Water and Solutions Notes from UW-Madison Chemistry

Distribution of Water on Earth

  • Distribution Overview

    • 71% of Earth's surface is covered by water

    • Water distribution on Earth can be categorized into various types:

    • Saline (oceans): 97% of Earth's water

    • Fresh water: 3%

      • Surface water: 0.3%

        • Lakes and rivers: 11% of fresh water

        • Glaciers: 68.7%

        • Other: 0.9%

    • Much of the fresh water is not potable or easily accessible


International Water Issues

  • Water Stress Globally

    • Over 2 billion people (more than a quarter of the global population) live in "water-stressed" countries


Local Water Concerns in Wisconsin

  • Raw Water

    • Raw water is untreated, unfiltered, and unsanitized

    • In 2018, prices for raw water reached as high as $60.99 for 2.5 gallons

  • Contaminants in Drinking Water

    • Arsenic has been detected in wells across every county in Wisconsin

    • High levels detected: >10 ppb (parts per billion)

    • Serious issues focus on specific regions with highest levels

    • Map highlights areas where arsenic exceeds safety standards

  • Salt Contamination

    • Salt from road treatments is leading to increased salinity in Madison's drinking water

  • PFAS Contamination

    • PFAS (Per- and polyfluoroalkyl substances), categorized as “forever chemicals,” are detected in Dane County due to contamination from firefighting foams used at local military bases and airports


Chemical Properties of Water

  • Water Molecule

    • Composition: H₂O (2 hydrogen atoms, 1 oxygen atom)

    • Oxygen has 6 valence electrons, while each hydrogen has 1

    • Molecule forms a bent shape due to electron pair repulsion

    • Bonding creates an angle of approximately 104.5°


Molecular Shape and Bond Angles

  • Electron Geometry

    • Bonds and lone pairs affect the shape and bond angles of a molecule

    • Shapes include:

    • Linear

    • Trigonal Planar

    • Tetrahedral

  • Typical Bond Angles

    • Water (H₂O): 104.5°

    • Ammonia (NH₃): 107.5°

    • Methane (CH₄): 109.5°

  • Understanding Molecular Shapes:

    • Determine based on bonding and non-bonding electron pairs

    • Lone pairs reduce bond angles because they take up more space compared to bonded pairs


ConcepTests: Analyzing Water Quality

  • Flint Water Crisis:

    • High levels of Lead (Pb) were found in drinking water during the crisis

  • Exam Questions:

    • Determining shapes and angles based on given molecular structures is critical for understanding molecular geometry

    • PCl₃ example:

    • Electron Pair Geometry: Tetrahedral

    • Molecular Shape: Trigonal Pyramidal

  • Draw and Analyze:

    • For SF₂:

    • Determine electron pair geometry and molecular shape from the Lewis structure