Properties of water

• Water


  • Biological medium on Earth
  • All living organisms require water more than any other substance
  • Most cells in water - cells are ~70–95% water
  • The abundance of water is the main reason the Earth is habitable
• Polarity of water molecules


  • Opposite ends have opposite charges
  • Polarity - hydrogen bonds with each other
• Models of water molecule


  • Bohr Model
  • Space-Filling Model
  • Ball-and-Stick Model
• Hydrogen bonds


  • Cohesion - polarity of water allows water molecules to be attracted to one another
  • Responsible for many of water's important physical properties
• Why is it unlikely that two neighbouring water molecules would be arranged like this?
• Six emergent properties of water


  • Cohesion - water molecules stick to other water molecules by hydrogen bonding
  • Adhesion - water molecules stick to other polar molecules by hydrogen bonding
  • High Specific Heat - water can absorb or release a large amount of heat with only a slight change in its own temperature
  • High Heat Of Vaporization - evaporative cooling helps stabilize temperatures
  • Lower Density Of Water - ice floats in liquid water
  • The Solvent of Life - water is a versatile solvent due to its polarity
• • Hydrophilic substance is one that has an affinity for water
  • Hydrophobic substance is one that does not have an affinity for water
  • Oil molecules are hydrophobic because they have relatively many nonpolar bonds
• Molecular mass


  Sum of all masses of all atoms in a molecule
• Mole (mol)


  6.02 x 10^23 molecules
• Molarity (M)


  Number of moles of solute per liter of solution
• Acid


  • Any substance that dissociates in water to increase the [H+] (and lower the pH)
  • The stronger an acid is, the more hydrogen ions it produces and the lower its pH
• Base


  Substance that combines with H+ dissolved in water, and thus lowers the [H+]
• • Though statistically rare, the dissociation of water molecules has a great effect on organisms
  • Changes in concentrations of H+ and OH– can drastically affect the chemistry of a cell
• Buffers


  • Substance that resists changes in pH
  • Act by releasing hydrogen ions when a base is added and absorbing hydrogen ions when acid is added
  • Overall effect of keeping [H+] relatively constant
• • Physiological buffer - carbonic acid (in blood)
  • pH of body fluids - narrow range