Water treatment, air quality, soil and agriculture, fuels and combustion, plastics and recycling, industrial processes, chemical risk, and sustainability — chemistry applied at the scale of civilization
Chemistry does not stay in the laboratory. It flows through rivers, rises into the atmosphere, feeds crops, powers cities, and shapes the products that billions of people depend on every day. Every glass of clean drinking water, every breath of manageable city air, every bag of fertilizer spread across a field, and every piece of recycled plastic represents chemical knowledge applied at an enormous scale.
Chapter 27 connects the principles you have studied throughout this course to the real world of water systems, air quality, soils, fuels, materials, manufacturing, and public health. This chapter is not a chemistry elective. It is chemistry in its most consequential form. When chemists understand the mechanisms behind pollution, contamination, combustion, and corrosion, they gain the ability to design systems that protect people, reduce harm, and use resources wisely.
This chapter approaches these topics through evidence, measurement, mechanism, and tradeoffs. Chemistry does not always produce clean and simple answers to environmental problems. What it does produce is the ability to describe problems precisely, measure them accurately, and evaluate potential solutions honestly.
Environmental problems are chemical problems at their core. When a lake becomes too acidic for fish to survive, that is an acid-base problem. When iron bridges corrode in salty air, that is a redox problem. When drinking water contains lead ions, that is an ionic chemistry problem. When the atmosphere traps more heat than it used to, that involves the molecular structure and infrared behavior of gases. Chemists are essential to identifying, measuring, explaining, and mitigating these problems.
Every structure of modern civilization depends on chemistry. The food supply depends on fertilizers, pesticides, soil amendments, and food preservation chemistry. Water supplies depend on treatment processes that remove pathogens, dissolved solids, and toxic compounds. Energy systems depend on the combustion of fuels, the electrochemistry of batteries, and increasingly on photovoltaic and hydrogen-based technology. Materials — from steel to concrete to glass to semiconductors to pharmaceuticals — are products of chemical understanding applied at industrial scale.
When chemists talk about environmental and industrial chemistry, they are studying the intersection of chemical knowledge with large-scale human systems. This is different from studying a single reaction in a beaker. The same principles of bonding, reaction, equilibrium, solubility, acid-base chemistry, and thermochemistry still apply. But the scale changes everything. A reaction that produces a small amount of carbon dioxide in a laboratory produces billions of tons of it each year across the global energy system. A reaction that makes a useful solvent in a factory may also produce a toxic byproduct that contaminates groundwater for decades.
A single log burning in a fireplace produces soot and gases. That same combustion chemistry, operating through millions of vehicles and power plants, produces enough particulate matter and nitrogen oxides to change the air quality of entire cities. Understanding chemistry at civilization scale means learning to ask not just "Does this reaction occur?" but "What are the products? Where do they go? What happens to them in the environment? Who is affected? What are the tradeoffs?"
Environmental problems are chemical problems at their core. When a lake becomes too acidic for fish to survive, that is an acid-base problem. When iron bridges corrode in salty air, that is a redox problem. When drinking water contains lead ions, that is an ionic chemistry problem. When the atmosphere traps more heat than it used to, that involves the molecular structure and infrared behavior of gases. Chemists are not the only professionals who address these problems, but they are essential to identifying, measuring, explaining, and mitigating them.
Responsible environmental chemistry requires honesty about what is known and what is uncertain. It requires measuring concentrations accurately, reporting data carefully, and distinguishing between chemical facts and policy choices. Policy choices — decisions about acceptable risk, economic tradeoffs, and regulatory standards — involve more than chemistry. But those decisions should be informed by chemistry.
Water is the most important solvent on Earth. Liquid water covers most of the planet's surface, moves through the atmosphere as vapor, and fills the bodies of every living organism. Water's chemical properties — its polarity, its ability to form hydrogen bonds, its capacity to dissolve ionic compounds and polar molecules, its resistance to temperature change — make it uniquely suited for the roles it plays in chemistry, biology, geology, and engineering.
Water molecules are polar. Each water molecule has a partial negative charge near the oxygen atom and partial positive charges near the hydrogen atoms. This polarity allows water to dissolve ionic compounds by surrounding and separating their ions. The partial negative oxygen atoms attract cations; the partial positive hydrogen atoms attract anions. Polar covalent compounds also dissolve in water because their charges interact with water's polar structure.
