff: Waste disposal and stormwater can carry plastics into waterways.
Industrial Effluents: Manufacturing processes may release plastic particles.
Aquaculture: Use of plastic materials in fish farming operations.
Recreational Activities: Activities like boating and fishing contribute to plastic waste.

Environmental Impact:

Wildlife Ingestion: Aquatic organisms can mistake plastics for food, leading to injury or death.
Habitat Disruption: Accumulation of plastics can alter physical habitats, affecting species diversity.
Chemical Contamination: Plastics can leach harmful additives and adsorb pollutants from water, introducing toxins into the food chain.

Human Health Risks:

Consumption of Microplastics: Presence in drinking water and seafood can lead to ingestion of plastics and associated chemicals.
Chemical Exposure: Toxic substances from plastics may disrupt endocrine systems and contribute to various diseases.

Mitigation Strategies:

Reducing Plastic Use: Promoting alternatives to single-use plastics and encouraging recycling.
Clean-Up Initiatives: Organizing efforts to remove plastic debris from water bodies.
Regulatory Measures: Implementing laws to control plastic production, usage, and disposal.
Research and Innovation: Developing biodegradable plastics and advanced filtration technologies.

Sewage in Natural Water

Sewage contamination in natural water is a significant environmental issue, resulting from the discharge of untreated or inadequately treated wastewater. It introduces a range of pollutants, including organic matter, nutrients, pathogens, and chemicals.

Composition of Sewage:

Organic Matter: Includes human waste, food residues, and other biodegradable materials.
Nutrients: Nitrogen and phosphorus compounds that can lead to eutrophication.
Pathogens: Bacteria, viruses, and protozoa that pose health risks.
Chemicals: Pharmaceuticals, personal care products, and household chemicals.

Sources of Sewage Pollution:

Domestic Sewage: From households via septic tanks or municipal sewer systems.
Industrial Waste: Production facilities discharging contaminated effluents.
Agricultural Runoff: Animal waste and fertilizers entering water bodies.
Stormwater: Carrying sewage overflows into natural waters during heavy rainfall.

Environmental and Health Impacts:

Eutrophication: Excess nutrients promote algal blooms, reducing oxygen levels and harming aquatic life.
Waterborne Diseases: Pathogens in sewage can cause illnesses such as cholera, dysentery, and hepatitis.
Toxicity: Chemicals in sewage can be toxic to aquatic organisms and accumulate in the food chain.
Habitat Degradation: Sewage pollution can alter physical environments, affecting biodiversity.

Treatment and Remediation:

Primary Treatment: Physical removal of solids through screening and sedimentation.
Secondary Treatment: Biological processes that degrade organic matter, such as activated sludge systems.
Tertiary Treatment: Advanced purification methods, including filtration, disinfection, and nutrient removal.
Constructed Wetlands: Natural systems that treat wastewater through plant uptake and microbial activity.

Microbes in Natural Water

Microorganisms, including bacteria, viruses, and protozoa, are inherent components of natural water ecosystems. While many microbes are beneficial, pathogens present significant health risks when water becomes contaminated.

Types of Microbial Contaminants:

Bacteria: Such as Escherichia coli (E. coli) and Salmonella, which can cause gastrointestinal illnesses.
Viruses: Including norovirus and hepatitis A, responsible for various infections.
Protozoa: Like Giardia and Cryptosporidium, which lead to severe diarrheal diseases.

Sources of Microbial Pollution:

Human Sewage: Direct discharge of untreated wastewater introduces pathogens.
Agricultural Runoff: Animal waste can carry microbes into water bodies.
Wildlife: Natural animal activities contribute to microbial presence.
Industrial Processes: Certain industries may release microbial contaminants.

Impact on Ecosystems and Human Health:

Ecosystem Balance: High microbial loads can disrupt aquatic ecosystems and reduce biodiversity.
Waterborne Diseases: Consumption of contaminated water can lead to outbreaks of illnesses, posing significant public health challenges.
Economic Costs: Medical treatments and lost productivity due to waterborne diseases incur substantial economic burdens.

Detection and Monitoring:

Culture-Based Methods: Growing microbes on selective media to identify presence.
Molecular Techniques: Using PCR and DNA sequencing for precise identification of pathogens.
Indicator Organisms: Measuring levels of non-pathogenic bacteria like E. coli to assess water quality.

