Chapter 3- Water and Life

characteristics of a water molecule

  • polar molecule- overall charge is unevenly distributed

  • v shape

  • two hydrogens and one oxygen

  • slight positive charge on each hydrogen atom

  • slight negative charge on the oxygen atom

  • each hydrogen atom is bonded to a more electronegative oxygen atom by a polar covalent bond- electrons in polar covalent bonds spend more time closer to the oxygen, rather than the hydrogen

properties of water

  • cohesion and adhesion- due to hydrogen bonding

    • creates a more structurally organized liquid

    • cohesion- hydrogen bonding between like molecules (water molecules to other water molecules)

    • adhesion- hydrogen bonding between unlike molecules (water molecules attracted to surrounding surfaces)

      • adhesion of water to vessel walls counters the pull of gravity

      • adhesion causes transpiration- the movement of water from plants root to leaf

        • water molecules cling to each other through cohesion, and sing to xylem tubes through adhesion

  • high surface tension- due to hydrogen bonding

    • surface tension is the measure of how difficult it is to break surface of liquid

      • permits container to be filled with water above its rim

      • permits bugs to walk on water surface

    • water has a greater surface tension than other liquids

    • occurs at the interphase between water and air

  • high specific heat (heat capacity)

    • it takes a lot of heat to raise water temperature

      • much of the heat energy is used to break the hydrogen bonds before water molecules can move faster

    • heat capacity helps to moderate temperature

      • large bodies of water can absorb the heat of the sun and store it while barely warming up

        • this keeps a stable marine and land environment and keeps earths temperature within limits that permit life

  • high heat of vaporization- due to hydrogen bonding

    • the quantity of heat a liquid must absorb for one gram of it to be converted from the liquid to gaseous state

      • the vaporization of water requires lots of heat

    • evaporative cooling- occurs because the molecules with the most energy (heat) are the most likely to leave as gas

      • this prevents overheating in terrestrial organisms (organism that lives on land) (why sweating cools the body)

      • has cooling effects on the environment (creates stable temperatures for life)

  • ice floats on water

    • unlike most substances, solid water is less dense than liquid water

      • water is most dense at 4 degrees celsius

      • in this temperature range, molecules are moving too slowly to break hydrogen bonds

      • by 0 degrees celsius, each water molecule is hydrogen bonded to 4 other molecules (locked into crystal lattice structure)

    • insulation by ice

      • floating ice insulates liquid water below, life exists under frozen surface

  • solvent of life (universal solvent)

    • water can be used to dissolve other polar substances (polar= molecule with uneven distribution of electron density)

    • water can be used to dissolve ionic substances

      • it is considered the solvent in life because it dissolves many things in a body

    • solution- a liquid that is a completely homogenous mixture of two or more substances

      • solvent- dissolving agent

      • solute- substance being dissolved

acids and bases

  • the disassociation of water molecules leads to formation of acids and bases

    • acid- a substance that increases the hydrogen ion concentration of a solution

    • base- a substances that reduces the hydrogen ion concentration of a solution

the pH scale

  • measure of acidity or alkalinity

  • measure of concentration of H+

    • pH 7= neutral; pH < 7= acid; pH > 7= base

buffers

  • a substance that minimizes changes in the concentration of H+ and OH- in a solution

  • buffers prevent large changes in pH

  • buffers work by accepting H+ ions when the pH drops and donation H+ ions when the pH increases