Nonpolar substances do not dissolve well in water. Oils, waxes, and many organic compounds are nonpolar and form separate layers or droplets when added to water. This distinction — polar dissolves in polar, nonpolar dissolves in nonpolar — determines which substances are carried in water systems and which accumulate in fatty tissues or organic sediments.
Natural water is never purely H₂O. Rainwater absorbs carbon dioxide from the atmosphere, forming carbonic acid (H₂CO₃), which gives natural precipitation a slightly acidic pH of around 5.6. As water moves over and through rock, it dissolves mineral compounds, picking up calcium ions (Ca²⁺), magnesium ions (Mg²⁺), sodium ions (Na⁺), bicarbonate ions (HCO₃⁻), sulfate ions (SO₄²⁻), and many other dissolved species. Water also picks up organic material from decaying plant and animal matter, suspended sediments, and dissolved gases.
| Water Type | Characteristic | Practical Effect |
|---|---|---|
| Hard water | High concentrations of Ca²⁺ and Mg²⁺ ions | Not harmful to health; reacts with soap to form scum; precipitates as CaCO₃ scale in pipes and water heaters |
| Soft water | Few dissolved mineral ions | Lathers easily with soap; may be more corrosive to copper pipes due to lack of buffering mineral content |
| Category | Examples | Source / Concern |
|---|---|---|
| Biological contaminants | Bacteria, viruses, parasites | Addressed through disinfection with chlorine compounds or ozone |
| Inorganic chemical contaminants | Pb²⁺, Hg²⁺, As, Cd²⁺, NO₃⁻ | Industrial discharge, agricultural runoff, corroded plumbing, natural mineral deposits |
| Organic chemical contaminants | Pesticides, solvents, pharmaceuticals, petroleum compounds | Difficult to remove; often nonpolar or structurally resistant to breakdown |
| Nutrient pollution | Excess nitrogen and phosphorus compounds | From fertilizers and sewage; causes algal blooms and eutrophication |
| pH imbalance | Acid rain, mine drainage | Makes water too acidic for aquatic organisms |
Excess nitrogen and phosphorus cause rapid algae growth (an algal bloom). When the algae die and decompose, bacteria consume dissolved oxygen to break down the organic matter, leaving too little oxygen for fish and other aquatic life. This chain of events — nutrients → algae → decomposition → oxygen loss — is called eutrophication.
Municipal water treatment systems use a series of chemical and physical processes to make water safe for drinking.
Al₂(SO₄)₃ + 6 H₂O → 2 Al(OH)₃ + 3 H₂SO₄
| Step | Process | What It Does |
|---|---|---|
| 1 | Sedimentation | Floc-particle aggregates settle to the bottom of large holding tanks and are removed as sludge |
| 2 | Filtration | Water passes through layers of sand, gravel, and activated charcoal, which adsorbs dissolved organic compounds, chlorine byproducts, and some contaminants onto its enormous surface area |
| 3 | Disinfection | Chlorine, ozone, or UV light kills remaining pathogens |
| 4 | pH adjustment | Brings treated water to a slightly alkaline pH (about 7.5–8.5) to minimize pipe corrosion |
| 5 | Fluoridation | Adds fluoride ions at low concentration (about 0.7 mg/L in the U.S.) to strengthen tooth enamel |
Activated charcoal works by adsorption — molecules stick to the surface of the charcoal. This is different from absorption, in which molecules are taken into the interior of a material. Activated charcoal's enormous surface area (created by its porous structure) is what makes it effective at removing dissolved organic compounds.
Chlorine gas (Cl₂) or sodium hypochlorite (NaOCl) is a powerful oxidizing agent that destroys bacterial cell membranes and inactivates viruses:
Cl₂ + H₂O → HCl + HOCl
Hypochlorous acid (HOCl) is the active disinfecting species — it is more effective than the hypochlorite ion (OCl⁻), which forms in alkaline water. This is why maintaining proper pH in treated water matters.
Chlorine treatment can react with naturally occurring organic compounds in water to form disinfection byproducts such as trihalomethanes (THMs), which are of concern at high concentrations. Treatment plants monitor and control DBP levels carefully. Ozone (O₃) and ultraviolet (UV) light are alternatives that avoid chlorinated byproducts, but both require a follow-up residual disinfectant to protect water as it travels through distribution pipes.