Control and Prevention Strategies:

Water Treatment: Implementing effective filtration, disinfection, and purification processes.
Sanitation Infrastructure: Developing and maintaining adequate sewage systems to prevent contamination.
Public Awareness: Educating communities on the importance of proper waste disposal and hygiene practices.
Regulatory Standards: Enforcing water quality guidelines to limit microbial contamination.

Fertilizers in Natural Water

Fertilizers are essential for enhancing agricultural productivity, but their runoff into natural water bodies can lead to significant environmental issues. The primary constituents of fertilizers—nitrogen, phosphorus, and potassium—can disrupt aquatic ecosystems when present in excess.

Types of Fertilizers:

Organic Fertilizers: Derived from natural sources like manure and compost.
Inorganic Fertilizers: Manufactured chemicals, such as ammonium nitrate and superphosphate.

Mechanism of Pollution:

Runoff: Rainfall or irrigation can wash excess fertilizers from agricultural fields into nearby water bodies.
Leaching: Fertilizer components seep through the soil, contaminating groundwater reservoirs.

Environmental Impacts:

Eutrophication: Elevated nutrient levels, especially nitrogen and phosphorus, lead to excessive algal growth, depleting oxygen levels and causing dead zones.
Algal Blooms: Harmful algal blooms can produce toxins detrimental to aquatic life and human health.
Water Quality Degradation: Increased turbidity and chemical imbalances affect the suitability of water for drinking and recreation.
Loss of Biodiversity: Oxygen-depleted environments cannot support diverse aquatic species.

Human Health Consequences:

Drinking Water Contamination: High nitrate levels can cause conditions like methemoglobinemia (blue baby syndrome) in infants.
Exposure to Toxins: Some algal blooms produce toxins that can contaminate seafood and pose health risks upon consumption.

Mitigation Measures:

Controlled Fertilizer Application: Applying fertilizers at optimal rates and times to minimize runoff.
Buffer Strips: Establishing vegetation barriers between agricultural fields and water bodies to absorb excess nutrients.
Precision Agriculture: Utilizing technology to apply fertilizers more efficiently and reduce waste.
Alternative Fertilizers: Developing and using fertilizers with lower environmental impacts.

Regulatory and Policy Approaches:

Nutrient Management Plans: Creating strategies for balanced fertilizer use to prevent environmental harm.
Water Quality Standards: Implementing and enforcing limits on nutrient concentrations in water bodies.
Incentives for Sustainable Practices: Providing financial or other incentives for farmers adopting eco-friendly fertilization methods.

Advanced Concepts

Theoretical Aspects of Substances in Natural Water

The behavior and impact of various substances in natural water can be understood through fundamental chemical principles and equations. This section delves deeper into the theoretical underpinnings of dissolved oxygen, metal solubility, plastic degradation, sewage composition, microbial dynamics, and fertilizer interactions in aquatic environments.

Dissolved Oxygen Equilibrium:

The solubility of oxygen in water is governed by Henry's Law, which states that the amount of dissolved gas is proportional to its partial pressure in the atmosphere:

$$ C = k_H \cdot P_{O_2} $$

Where:

C: Concentration of dissolved oxygen (mg/L)
k_H: Henry's Law constant for oxygen in water ($\frac{mg}{L \cdot atm}$)
P_{O_2}: Partial pressure of oxygen (atm)

Temperature and salinity affect Henry's Law constant, thereby influencing oxygen solubility.

Metal Speciation and Solubility:

The solubility of metals in water is influenced by pH, redox potential, and the presence of complexing agents. Metals can exist in various oxidation states, forming different species:

$$ \text{Fe}^{2+} + 2\text{H}_2\text{O} \leftrightarrow \text{Fe(OH)}_2 + 2\text{H}^+ $$

This equilibrium highlights how pH changes can affect iron solubility, leading to precipitation or dissolution.

Polymer Degradation Kinetics:

Plastics degrade through photodegradation and hydrolysis. The rate of degradation can be modeled using first-order kinetics:

$$ \frac{dC}{dt} = -kC $$

Where:

C: Concentration of polymer
k: Degradation rate constant

Solving the equation gives:

$$ C(t) = C_0 \cdot e^{-kt} $$

This equation describes the exponential decay of polymer concentration over time.

