12BIO4 - Properties of Water

Why Study Water?

  • All life occurs in water (inside & outside cells).

  • Water has unique chemical and physical properties essential for life.

Chemistry of Water

  • H₂O molecules form hydrogen bonds (not chemically bonded):

    • +H attracted to O- slightly charged

    • Creates a "sticky" molecule (attracted to another water molecule).

Properties of Water

1. Cohesion & Adhesion

  • Cohesion: H-bonding between water molecules → surface tension, droplets, drinking straw.

    • flat surface because of water attracting to itself

    • causes round droplets to form as water falls from a facet with a low leak

  • Adhesion: H-bonding between water and other substances → capillary action, meniscus, water climbing paper towel.

    • meniscus: stuck to sides of the container it’s in such as glass, because its attracted

    • allows it to stick to dry surfaces such as a wooden countertop

    • water climbs up paper towel or cloth

  • Transpiration in plants relies on both cohesion & adhesion.

2. Good Solvent

  • Water is polar, making it a universal solvent.

  • Dissolves many molecules by surrounding + and – ions.

  • Hydrophilic: substances attracted to water (polar).

  • Hydrophobic: substances not attracted to water (nonpolar, e.g., fats).

3. Ice is Less Dense than Liquid Water

  • Most substances are denser as solids, but ice floats.

  • H-bonds form a crystal lattice, making ice less dense.

  • Importance:

    • Lakes/oceans don’t freeze solid.

    • Surface ice insulates water below → life survives winter.

    • Seasonal turnover circulates nutrients.

4. High Specific Heat

  • Water resists temperature changes.

  • Takes a lot of energy to heat up/cool down.

  • Moderates Earth’s climate and body temperatures.

5. High Heat of Vaporization

  • Evaporation removes heat (evaporative cooling).

  • Organisms use this for temperature regulation (e.g., sweating).

    • when sweat evaporates, water absorbs heat from the body which has a cooling effect

Ionization of Water & pH

  • Water ionizes:
    H₂O ⇌ H⁺ + OH⁻

  • pH scale (1–14) measures acidity/basicity:

    • Neutral: [H⁺] = [OH⁻] (pH 7, pure water).

    • Acidic: [H⁺] > [OH⁻] (pH < 7).

    • Basic: [H⁺] < [OH⁻] (pH > 7).

pH Scale Examples

  • pH 1: Stomach acid, lemon juice.

  • pH 3: Vinegar, cola, beer.

  • pH 5: Rainwater, black coffee.

  • pH 7: Pure water, blood.

  • pH 8: Seawater.

  • pH 11: Household ammonia.

  • pH 13–14: Oven cleaner, sodium hydroxide.

Key Takeaways:

  • Water’s hydrogen bonding creates its unique properties.

  • Cohesion/adhesion enable water transport in plants.

  • Ice floats → ecosystems survive winter.

  • High specific heat & heat of vaporization regulate temperature.

  • Water ionization underpins pH and life chemistry.