Chapter 12: Environmental Chemistry - Water

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Chapter Overview & SLOs

What is water pollution? Water pollution is the contamination of water bodies (lakes, rivers, oceans, aquifers, and groundwater) by harmful substances, making water unsafe for human consumption, aquatic life, and other uses.

What are the primary sources of water pollution?

• Industrial wastes: Industries discharge toxic heavy metals and chemicals.
  • Heavy metals: Lead (Pb), Cadmium (Cd), Mercury (Hg) — highly toxic, bioaccumulate in food chains.
  • Leather tanneries: Release carcinogenic Chromium (VI) salts ($CrO_4^{2-}$, $Cr_2O_7^{2-}$).
  • Other industries: Textiles (dyes), paper mills (organic compounds), chemical plants (acids, bases).
• Household wastes (domestic sewage): Detergents (contain phosphates that cause eutrophication), human waste (contains pathogenic microorganisms), food waste, and cleaning chemicals.
• Agricultural runoff: Fertilizers (NPK) and pesticides washed from fields into water bodies, causing eutrophication (excessive algae growth due to nutrient enrichment).
• Livestock waste: Manure from animal farms contaminates water with bacteria and excess nutrients.

What are water-borne diseases? Diseases caused by pathogenic microorganisms (bacteria, viruses, protozoa) transmitted through contaminated water.

| Type | Disease | Pathogen | Symptoms | |------|---------|----------|----------| | Bacterial | Typhoid | *Salmonella typhi* | High fever, abdominal pain | | Bacterial | Cholera | *Vibrio cholerae* | Severe diarrhea, dehydration | | Viral | Hepatitis A | Hepatitis A virus | Jaundice, liver inflammation | | Viral | Jaundice | Various viruses | Yellowing of skin/eyes | | Protozoal | Amoebic dysentery | *Entamoeba histolytica* | Bloody diarrhea, abdominal pain |

How is water treated for safe use?

• Coagulation (Flocculation):
  • Aluminum sulfate (alum, $Al_2(SO_4)_3$) is added to water along with slaked lime ($Ca(OH)_2$).
  • Reaction: $$Al_2(SO_4)_3 + 3Ca(OH)_2 \rightarrow 2Al(OH)_3 + 3CaSO_4$$
  • $Al(OH)_3$ forms a gelatinous precipitate that traps suspended particles, dirt, and bacteria, causing them to settle (sedimentation).
• Sedimentation: Heavy particles settle at the bottom.
• Filtration: Water passes through sand and gravel filters to remove remaining particles.
• Chlorination: Chlorine gas ($Cl_2$) or sodium hypochlorite ($NaOCl$) is added to destroy pathogenic bacteria and viruses. Used for swimming pools and drinking water disinfection.
• Aeration: Air is bubbled through water to remove dissolved gases (like $H_2S$) and volatile organic compounds, and to improve taste.

What are chemical indicators for water?

• pH of distilled water: 7 (neutral)
• Anhydrous copper(II) sulphate ($CuSO_4$): White solid. When hydrated, it turns blue ($CuSO_4 \cdot 5H_2O$). Used as a test for water.

What is water scarcity in Pakistan? Pakistan is facing severe water scarcity due to various factors:

• Rapid population growth: Increasing demand for fresh water.
• Climate change: Affecting rainfall patterns, reducing glacial melt in the Himalayas, and increasing frequency of droughts.
• Inadequate flood irrigation methods: Wastage of water, low irrigation efficiency.
• Illegal drilling (illegal wells and tube wells): Depleting groundwater aquifers.
• Poor water management: Lack of dams and reservoirs for water storage, water theft, and outdated infrastructure.

What are NPK fertilizers? NPK fertilizers are compound fertilizers containing the three essential macronutrients for plant growth:

• N (Nitrogen): Promotes leafy growth and protein synthesis.
• P (Phosphorus): Promotes root development, flowering, and fruiting.
• K (Potassium): Promotes overall plant health, disease resistance, and water regulation.
• Examples: 10-10-10, 20-20-20 (numbers represent percentage of N-P-K).
• However, excess NPK runoff causes eutrophication (algal blooms) in water bodies.

How can we prevent water-borne diseases?

• Proper sanitation and sewage treatment
• Safe drinking water storage and purification (boiling, chlorination, filtration)
• Hand washing with soap after using toilet and before eating
• Proper food hygiene and cooking

These notes are strictly aligned with the Student Learning Outcomes (SLOs) for the FBISE 2026 annual examination.

• How do we identify sources of water contamination? Categorize pollutants into industrial wastes (heavy metals Lead Pb, Cadmium Cd, Mercury Hg; leather tanneries Chromium VI salts), household wastes (detergents, human waste), and agricultural runoff (pesticides, NPK fertilizers, livestock waste).
• How do we classify and prevent water-borne illnesses? Distinguish between diseases caused by bacteria (Typhoid, Cholera), viruses (Hepatitis, Jaundice), and protozoa (Amoebic dysentery), and emphasize the importance of sanitation, hygiene, and hand-washing with soap for prevention.
• How do chemical processes aid in water treatment? Formulate the chemical equation for coagulation using aluminum sulfate: $$Al_2(SO_4)_3 + 3Ca(OH)_2 \rightarrow 2Al(OH)_3 + 3CaSO_4$$ and explain the roles of aeration (removing dissolved gases) and chlorination (destroying pathogenic bacteria) in water sterilization.
• How do we address water resource challenges in Pakistan? Evaluate the necessity of effective management practices, infrastructure development (dams, reservoirs), and public awareness to combat declining water resources caused by rapid population growth, climate change, inadequate flood irrigation methods, and illegal drilling.

Frequently Asked Questions (FAQ)