Fluoridation strengthens tooth enamel by replacing hydroxide ions in the mineral hydroxyapatite with fluoride ions, forming fluorapatite, which is more resistant to acid attack.
| Stage | Process |
|---|---|
| Primary treatment | Removes large solids through screening and sedimentation |
| Secondary treatment | Biological processes — bacteria consume dissolved organic matter, converting it to CO₂, water, and bacterial biomass (sludge) |
| Tertiary treatment | Addresses remaining nutrients, pharmaceuticals, pathogens, and fine particles using chemical precipitation, activated charcoal, or advanced filtration |
The chemical goal throughout wastewater treatment is to return water to a condition safe for the receiving environment, reducing biochemical oxygen demand (BOD), nutrient concentrations, and pathogen levels.
The atmosphere is a complex chemical system. It contains nitrogen (N₂, approximately 78%), oxygen (O₂, approximately 21%), argon (Ar, about 0.9%), and trace gases including carbon dioxide (CO₂), methane (CH₄), ozone (O₃), water vapor, and many other compounds. Human activities add pollutants to this system, changing its chemistry in ways that affect human health, ecosystems, materials, and climate.
| Type | Pollutant | Source |
|---|---|---|
| Primary released directly from a source | Carbon monoxide (CO) | Incomplete combustion |
| Sulfur dioxide (SO₂) | Burning coal, smelting metal ores | |
| Nitrogen oxides (NOₓ) | High-temperature combustion in engines and power plants | |
| Particulate matter (PM) | Combustion, construction, industrial processes | |
| Volatile organic compounds (VOCs) | Fuels, solvents, paints, industrial processes | |
| Lead (Pb) | Historically leaded gasoline; smelting | |
| Secondary formed by atmospheric reactions | Ground-level ozone (O₃) | NOₓ + VOCs + sunlight |
Ground-level ozone forms in the atmosphere when nitrogen oxides and volatile organic compounds react in the presence of sunlight:
NOₓ + VOCs + sunlight → O₃ + other products
The resulting brownish haze — ground-level ozone, nitrogen dioxide, and particulate matter interacting in sunlight — is called photochemical smog.
Ozone in the upper atmosphere (stratosphere) protects life from ultraviolet radiation and is beneficial. Ozone at ground level is a powerful oxidant that irritates lung tissue, damages crops, and degrades rubber and plastics — it is harmful. Do not confuse the two: same molecule, very different consequences depending on location.
Coal, oil, and natural gas contain sulfur compounds. When burned, the sulfur is oxidized to sulfur dioxide, which can then be further oxidized and combine with water to form sulfuric acid.
S + O₂ → SO₂2 SO₂ + O₂ → 2 SO₃SO₃ + H₂O → H₂SO₄4 NO + 3 O₂ + 2 H₂O → 4 HNO₃Power plants reduce SO₂ emissions using scrubbers, which react flue gases with limestone or lime slurry:
CaCO₃ + SO₂ → CaSO₃ + CO₂ or Ca(OH)₂ + SO₂ → CaSO₃ + H₂O
The calcium sulfite produced can be further oxidized to calcium sulfate (gypsum), which has commercial uses in construction.
Carbon monoxide (CO) is produced by incomplete combustion — combustion that occurs without enough oxygen to fully oxidize carbon to carbon dioxide.
| Combustion Type | Reaction | Product |
|---|---|---|
| Complete combustion | C + O₂ → CO₂ | Carbon dioxide |
| Incomplete combustion | 2 C + O₂ → 2 CO | Carbon monoxide |
Carbon monoxide binds to hemoglobin in the blood much more strongly than oxygen does. Hemoglobin bound to CO cannot carry oxygen to body tissues. Exposure to high concentrations of CO causes oxygen deprivation, loss of consciousness, and can be fatal. Proper ventilation and combustion engineering minimize CO emissions.
Particulate matter (PM) is classified by particle size. PM₁₀ refers to particles less than 10 micrometers in diameter; PM₂.₅ refers to particles less than 2.5 micrometers. Smaller particles penetrate more deeply into lung tissue and are more damaging to health. Particulates come from combustion of solid fuels, diesel engines, construction dust, wildfires, and industrial processes, and are associated with respiratory and cardiovascular disease at elevated concentrations.
