The Properties of Water

• what are main properties of water?

  • consists of two hydrogen atoms covalently bonded to an oxygen atom
  • electrons aren't shared equally in this bond, so the oxygen has a slight negative charge and the hydrogen a slight positive charge
• why does water have a much higher b.p. than hydrogen sulphide?

  H2O molecules can form strong hydrogen bonds with each other (intermolecular attractions that require more energy to break), H2S molecules don't have H bonds (only weaker dipole-dipole forces) so take less energy to break
• why does ice float?

  • ice is less dense than water, because hydrogen bonds formed (hydrogen bonds fix the positions of the polar molecules slightly further apart than the average distance in the liquid state)
• Two advantages of ice floating in water:

  • forms an insulating layer on surface of bodies of water, protecting aquatic life from freezing temperatures
  • the process of freezing and thawing plays a critical role in global climate regulation
• why are plants able to draw water up their stems?

  • Cohesive properties - moves as one mass because H2O molecules are attracted to each other
  • Adhesive properties - molecules are attracted to other materials
• why is water a good solvent?

  • it is a polar molecule
  • many solutes in an organism can be dissolved in water
  • acts as a medium for chemical reactions and also helps transport dissolved compounds into and out of cells
• why is water a good transport medium?

  • cohesion between water molecules
  • adhesion occurs between water molecules and other polar molecules and surfaces
  • capillary action - water can rise up a narrow tube against the force of gravity
• why does water have a high specific heat capacity and high latent heat of vaporisation?

  • due to hydrogen bonds between water molecules - need to absorb a lot of energy to break
• How do hydrogen bonds form?

  • the slight negative charge on oxygen atoms attract to the slight positive charge on hydrogen atoms of other water molecules
  • is between a H atom and a highly electronegative atom (O, N or F)
• what molecules don't dissolve in water?

  Hydrophobic molecules (non-polar)
• why is it important that water is a liquid over a large range of temperatures?

  so that it can retain its properties for a larger proportion of time
• what are advantages of water being liquid at body temperature?

  • provides a reaction medium for chemical reactions
  • forms a major component of tissue in organisms
• what are advantages of water having a high specific heat capacity?

  • provides a stable environment for aquatic organisms
  • enzymes-controlled reactions need stable temperatures
• what are advantages of water having a high latent heat of vaporisation?

  • some animals are cooled when sweat evaporates
  • plants are cooled when water evaporates from leaves