Water Properties

The Extraordinary Properties of Water

  • Water Molecule Structure

    • A water molecule (H2O) consists of three atoms: one oxygen and two hydrogen.

    • Diagram: H H O

  • Polarity of Water

    • Oxygen atom attracts more electrons than hydrogen, giving it a slight negative charge, while hydrogen has a slight positive charge.

    • The polarity of water molecules contributes to its unique properties, despite being electrically neutral (equal e- and p+).

  • Hydrogen Bonds

    • Formed between the highly electronegative oxygen of one water molecule and the hydrogen of another.

    • A single hydrogen bond is weak, but collectively, many hydrogen bonds create strength.

    • Attraction between the negative oxygen end of one molecule and the positive hydrogen end of another creates hydrogen bonds.

Properties of Water

  • Boiling and Freezing Points

    • At sea level, pure water boils at 100 °C and freezes at 0 °C.

    • Boiling temperature decreases at higher elevations due to lower atmospheric pressure.

  • Key Properties of Water:

    • Cohesion

    • Adhesion

    • High Specific Heat

    • High Heat of Vaporization

    • Less Dense as a Solid

Cohesion and Adhesion

  • Cohesion

    • Attraction between like substances (water molecules).

    • Results in surface tension which allows insects to walk on water.

  • Adhesion

    • Attraction between different substances (e.g., water and glass, soil, plant tissues).

    • Capillary Action: Water molecules tow each other along, crucial in processes like transpiration in plants and the absorption in paper towels.

    • Water’s adhesive properties lead to its ability to form spheres and cling to surfaces like spiderwebs.

  • High Specific Heat

    • Amount of energy needed to change the temperature of 1g of a substance by 1°C.

    • Water resists temperature changes, can absorb/release large amounts of heat without a significant change in temperature.

High Heat of Vaporization and Density of Ice

  • High Heat of Vaporization

    • Energy required to convert 1g of a substance from liquid to gas is significant (540 cal/g for water).

    • Hydrogen bonds must be broken for evaporation to occur.

    • Evaporation of water removes heat, contributing to the cooling effect.

    • Water vapor creates a thermal layer around the Earth, moderating temperature.

  • Less Dense as a Solid

    • Ice is less dense than liquid water, allowing it to float.

    • Liquid water constantly forms and breaks hydrogen bonds, whereas ice forms a stable crystal lattice structure.

Homeostasis and Mixtures

  • Homeostasis

    • Ability to maintain stable internal conditions despite external changes, aided by water's properties:

      • A good insulator

      • Resists temperature change

      • Acts as a universal solvent

      • Functions as a coolant

      • Ice insulates against extreme temperatures.

  • Solutions and Suspensions

    • Solutions: Homogeneous mixtures where ions disperse evenly in water.

      • Solute: Substance being dissolved.

      • Solvent: Substance doing the dissolving.

    • Suspensions: Mixtures where particles do not dissolve but remain suspended in water, preventing settling out (e.g., muddy water).

  • Dissociation of Water

    • Water dissociates into Hydrogen ions (H+) and Hydroxide ions (OH-) at a ratio of 1 in 550 million molecules.

The pH Scale and Buffers

  • pH Scale

    • Measures the concentration of H+ ions, ranging from 0 to 14.

      • pH 7: Neutral.

      • pH 0 to 7: Acidic (more H+ ions).

      • pH above 7 to 14: Basic (more OH- ions).

    • Each unit change on the pH scale represents a tenfold change in ion concentration (e.g., pH 3 is 1000 times stronger than pH 6).

  • Acids and Bases

    • Strong acids: pH of 1-3; produce large amounts of H+ ions.

    • Strong bases: pH of 11-14; contain more OH- ions and fewer H+ ions.

  • Buffers

    • Weak acids or bases that stabilize pH by reacting with strong acids/bases to maintain a balanced environment.