Biochemical Oxygen Demand (BOD):

BOD measures the amount of oxygen required by microorganisms to decompose organic matter in water. It is indicative of the organic pollution level:

$$ \text{BOD} = \frac{\Delta \text{Oxygen Consumption} \times \text{Volume of Oxygen Consumed}}{\text{Volume of Sample}} $$

A high BOD signifies high levels of organic pollutants, leading to reduced DO levels and potential hypoxia.

Nutrient Cycling and Eutrophication:

The nitrogen and phosphorus cycles are critical in understanding fertilizer impacts. Excess nutrients can lead to eutrophication, modeled by reaction kinetics:

$$ \text{Nutrient Concentration} = \frac{N_0}{1 + kt} $$

Where:

N_0: Initial nutrient concentration
k: Rate constant
t: Time

This equation illustrates the decrease in nutrient concentration over time due to biological uptake and other removal processes.

Complex Problem-Solving: Calculating Oxygen Consumption

Consider a scenario where a wastewater treatment plant discharges sewage into a river. The sewage has a BOD of 300 mg/L, and the discharge flow rate is 500 m³/day. The river has a flow rate of 10,000 m³/day with an initial DO of 8 mg/L. Assuming complete mixing, calculate the final DO concentration in the river after discharge.

Given:

BOD of sewage, $B_0 = 300 \text{ mg/L}$
Discharge flow rate, $Q_s = 500 \text{ m³/day}$
River flow rate, $Q_r = 10,000 \text{ m³/day}$
Initial DO of river, $DO_{initial} = 8 \text{ mg/L}$

Assumptions:

Complete mixing occurs immediately after discharge.
BOD is entirely consumed by the microorganisms, leading to oxygen demand.

Calculation Steps:

Determine the mass of DO consumed by sewage:

$$ \text{Mass}_{\text{DO consumed}} = B_0 \times Q_s = 300 \text{ mg/L} \times 500,000 \text{ L/day} = 150,000,000 \text{ mg/day} = 150 \text{ kg/day} $$

Calculate the total flow after mixing:

$$ Q_{total} = Q_r + Q_s = 10,000 + 500 = 10,500 \text{ m³/day} $$

Determine the dilution of BOD in the river:

The BOD from sewage dilutes in the total flow:

$$ \text{BOD}_{\text{final}} = \frac{B_0 \times Q_s}{Q_{total}} = \frac{300 \times 500}{10,500} \approx 14.29 \text{ mg/L} $$

Calculate the oxygen deficit caused by BOD consumption:

$$ \text{Oxygen deficit} = \text{BOD}_{\text{final}} = 14.29 \text{ mg/L} $$

Determine the final DO concentration:

$$ DO_{final} = DO_{initial} - \text{Oxygen deficit} = 8 - 14.29 = -6.29 \text{ mg/L} $$

Interpretation: A negative DO indicates complete oxygen depletion, leading to hypoxic conditions detrimental to aquatic life.

Interdisciplinary Connections: Chemistry and Environmental Engineering

The study of substances in natural water intersects with various disciplines, notably environmental engineering, biology, and public health. Understanding the chemical interactions and transformations of pollutants informs the design of treatment systems and the implementation of environmental conservation strategies.

Environmental Engineering:

Water Treatment Technologies: Chemical principles guide the development of filtration, coagulation, and disinfection methods to remove contaminants.
Pollution Control: Designing systems to prevent industrial and agricultural pollutants from entering natural water bodies.

Biology:

Aquatic Ecology: Studying the impact of chemical pollutants on aquatic organisms and ecosystem dynamics.
Microbial Ecology: Exploring how microbes degrade pollutants and contribute to nutrient cycling.

Public Health:

Water Quality Standards: Establishing guidelines to protect human health from contaminated water sources.
Disease Prevention: Implementing measures to reduce the spread of waterborne pathogens.