The stratospheric ozone layer absorbs ultraviolet (UV) radiation from the sun, preventing much of it from reaching Earth's surface. Without this protection, rates of skin cancer, cataracts, and damage to plant and animal life would increase dramatically.
Ozone is created in the stratosphere when UV radiation splits oxygen molecules:
O₂ + UV → 2 O
O + O₂ → O₃
Naturally, ozone is also broken down by UV, maintaining a dynamic balance.
CF₂Cl₂ + UV → CF₂Cl + ClCl + O₃ → ClO + O₂
ClO + O → Cl + O₂
Soil is not simply dirt. It is a complex chemical system containing mineral particles, organic matter, water, gases, and living organisms. Healthy soil chemistry supports plant growth and agricultural productivity. Poor soil chemistry — whether from nutrient depletion, acidification, contamination, or salinization — makes agriculture difficult or impossible.
| Nutrient | Role in the Plant | Absorbed Form |
|---|---|---|
| Nitrogen (N) | Proteins, nucleic acids, chlorophyll | NO₃⁻ or NH₄⁺ |
| Phosphorus (P) | ATP, DNA, cell membranes | H₂PO₄⁻ or HPO₄²⁻ |
| Potassium (K) | Enzyme function, water regulation, ion transport | K⁺ |
| Secondary macronutrients | Various structural and regulatory roles | Ca²⁺, Mg²⁺, SO₄²⁻ |
| Micronutrients | Needed in very small amounts | Fe, Mn, Zn, Cu, B, Mo |
The major macronutrients — nitrogen, phosphorus, and potassium — are abbreviated NPK on fertilizer labels. Although N₂ gas makes up most of the atmosphere, plants cannot use gaseous nitrogen directly; it must be converted to nitrate or ammonium ions first.
Nitrogen fixation — converts atmospheric N₂ into NH₃ or NH₄⁺. Carried out by certain bacteria (e.g., Rhizobium species in legume roots) or industrially by the Haber-Bosch process (Section 27.7).
Nitrification — bacterial conversion of NH₄⁺ to NO₂⁻ and then to NO₃⁻, the primary form most crops absorb.
Denitrification — converts NO₃⁻ back to N₂ or N₂O gas in waterlogged, low-oxygen soils, returning nitrogen to the atmosphere but reducing plant-available nitrogen.
Decomposition — organic matter is broken down by soil bacteria and fungi, releasing nitrogen as NH₄⁺ back into the available pool.
Soil pH affects which nutrients are available to plants. Most crops grow best in slightly acidic to neutral soil, roughly pH 6 to 7.
| Soil Condition | Effect on Nutrient Availability |
|---|---|
| Low pH (acidic) | Aluminum and manganese become more soluble and can reach toxic concentrations; phosphate becomes less available; calcium and magnesium leach more easily |
| High pH (alkaline) | Iron, manganese, zinc, and copper become less soluble and may cause deficiency even when present in the soil |
Farmers raise soil pH by adding lime (calcium carbonate, CaCO₃, or calcium hydroxide, Ca(OH)₂). They lower soil pH by adding elemental sulfur, which oxidizes in soil to form sulfuric acid.
Commercial fertilizers supply nutrients in forms plants can absorb. Nitrogen fertilizers include ammonia (NH₃), ammonium nitrate (NH₄NO₃), urea (CO(NH₂)₂), and ammonium sulfate ((NH₄)₂SO₄). Potassium fertilizers come mainly from mined deposits of potassium chloride (KCl) or potassium sulfate (K₂SO₄).
Ca₃(PO₄)₂ + 2 H₂SO₄ → Ca(H₂PO₄)₂ + 2 CaSO₄Excess nitrogen and phosphorus that are not absorbed by crops wash into rivers and lakes, causing eutrophication (Section 27.2). Careful nutrient management — applying the right amount at the right time in the right place — reduces runoff while maintaining crop productivity.
