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									Ch. 17: Water and
       Aqueous Systems
• Ch. 17.1 Liquid Water and Its
           Properties
• Ch. 17.2 Water Vapor and Ice
• Ch. 17.3 Aqueous Systems
• Ch. 17.4 Heterogeneous
           Aqueous Systems
Ch. 17.1: Liquid Water and Its
          Properties

• Water is a unique compound
  –   Covers 75% of Earth’s surface
  –   A simple triatomic molecule
  –   Highly polar with a bent shape
  –   Water molecules are attracted to one another
      by intermolecular attractions, mainly
      hydrogen bonding, which causes:
       •   High surface tension
       •   High specific heat capacity
       •   High heat of vaporization
       •   High boiling point
Ch. 17.1: Liquid Water and Its
          Properties

• Surface Properties
  – The surface of H2O acts like a skin
  – Surface tension is a result of
    hydrogen bonding
  – Water is cohesive, especially at
    the surface
    • Water cannot form bonds with the air
    • Instead, molecules are pulled inward
      – Explains why drops of H2O are spherical
Ch. 17.1: Liquid Water and Its
          Properties
• Surface Properties
  – All liquids have a surface tension, but
    water’s is higher than most
  – It is possible to lower the surface
    tension of water by adding a surfactant
    • A wetting agent such as soap or detergent
    • The detergent molecules interfere with the
      attraction between the water molecules
• Hydrogen bonding also explains
  water’s unusually low vapor pressure
  – Limits water’s ability to vaporize or
    evaporate
Ch. 17.1: Liquid Water and Its
          Properties

• Specific heat capacity
  – It takes 4.18J (1 cal) to raise the
    temperature of 1 gram of water 10C
  – This is the specific heat capacity of water
     • The specific heat capacity of water is nearly
       constant between 00C and 1000C
     • Because of hydrogen bonding, the specific
       heat capacity of H2O is very high
     • Helps moderate daily air temp around large
       bodies of H2O
        – Water absorbs heat from warmer surroundings,
          which lowers the air temperature
        – At night, heat is transferred from the warmer
          water to the surrounding air
Ch. 17.2 Water Vapor and Ice

• Evaporation and Condensation
 – Water absorbs a large amount of
   heat as it evaporates/vaporizes
   • Heat of vaporization is the energy
     needed to convert 1g of substance
     from a liquid to a gas at the boiling
     point
     – Hydrogen bonds must be broken before the
       liquid changes to the gaseous state
 Ch. 17.2 Water Vapor and Ice

• Evaporation and Condensation
 – The reverse of vaporization is
   condensation
   • The heat of condensation is equal to
     the heat of vaporization of water
   • Heat is released during condensation,
     gained during evaporation
 – Evaporation and condensation are
   important to regional temperatures
   on Earth
 Ch. 17.2 Water Vapor and Ice

• Boiling point
  – Water has a very high boiling point
    • Due to hydrogen bonding
    • Molecular compounds of low molar mass are
      usually gases or liquids and have low boiling
      points at normal atmospheric pressure
       – Water is an exception
    • It takes a great deal of heat to to disrupt the
      bonding between the molecules in water
       – If this were not true, water would be a gas at
         the usual temperatures found on Earth
Ch. 17.2 Water Vapor and Ice

• Ice
  – Liquids usually contract as they
    cool
    • Density increases while mass stays
      constant
    • Eventually the liquid will solidify
    • Because the density of the solid is
      greater than the liquid, the solid will
      sink
Ch. 17.2 Water Vapor and Ice

• Ice
  – As water cools, at first it behaves
    like a typical liquid
    • It contacts slightly and it’s density
      gradually increases (until 40C)
    • Then the density begins to decrease
        – Water no longer behaves like a typical liquid
        – Ice has a 10% lower density than water at
          00C
        – As a result, ice floats (see pg. 481)
        – Ice is one of only a few solids that floats in
          it’s own liquid
Ch. 17.2 Water Vapor and Ice


• Ice
  – The fact that ice floats has
    important consequences for living
    organisms
    • Acts as an insulator in bodies of water
  – Water molecules require a
    considerable amount of kinetic
    energy to return to the liquid state
    • Known as heat of fusion
    • Very high in water, compared to other
      low molar mass molecules
Ch. 17.3 Aqueous Solutions

• Solvents and solutes
  – Water samples containing
    dissolved substances are called
    aqueous solutions
    • The dissolving medium is the solvent
    • The dissolved particles are the solute
    • Solutes and solvents may be solids,
      liquids or gases
  – Solutions are homogeneous
    mixtures
    • They are stable mixtures
Ch. 17.3 Aqueous Solutions