Comparison Table

Substance	Sources	Environmental Impact	Human Health Risks
Oxygen	Atmospheric diffusion, photosynthesis, water movement	Essential for aquatic life; influences ecosystem health	Insufficient DO can lead to hypoxia affecting fisheries and water quality
Metals	Industrial effluents, mining runoff, agricultural runoff, urban runoff	Toxicity to aquatic organisms, bioaccumulation, habitat degradation	Lead and mercury exposure can cause neurological and developmental issues
Plastics	Urban littering, industrial discharges, microplastics from degradation	Wildlife ingestion, habitat disruption, chemical contamination	Microplastics ingestion can lead to chemical exposure and endocrine disruption
Sewage	Domestic wastewater, industrial discharges, agricultural runoff, stormwater overflows	Eutrophication, oxygen depletion, habitat degradation	Waterborne diseases, exposure to toxic chemicals
Microbes	Sewage, agricultural runoff, wildlife, industrial processes	Ecosystem imbalance, water quality degradation	Gastrointestinal and other waterborne diseases
Fertilizers	Agricultural runoff, leaching from fields	Eutrophication, algal blooms, loss of biodiversity	Methemoglobinemia from nitrates, toxin exposure from algal blooms

Summary and Key Takeaways

Various substances in natural water, including oxygen, metals, plastics, sewage, microbes, and fertilizers, significantly impact environmental and human health.
Understanding the sources, behaviors, and effects of these substances is crucial for effective water quality management.
Advanced chemical principles and interdisciplinary approaches are essential for addressing water pollution challenges.
Implementing mitigation strategies and adhering to regulatory standards can mitigate the adverse effects of water contaminants.

Examiner Tip

Tips

Use the mnemonic **"POLLUTE"** to remember key pollutants in water:
**P**lastics, **O**xygen, **L**ead (Metals), **L**earn (Sewage), **U**rban Runoff (Microbes), **T**oxins (Fertilizers), **E**utrophication. Understanding each component helps in tackling water pollution effectively.

Did You Know

1. Microplastics have been found in some bottled water brands, highlighting the pervasive nature of plastic pollution.
2. Certain algae blooms can produce toxins that remain in the water even after the algae die off.
3. Wetlands act as natural water filters, removing pollutants like heavy metals and excess nutrients from water.

Common Mistakes

1. **Ignoring Units:** Students often mix units when calculating concentrations. Always ensure consistency, e.g., converting m³ to L.
2. **Misapplying Henry's Law:** Remember that Henry's Law applies only to gases at equilibrium with their partial pressures. Incorrectly applying it to non-volatile substances leads to errors.
3. **Overlooking pH Effects:** Neglecting how pH affects metal solubility can result in incorrect conclusions about metal toxicity.

FAQ

What is the role of dissolved oxygen in aquatic ecosystems?

Dissolved oxygen is essential for the respiration of aquatic organisms. High levels support healthy ecosystems, while low levels can lead to hypoxia, causing harm to marine life.

How do heavy metals enter natural water bodies?

Heavy metals enter water bodies through natural processes like rock weathering and anthropogenic activities such as industrial discharges, agricultural runoff, and improper waste disposal.

What distinguishes microplastics from macroplastics?

Microplastics are plastic particles smaller than 5mm, often resulting from the breakdown of larger plastics, whereas macroplastics are visible pieces like bottles and bags.

Why is sewage treatment important for natural water?

Sewage treatment removes contaminants, pathogens, and excess nutrients from wastewater, preventing eutrophication and protecting public health by ensuring cleaner water bodies.

How do fertilizers contribute to eutrophication?

Excess fertilizers run off into water bodies, introducing high levels of nitrogen and phosphorus, which stimulate excessive algal growth. When these algae die and decompose, they deplete dissolved oxygen, leading to eutrophication.

1. Acids, Bases, and Salts

1.1 Salt Preparation

1.1.1 Making soluble salts by reaction with excess metal

1.1.2 Making soluble salts by reaction with excess base

1.1.3 Making soluble salts by reaction with excess carbonate

1.1.4 Making soluble salts by titration

1.2 Solubility Rules

1.2.1 Solubility of sodium, potassium, ammonium salts

1.2.2 Solubility of nitrates, chlorides, sulfates, carbonates, and hydroxides

1.3 Hydrated and Anhydrous Salts

1.3.1 Definition of hydrated and anhydrous substances

1.4 Insoluble Salts