Pesticides are chemicals that kill insects (insecticides), weeds (herbicides), or fungi (fungicides) that reduce crop yield. Some pesticides are biodegradable — they break down through chemical or biological reactions. Others are persistent — they resist breakdown and accumulate in soil, water, and food chains. Organochlorine pesticides such as DDT were found to accumulate in fatty tissues of animals through bioaccumulation, reaching harmful concentrations at higher levels of the food chain (biomagnification, discussed further in Section 27.8). Integrated pest management (IPM) combines targeted pesticide use with biological controls, crop rotation, and resistant plant varieties to minimize chemical use while controlling pests effectively.
Combustion is one of the most widely used chemical reactions in civilization. Fuels — coal, petroleum, natural gas, wood, and increasingly hydrogen and biofuels — are burned to release energy for heating, electricity generation, transportation, and manufacturing.
Fossil fuels are hydrocarbons and related organic compounds formed over millions of years from the remains of ancient organisms. They store chemical energy in carbon-hydrogen and carbon-carbon bonds.
| Fuel | Composition | Combustion Reaction |
|---|---|---|
| Natural gas | Primarily methane (CH₄), with some ethane and propane | CH₄ + 2 O₂ → CO₂ + 2 H₂O |
| Petroleum (crude oil) | Complex mixture of hydrocarbons, refined into gasoline, diesel, jet fuel, etc. | 2 C₈H₁₈ + 25 O₂ → 16 CO₂ + 18 H₂O (octane, approx.) |
| Coal | Mostly carbon, with hydrogen, sulfur, nitrogen, and trace metals | Produces CO₂, H₂O, SO₂, NOₓ, and ash |
Complete combustion of a pure hydrocarbon produces only carbon dioxide and water. In practice, fuels are impure and combustion conditions are imperfect, producing a range of products.
| Product | How It Forms | Effect |
|---|---|---|
| Carbon dioxide (CO₂) | Complete combustion of carbon-containing fuels | Primary greenhouse gas; absorbs and re-emits infrared radiation, warming the lower atmosphere |
| Water vapor (H₂O) | Combustion of hydrogen-containing fuels | Also a greenhouse gas |
| Carbon monoxide (CO) | Incomplete combustion | Binds hemoglobin, reducing oxygen transport (Section 27.3) |
| Sulfur dioxide (SO₂) | Sulfur impurities in coal and oil | Precursor to acid rain |
| Nitrogen oxides (NOₓ) | High combustion temperatures | Precursor to acid rain and ground-level ozone |
| Particulate matter | Solid fuel combustion, diesel engines | Respiratory and cardiovascular health effects |
| Unburned hydrocarbons / VOCs | Incomplete combustion, fuel evaporation | Contribute to ground-level ozone formation |
Hydrogen burns cleanly, producing only water:
2 H₂ + O₂ → 2 H₂O
Hydrogen is not a fossil fuel — it must be produced from water or other hydrogen-containing compounds using energy. Currently, most hydrogen is produced by steam reforming of methane, which still produces CO₂. Electrolysis of water using renewable electricity can produce hydrogen without carbon emissions.
Biofuels are derived from plant or animal matter. Ethanol (C₂H₅OH) made from corn or sugarcane can replace or blend with gasoline; biodiesel is made from vegetable oils or animal fats. Biofuels are renewable but still produce CO₂ on combustion. The net carbon effect depends on how much CO₂ was removed from the atmosphere by the plants during growth.
Plastics are organic polymers — long chains of repeating molecular units called monomers. They are lightweight, durable, chemically resistant, and easily molded. These properties have made plastics central to packaging, construction, medicine, electronics, transportation, and countless other applications.
Monomers containing double bonds join together without losing any atoms. Ethylene (C₂H₄) polymerizes to form polyethylene:
n CH₂=CH₂ → −(CH₂−CH₂)ₙ−
The subscript n indicates that the unit repeats many thousands of times. Common addition polymers include polyethylene (PE), polypropylene (PP), polyvinyl chloride (PVC), and polystyrene (PS).