• Solvents and solutes
  – Substances that dissolve most
    readily in water include ionic cmpds
    and polar covalent molecules
    • Non-polar molecules like grease do no
      dissolve in water
    • Non-polar molecules will dissolve in
      other non-polar molecules
Ch. 17.3 Aqueous Solutions

• The Solution Process
  – Solvation is the process that occurs
    when a solute dissolves
    • The negatively and positively charged
      particles are surrounded by solvent molecules
    • In some ionic cmpds, internal attractions are
      stronger than external attractions – these
      cmpds cannot be solvated and are said to be
      insoluble
  – The rule is “like dissolves like”
Ch. 17.3 Aqueous Solutions

• Electrolytes and nonelectrolytes
  – Cmpds that conduct an electric current
    in aqueous solution or the molten state
    are called electrolytes
    • All ionic cmpds are electrolytes
    • Some are insoluble in water
  – Cmpds that do not conduct an electric
    current are called nonelectrolytes
    • They are not composed of ions
    • Most carbon cmpds are nonelectrolytes
  – Some very polar molecular cmpds are
    nonelectrolytes in the pure state, but
    become electrolytes when they dissolve
Ch. 17.3 Aqueous Solutions

• Electrolytes and nonelectrolytes
  – Not all electrolytes conduct an
    electric current to the same degree
    • Some electrolytes are strong
      – When dissolved, almost all of the solute
        exists as separate ions
      – Ex: NaCl
    • Some electrolytes are weak
      – When dissolved, only a fraction of the solute
        exists as separate ions
      – Ex: HgCl2
      – See pg. 485
Ch. 17.3 Aqueous Solutions

• Water of hydration
  – The water in a crystal is called the water
    of hydration or water of crystallization
    • A cmpd that contains water is called a
      hydrate
    • When writing the formula, a dot is used to
      connect the formula of the cmpd and the
      number of water molecules per formula unit
    • Hydrates appear dry and are unchanged in
      normally moist air
    • When heated above 1000C, hydrates lose their
      water of hydration
Ch. 17.3 Aqueous Solutions

• Hydrates
 – The forces holding the H2O in
   hydrates is not very strong
   • Held by weak forces
   • Results in a higher that normal vapor
     pressure
     – If the vapor pressure is higher than the
       vapor pressure in the air, the hydrate will
       effloresce by losing the water of hydration
Ch. 17.3 Aqueous Solutions

• Hygroscopic substances
  – Some hydrated salts that have a low
    vapor pressure remove water from air to
    form higher hydrates
  – Salts and other substances that remove
    water from air are hygroscopic
    • Many are used as dessicants
    • Some cmpds are so hygroscopic that they
      become wet when exposed to air – these are
      called deliquescent cmpds
    • Remove enough H2O to dissolve completely
      and form solutions
    • Occurs when the soln formed has a lower
      vapor pressure than that of air
Ch. 17.4 Heterogeneous
         Aqueous Systems

• Suspensions
  – Mixtures from which particles settle out
    upon standing
• Colloids
  – Mixtures containing particles that are
    intermediate in size between
    suspensions and true solutions
    • The particles are in the dispersed phase
    • They are spread through the dispersion
      medium, which can be a solid, liquid or gas
    • See pg. 491 for examples
Ch. 17.4 Heterogeneous
         Aqueous Systems
• Colloids
  – Properties differ from suspensions and
    solutions
    • May be cloudy when concentrated, clear when
      dilute
    • Intermediated sized particles cannot be
      filtered and do not settle out
    • Exhibit the Tyndall effect – scattering of
      visible light in all directions
    • Colloids scintillate (flash light) when studied
      under a microscope
       – Due to the erratic movement of the particles
         that reflect light
       – This chaotic movement is known as Brownian
         motion
Ch. 17.4 Heterogeneous
         Aqueous Systems
• Colloids
  – Properties differ from suspensions and
    solutions
    • Colloids scintillate (flash light) when studied
      under a microscope
       – Due to the erratic movement of the particles
         that reflect light
       – This chaotic movement is known as Brownian
         motion
       – Caused by collisions of molecules, which
         prevent the colloidal properties from settling
Ch. 17.4 Heterogeneous
         Aqueous Systems

• Colloids
 – Colloids may also absorb ions
   onto their surface
     – All the particles in a particular
       system will have the same charge
     – Repulsion of like charges keep the
       colloids from forming aggregates
       » Adding an opposite charge will cause
         separation of the colloid
Ch. 17.4 Heterogeneous
         Aqueous Systems

• Emulsions
 – Colloidal dispersions of liquids in
   liquids
 – Requires an emulsifying agent
   • Ex: soap and detergents
   • Allow formation of colloidal
     dispersions between liquids that do
     not normally mix by forming bonds
     with the water molecules

								
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