Chemistry · 9th Grade · Thermodynamics and Kinetics · Weeks 19-27

Water Quality and Contaminants

Students will investigate common water contaminants and their chemical properties.

• Common Core State Standards
• HS-ESS3-3
• HS-PS1-2

About This Topic

Water quality is a chemistry topic with immediate real-world stakes, bridging core chemistry concepts (solubility, pH, chemical properties) with environmental science and public health. Common water contaminants include heavy metals such as lead and arsenic, nitrates from agricultural runoff, microplastics, chlorinated solvents, and pathogens. Each contaminant has distinct chemical properties that determine how it enters water systems, how it behaves once dissolved, and how difficult it is to remove through treatment.

Persistent pollutants such as per- and polyfluoroalkyl substances (PFAS) remain chemically stable in the environment because of their exceptionally strong carbon-fluorine bonds. Unlike most organic compounds, they do not break down readily through oxidation or hydrolysis pathways. Understanding why certain substances persist requires students to connect molecular structure to chemical reactivity, a core skill aligned to HS-PS1-2. Lead contamination, by contrast, involves oxidation chemistry where Pb metal dissolves into water under corrosive conditions.

Active learning is particularly valuable here because real-world water quality data is publicly available from the EPA and state environmental agencies. Analyzing actual water quality reports from local municipalities or historically contaminated sites makes the chemistry personally relevant and motivates deeper engagement with the underlying concepts.

Key Questions

1. Identify common chemical contaminants found in water sources.
2. Explain the health risks associated with various water contaminants.
3. Analyze the chemical properties that make certain substances persistent pollutants.

Learning Objectives

• Identify at least three common chemical contaminants in US drinking water sources, citing their origins.
• Explain the specific health risks associated with lead and nitrate contamination in drinking water.
• Analyze the chemical properties, such as bond strength and solubility, that contribute to the persistence of PFAS in the environment.
• Compare the chemical behaviors of heavy metals and organic pollutants in water systems.
• Evaluate the effectiveness of common water treatment methods for removing specific contaminants like chlorine or dissolved solids.

Before You Start

Chemical Bonding and Molecular Structure

Why: Understanding different types of chemical bonds (e.g., covalent, ionic) and how molecular structure influences properties is essential for explaining contaminant behavior.

Acids, Bases, and pH

Why: Knowledge of pH is fundamental to understanding water's corrosivity and its interaction with metal contaminants.

Chemical Reactions and Stoichiometry

Why: Students need to understand basic reaction types, such as oxidation-reduction, to explain how contaminants enter and transform in water.

Key Vocabulary

contaminant	A substance that pollutes or taints something, especially water, making it impure or unsafe.
solubility	The ability of a substance to dissolve in a solvent, such as water, forming a homogeneous solution.
persistent pollutant	A chemical substance that resists degradation, remaining in the environment for long periods due to its stable chemical structure.
oxidation	A chemical reaction involving the loss of electrons, often resulting in the formation of oxides or the dissolution of metals in water.
pH	A measure of the acidity or alkalinity of an aqueous solution, indicating the concentration of hydrogen ions.

Watch Out for These Misconceptions

Common Misconception

Students think that clean water means no chemicals are present.

What to Teach Instead

All water contains dissolved substances; the question is which ones and at what concentrations. The concept of maximum contaminant levels and the distinction between regulated contaminants and naturally occurring minerals helps students understand that safety is about concentration thresholds, not the presence or absence of any particular substance.

Common Misconception

Students believe that if a substance is odorless and colorless, it is safe to drink.

What to Teach Instead

Many of the most dangerous water contaminants, including lead, arsenic, nitrates, and PFAS, are odorless, colorless, and tasteless. Sensory properties are not reliable indicators of chemical safety. Data analysis activities using real water quality reports make this concrete.

Common Misconception

Students think water contaminants only come from industrial sources.

What to Teach Instead

Agricultural runoff introduces nitrates and pesticides; older building materials contribute lead; septic systems can introduce pathogens and pharmaceuticals. The gallery walk activity exposes students to the diverse sources of contamination in a structured and memorable way.