Condensation polymerization joins monomers with the release of a small molecule, usually water. Nylon is a condensation polymer formed from diamine and diacid monomers. Polyester (used in bottles labeled PET — polyethylene terephthalate) is another condensation polymer.
| Plastic | Resin Code | Properties & Uses |
|---|---|---|
| PET (polyethylene terephthalate) | 1 | Used for beverage bottles; one of the most commonly recycled plastics |
| HDPE (high-density polyethylene) | 2 | Stiff; used in bottles and pipes |
| PVC (polyvinyl chloride) | 3 | Rigid (pipes, window frames) or flexible with plasticizers (flooring, cables); contains chlorine, produces HCl and dioxins if burned |
| LDPE (low-density polyethylene) | 4 | Flexible; used in plastic bags and films |
| PP (polypropylene) | 5 | Heat-resistant; used in food containers, bottle caps, medical equipment |
| PS (polystyrene) | 6 | Used in disposable cups, food trays, packaging foam; brittle, difficult to recycle |
Plastics persist in the environment because most are not biodegradable — they are not broken down by soil bacteria or other biological processes on human timescales. Instead, plastics fragment into smaller and smaller pieces under UV radiation and physical stress, eventually becoming microplastics — particles smaller than 5 millimeters. Microplastics have been found in oceans, rivers, soil, drinking water, and animal tissues.
Plastics may contain additives (plasticizers, flame retardants, stabilizers) that can leach into food, water, or body tissues — bisphenol A (BPA) is one such compound studied for potential hormonal effects. Plastics floating in the ocean concentrate persistent organic pollutants (POPs) from seawater onto their surfaces, which marine organisms can then consume along with the plastic. Burning plastics in open fires or inadequate incinerators releases toxic compounds, including HCl from PVC and polycyclic aromatic hydrocarbons (PAHs) from other plastics.
| Method | Process | Limitation |
|---|---|---|
| Mechanical recycling | Sorting, cleaning, melting, and re-forming plastics | Most effective for single-resin streams; mixing different plastics produces poor-quality material because melting points and molecular structures are incompatible |
| Chemical recycling | Breaks polymer chains back into monomers via depolymerization, pyrolysis, or solvolysis | Can handle mixed plastics but requires significant energy input |
Recycling rates for most plastics remain low in most countries, largely because of sorting challenges, contamination, and economics. Reducing plastic use and designing products for recyclability from the start are complementary strategies.
Several industrial chemical processes are so large in scale and so important to civilization that every chemistry student should understand their basic chemistry.
N₂ + 3 H₂ ⇌ 2 NH₃ ΔH = −92 kJ/molThe Haber-Bosch process consumes enormous amounts of energy (mainly natural gas), and the nitrogen it releases into agriculture has large-scale effects on natural ecosystems, including eutrophication of waterways and emission of nitrous oxide (N₂O), a potent greenhouse gas.
S + O₂ → SO₂2 SO₂ + O₂ ⇌ 2 SO₃ This step is also an equilibrium. High pressure and lower temperature favor SO₃ production, but temperature must be high enough for the catalyst to function. About 99.5% conversion is achieved.SO₃ + H₂SO₄ → H₂S₂O₇
H₂S₂O₇ + H₂O → 2 H₂SO₄
2 H₂O + 2 e⁻ → H₂ + 2 OH⁻2 Cl⁻ → Cl₂ + 2 e⁻2 NaCl + 2 H₂O → Cl₂ + H₂ + 2 NaOHCrude oil is a complex mixture of thousands of different hydrocarbons with different chain lengths, structures, and boiling points. Petroleum refining separates and transforms these compounds into useful products.
| Fraction | Carbon Range | Use |
|---|---|---|
| Petroleum gas | C₁–C₄ | Fuel gas |
| Gasoline | C₅–C₁₂ | Motor fuel |
| Kerosene / jet fuel | C₁₀–C₁₆ | Aviation and heating fuel |
| Diesel fuel | C₁₄–C₂₀ | Diesel engines |
| Heavy fuel oil and lubricants | C₂₀–C₅₀ | Industrial fuel, lubrication |
| Bitumen / asphalt | C₅₀+ | Road surfacing, roofing |
Cracking breaks long-chain hydrocarbons into shorter, more valuable molecules. Thermal cracking uses heat; catalytic cracking uses a zeolite catalyst (fluid catalytic cracking, or FCC, is widely used to increase gasoline yield from heavier fractions). Reforming rearranges molecular structures to produce branched and aromatic hydrocarbons with higher octane ratings. Treating removes sulfur compounds by reacting them with hydrogen over a catalyst to produce hydrogen sulfide (H₂S), which is recovered and converted to elemental sulfur or sulfuric acid.
