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Chapter 17. Water. Section 17.1. Water is a very polar molecule. The oxygen is electronegative. The hydrogens are electropositive. Water molecules are attracted to each other by Hydrogen bonds. The results of these bonds are that water has a: High surface tension

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section 17 1
Section 17.1
  • Water is a very polar molecule.
    • The oxygen is electronegative.
    • The hydrogens are electropositive.
  • Water molecules are attracted to each other by Hydrogen bonds. The results of these bonds are that water has a:
    • High surface tension
    • High specific heat capacity
    • High heat of vaporization
    • High boiling point.
why does water form a ball on a sheet of paper
Why does water form a ball on a sheet of paper?
  • Hydrogen bonds!
  • Water has a high surface tension:
    • The inward force or pull that tends to minimize the surface area of a liquid.
  • You can decrease the surface tension by adding a surfactant:
    • A wetting agent such as soap or detergent.
      • These detergents interfere with the hydrogen bonding.
why does water have a low vapor pressure
Why does water have a low vapor pressure?
  • Hydrogen Bonds!
  • The water molecules hold onto each other so that water doesn’t escape into the gaseous phase.
why does water have a high specific heat capacity
Why does water have a high specific heat capacity?
  • Hydrogen Bonds!!
  • It takes 4.18 J (1 cal) to raise 1 g of water 1˚C.
    • Do metals have a higher or lower heat capacity?
      • Lower
  • Why does this factor help to moderate?
    • Our temperatures – summer – relatively mild and winter relatively mild.
section 17 2
Section 17.2
  • Heat of Vaporization:
    • Amount of energy needed to convert 1 g of a substance from a liquid to a gas at the boiling point.
  • Condensation:
    • Going from a gas to a liquid – the amount of energy removed in order to convert 1 g of a substance from a gas to a liquid at the boiling point.
  • Molecular compounds that have low molecular mass are usually gas or liquids at room temperature and have low boiling points. (NH3 boils at -33˚C)
    • Water, a molecular compound with low molar mass, is high (boils at 100˚C). Why?
what happens when a liquid cools
What happens when a liquid cools?
  • The molecules move closer and closer, the density increases, and eventually the liquid turns to a solid and sinks.
  • Water behaves like this for only so long. At 4˚C, water is the densest it will be. Below this temperature water is actually decreasing in density.
  • When water reaches 0˚C, water turns to a solid and floats.
    • Why?
      • When ice forms, the hydrogen bonds make the molecules form a honeycomb type of structure. This makes it less dense.
  • What would happen if ice were to become more dense than liquid water in real life?
section 17 3
Section 17.3
  • Aqueous solutions:
    • Water samples containing diessolved substances.
    • Solvent:
      • Dissolving medium
    • Solute:
      • Dissolved particles
    • Substances that dissolve readily in water:
      • Ionic compounds
      • Polar covalent molecules
      • Nonpolar covalent molecules don’t (oil, grease)
    • Solvation:
      • The process that occurs when a solute dissolves.
like dissolves like
“Like Dissolves Like”

+ =

- =

H2O =

This occurs until all of the solid chunk have been “carried off”,

dissolved, by the dissolving medium.

slide10
Electrolytes:
    • Compounds that conduct an electric current in aqueous solutions or the molten state.
      • Ex: ionic compounds
    • Strong Electrolytes:
      • Almost all of the solute exists as separate ions and conducts a strong current.
    • Weak Electrolytes:
      • Only a fraction of the solute exists as ions and conducts a weak current.
  • Nonelectrolytes:
    • Compounds that do not conduct an electric current in either the aqueous or molten states.
      • Ex: molecular compounds
electrolyte vs nonelectrolyte
Electrolyte vs. Nonelectrolyte

Electrolytes conduct electricity

Non-electrolytes do not

slide12
Water of hydration:
    • The water in a crystal or water of crystallization.
      • A compound containing water of hydration is a hydrate.
        • Ex: copper (II) sulfate pentahydrate
          • CuSO4ּ5H2O
  • Effloresce:
    • If a hydrate has a vapor pressure higher than that of the water vapor in the air, the hydrate will effloresce by loosing the water of hydration.
      • For simplicity sakes: if its humid, water stays with the compound and if it’s dry, the water evaporates.
    • Example: CuSO4ּ5H2O has a vapor pressure of 1.0 kPa. The water vapor at room temperature is ~1.3 kPa. Will the compound effloresce?
      • No, the vapor pressure is too high.
drying agents
Drying Agents
  • Some compounds have a low vapor pressure and remove water from the air.
  • Hygroscopic:
    • Salts and other compounds that remove moisture from the air.
      • CaCl2
    • Hygroscopic substances are used as drying agents or desiccants.
      • Ex.: silica gel
    • Deliquescent compounds:
      • Remove sufficient water from the air to dissolve completely and form solutions.
        • Ex: NaOH and Damp Rid
section 17 4
Section 17.4
  • Suspensions:
    • Mixtures from which particles settle out upon standing.
      • Ex.: sandy water
    • The particles are bigger than in solutions (100 x bigger)
    • Exhibits Tyndall Effect:
      • Scattering of light in all directions
    • Heterogeneous mixtures – 2 substances clearly identified.
colloids
Colloids
  • Colloids:
    • Heterogeneous mixtures containing particles that are intermediate in size. They are between those of suspensions and true solutions. (between 1nm and 100nm)
    • Particles are the dispersed phase.
    • Spread throughout the dispersion medium. (they can be solids, liquids, or gases)
      • Ex.: glue, jello, paint, aerosol sprays, smoke, fog
colloid cont
Colloid cont.
  • Cloudy or milky appearance when concentrated, but when dilute appear clear.
  • Exhibit the Tyndall Effect
  • Particles exhibit Brownian Motion:
    • Chaotic movement of colloidal particles
      • These movements are the result of bumping into water molecules and keeps them from settling out.
      • Ex: bumper cars
emulsions
Emulsions
  • Emulsions:
    • Colloidal dispersions of a liquid in a liquid.
      • i.e. particles group together because of the same charge.
      • Ex.: oil in water
        • The soap and detergents are emulsifying agents (they cause an emulsion to form)

O

Polar end

Attracted to the water

Non polar end

Attracted to

grease

OH

Palmitic acid – used in making soaps

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