Active Learning Ideas

Data Analysis: Reading Water Quality Reports

Provide groups with a consumer confidence report from a local water utility alongside EPA maximum contaminant levels. Students identify which contaminants are present, compare values to legal limits, and classify each as biological, chemical, or physical, then present findings and propose questions for further investigation.

40 min·Small Groups

Case Study Analysis: Flint Water Crisis Chemistry

Guide students through the chemistry of the Flint, Michigan water crisis, focusing on how lead leached from pipes due to corrosive water conditions. Students identify the chemical factors that increased corrosivity, then analyze what could have been done differently at each stage using their chemistry knowledge.

45 min·Small Groups

Gallery Walk: Contaminant Chemistry Profiles

Post stations with profiles of different contaminants (lead, nitrates, PFAS, arsenic, microplastics), each showing molecular structure, source, health effects, and persistence data. Students rotate with a graphic organizer to compare chemical properties and identify which contaminants are most persistent and why.

35 min·Small Groups

Think-Pair-Share: Why PFAS Is Difficult to Remove

Provide a brief reading on PFAS chemistry and ask students to identify the structural feature that makes PFAS resistant to environmental breakdown. Partners compare explanations and together write a two-sentence summary connecting C-F bond strength to the concept of environmental persistence.

20 min·Pairs

Real-World Connections

• Environmental chemists working for municipal water treatment plants, such as the DC Water facility, analyze water samples daily to ensure compliance with EPA standards and protect public health from contaminants like lead and disinfection byproducts.
• Public health officials investigate outbreaks of waterborne illnesses, like the 2014 Flint water crisis, to determine the source of contamination (e.g., lead leaching from pipes) and implement remediation strategies.
• Toxicologists assess the long-term health impacts of exposure to persistent chemicals like PFAS found in firefighting foam or non-stick cookware, informing regulatory decisions for industries and consumer products.

Assessment Ideas

Quick Check

Provide students with a list of common water contaminants (e.g., lead, nitrates, chlorine, PFAS). Ask them to select two and write a brief explanation of their chemical properties and one associated health risk.

Discussion Prompt

Pose the question: 'Why are some chemicals, like PFAS, considered 'forever chemicals' while others, like iron in hard water, are more easily managed?' Facilitate a discussion focusing on bond strength, reactivity, and environmental persistence.

Exit Ticket

Students complete the sentence: 'A key chemical property that makes a substance a persistent pollutant is ______ because ______.' They should also list one example of a persistent pollutant.

Frequently Asked Questions

What are PFAS chemicals and why are they called forever chemicals?

PFAS (per- and polyfluoroalkyl substances) are synthetic compounds with very strong carbon-fluorine bonds, among the strongest in organic chemistry. These bonds resist breakdown by most environmental processes, including microbial degradation, heat, and UV light. As a result, PFAS accumulate in the environment and in living organisms rather than degrading over time.

How does lead get into drinking water?

Lead rarely enters water at its source but leaches from lead pipes, lead solder in plumbing joints, or lead-based fixtures as corrosive water flows through them. The chemistry involves oxidation of lead metal to Pb2+ ions, which dissolve into the water supply. The problem is worst in older buildings and cities with aging infrastructure like the situation in Flint, Michigan.

What are the health risks of nitrate contamination in water?

Nitrates are most dangerous to infants under six months, causing methemoglobinemia (blue baby syndrome) by interfering with oxygen transport in blood. For adults, chronic exposure has been linked to increased risk of certain cancers. Nitrates enter water primarily from fertilizer runoff and septic systems, particularly in agricultural regions.

How does studying water quality help students learn chemistry concepts?

Water quality connects solubility, pH, oxidation-reduction, and molecular structure to issues students encounter in daily life and the news. When students analyze real water quality data from their own community, chemistry stops being abstract. Active learning approaches like case studies and real data analysis also build the critical reading skills emphasized in the CCSS ELA-Literacy standards aligned to this topic.