Every substance is toxic at some dose. Water causes hyponatremia (low blood sodium) if consumed in excessive quantities. Caffeine is lethal at very high doses. The difference between a medicine and a poison often lies in the dose. This foundational principle of toxicology — associated with the 16th-century physician Paracelsus — is sometimes stated as: "The dose makes the poison."
| Measure | Definition | Example |
|---|---|---|
| LD₅₀ | Lethal dose for 50% of a test population (mg/kg body weight); lower LD₅₀ = more acutely toxic | Botulinum toxin ≈ 1–2 ng/kg; table salt (NaCl) ≈ 3,000 mg/kg |
| LC₅₀ | Lethal concentration for 50% of a test population | Used for inhaled gases or dissolved water pollutants |
| NOAEL | No Observed Adverse Effect Level — the highest dose at which no harmful effects are detected | Used with safety factors to set exposure limits for pesticides, additives, and pollutants |
| Type | Cause | Example |
|---|---|---|
| Acute toxicity | A single large exposure | Rapid symptoms; can cause death |
| Chronic toxicity | Repeated small exposures over time | Lead, mercury, and arsenic — accumulate in the body or cause slow damage |
| Route | Description | Most Relevant For |
|---|---|---|
| Inhalation | Breathing in gases, vapors, or particles | Workplace chemical exposure, air pollution |
| Ingestion | Swallowing | Food and water contamination |
| Dermal contact | Skin absorption | Organic solvents, lipid-soluble compounds |
Some substances are not easily broken down by biological systems. They accumulate in fatty tissues because they are lipid-soluble and nonpolar. As larger animals eat many smaller ones, the concentration in fat tissue increases at each step in the food chain. This is biomagnification.
Risk has two components: hazard (how harmful the substance can be) and exposure (how much and how often people are actually exposed). A very hazardous substance with zero exposure poses no risk. A mildly hazardous substance with constant high exposure may pose significant risk.
Regulatory agencies combine hazard assessment (toxicology studies, epidemiology) with exposure assessment (measured concentrations in air, water, food, consumer products) to estimate risk and set standards — often with large safety factors to protect sensitive populations such as children and people with health conditions.
Chemistry has created enormous benefits for human civilization — fertilizers that feed billions, medicines that prevent and cure disease, materials that build shelter and infrastructure, processes that provide clean water and sanitation. Chemistry has also created problems — persistent pollutants, toxic waste, greenhouse gas emissions, resource depletion. The same knowledge and tools that produce harm can be redirected to prevent it.
Developed by Paul Anastas and John Warner, the twelve principles of green chemistry guide the design of chemical products and processes that reduce or eliminate the use and generation of hazardous substances. Selected principles include:
• Prevent waste rather than treating it after the fact
• Design syntheses to use and produce substances with little or no toxicity
• Design chemicals that degrade after use into innocuous products
• Use renewable feedstocks rather than fossil fuels
• Use catalysts rather than stoichiometric reagents to improve efficiency
• Minimize energy use and use renewable energy
• Design processes to minimize the potential for accidents, explosions, and fires
Green chemistry is not a compromise of chemical effectiveness — it is a design challenge. Developing a synthesis that is both effective and clean is harder than developing one that is merely effective. It requires deep understanding of reaction mechanisms, thermodynamics, kinetics, and materials chemistry.
Sustainability in chemistry means meeting current needs without compromising the ability of future generations to meet their needs. This requires thinking about resources (are the materials renewable?), energy (where does it come from and at what cost?), waste (where does it go and what does it do?), and toxicity (who is harmed and how?).
| Area | Chemistry Involved |
|---|---|
| Renewable energy chemistry | Photovoltaic cells (semiconductor chemistry), wind turbine materials (polymer composites, rare earth magnets), battery chemistry (lithium-ion, solid state), hydrogen fuel cells |
| Carbon capture and storage | Chemical and geological methods for capturing CO₂ from combustion and sequestering it underground or converting it to useful materials |
| Biobased materials | Replacing petroleum-derived polymers with materials from plant biomass, including bioplastics made from polylactic acid (PLA) derived from corn starch or sugarcane |
| Circular economy | Designing products and industrial systems so materials cycle continuously through use and recovery rather than ending as waste |
Public health and chemistry are deeply intertwined. Major public health advances of the twentieth century include:
| Advance | Chemical Basis |
|---|---|
| Clean water and sanitation | Water treatment chemistry (chlorination, coagulation, filtration) dramatically reduced deaths from waterborne diseases such as cholera and typhoid |
| Vaccines and pharmaceuticals | Drug development and manufacturing chemistry extended lifespans and reduced suffering from infectious and chronic disease |
| Pesticide chemistry | Insecticide use dramatically reduced malaria transmission through mosquito control (though environmental consequences required careful management) |
| Food preservation | Chemical preservatives, controlled atmosphere storage, and refrigeration reduced food waste and food-borne illness |
| Lead removal | Removing lead — a neurotoxin — from gasoline, paint, and plumbing measurably improved public health outcomes, particularly for children |
Chemistry gives you a more powerful lens through which to view the world. You can read a report about drinking water quality and understand what the contaminants are and why they matter. You can evaluate a claim about air pollution and assess whether the chemistry behind it is sound. You can read a product label and understand what chemicals it contains. This does not mean every chemical question has a simple answer — many real-world problems involve incomplete information, economic constraints, and genuine disagreement among experts. What chemistry gives you is the ability to recognize what is known, what is uncertain, what requires more evidence, and what the chemical mechanisms actually are.
Use the data tables and descriptions below to answer each question. Show your reasoning.
| Treatment Stage | Turbidity (NTU) | Coliform Bacteria (per 100 mL) | pH |
|---|---|---|---|
| Source water | 85 | 4,200 | 7.2 |
| After coagulation/sedimentation | 12 | 1,800 | 6.8 |
| After filtration | 1 | 320 | 6.9 |
| After chlorination and pH adjustment | 0.1 | 0 | 7.6 |
| Day | O₃ (ppb) | NO₂ (ppb) | Hours of Sunlight |
|---|---|---|---|
| Monday | 45 | 38 | 5 |
| Tuesday | 62 | 42 | 9 |
| Wednesday | 88 | 48 | 11 |
| Thursday | 41 | 35 | 4 |
| Friday | 97 | 52 | 12 |
| Saturday | 75 | 44 | 10 |
| Sunday | 38 | 30 | 3 |
| Property | Measured Value | Optimal Range |
|---|---|---|
| pH | 5.2 | 6.0–7.0 |
| Nitrate-N (mg/kg) | 8 | 20–40 |
| Phosphorus (mg/kg) | 22 | 25–50 |
| Potassium (mg/kg) | 180 | 150–250 |
| Lead (mg/kg) | 280 | < 400 |
In 2014, the city of Flint, Michigan changed its water supply from treated Lake Huron water to water from the Flint River. The Flint River water was more corrosive because it had higher chloride concentrations and different chemistry than the previous source water. The new water also was not treated with corrosion inhibitors.
The previous water source had deposited a protective scale of calcium carbonate inside aging lead pipes and brass fixtures throughout the city. The more corrosive Flint River water began dissolving that scale, exposing the underlying lead and brass to direct contact with water. Lead (as Pb²⁺ ions) leached from the pipes into drinking water.
Within months, lead concentrations in the water of many Flint homes exceeded the EPA action level of 15 ppb (micrograms per liter). In some homes, concentrations exceeded 100 ppb. Lead is a neurotoxin with no safe level of exposure for children. Studies found elevated blood lead levels in children throughout the affected area.
For each scenario, identify the relevant chemical risk, the potential benefit, and a possible approach that reduces risk while maintaining some benefit. There is often no perfect answer — explain your reasoning clearly.
Answer each question in one to three sentences.
Chapter 27 applied chemistry to large-scale systems of civilization — water, air, soil, fuels, materials, and industry. Chapter 28 turns to the smallest but most intricate chemical system: the living cell. Biological and Medical Chemistry explores how the principles of bonding, polarity, acid-base chemistry, enzyme kinetics, pH, and molecular structure play out inside living organisms. You will examine water's unique role in biology, study the four major classes of biological molecules (carbohydrates, lipids, proteins, and nucleic acids), explore how enzymes work as biological catalysts, and connect the chemistry of buffers and pH to the function of the human body. Chapter 28 also examines how medicines work at the molecular level, completing the arc from atoms and reactions to the chemistry that sustains life itself.