Water and aqueous systems

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Transcript Water and aqueous systems

Water and aqueous
systems
Water is unique
 High
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polarity
Forms extensive hydrogen bonds with other
molecules
High surface tension
Low vapor pressure
High specific heat
High heat of vaporization
High melting point
 All
because of hydrogen bonds!
Properties of water
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Specific heat of water:
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Evaporization and condensation:
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4.184 J/g°C
∆Hevp = 2.26 kJ/g
∆Hcond = -2.26 kJ/g
High boiling point: 100°C
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Most molecules with a low molecular mass have low
boiling points.
Ex: CH4 boils at -164°C, and ammonia at -33.3°C
Why does ice float?
 When
most liquids cool they contract and
become more dense.
 Water does this until about 4°C, then
arranges itself like a honey comb.
 Ice is less dense than water so it floats.
Aqueous solutions
 Anything
dissolved in water.
 Solvent: what does the dissolving
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Ex: water, paint thinners, mineral spirits.
 Solute:
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what gets dissolved
Ex: salt, the paint, etc.
 Solvation:
the process that occurs when a
solute dissolves in a solution.
How does this work?
 Water
is polar (positive on one end and
negative on the other)
 The positive part of the water is attracted
to the negative part of the solute and vise
versa.
 It surrounds the solute and breaks it apart.
Why can’t I dissolve oil in water?
I
can dissolve it in gasoline.
 “like dissolves like”
 Water is polar, it will only dissolve polar
molecules and ionic compounds (because
of the difference of charges.
 Gasoline is non-polar, and so is oil.
Electrolytes
 Electrolytes
conduct electricity when
dissolved in water. All ionic compounds
are electrolytes.
 Nonelectrolytes do not.
Water of hydration
 Water
of hydration: Water is an important
part of many different crystals. This is the
water in the crystal.
 Hydrate: any compound that contains
water.
 Effloresce: when a hydrate has a higher
vapor pressure, and loses it’s water
Water of hydration (cont)
 Hydroscopic:
remove moisture from the
air.
 Desiccants: hydroscopic substances used
to keep things dry. (the little packet that
says “do not eat”)
 Deliquescent: remove enough water from
the air to dissolve completely
Heterogenous systems
 Suspensions:
mixtures from which particle
settle out upon standing
 Colloids: Particles are smaller than a
suspension, but larger than a solution.
These cannot be filtered out and will not
settle out.
 Tyndall effect: Scattering of visible light in
all directions
More!
 Brownian
Motion: Chaotic movement of
colloidal particles, it keeps them from
settling.
 Emulsions: colloidal dispersions of liquids
on liquids. There must be an emulsifying
agent to form the mixture and keep it
stable.
Solubility
 Saturated
solution: contains the maximum
amount of solute for a given amount of
solvent at a constant temperature.
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You can’t add any more sugar to your koolaid, because it won’t dissolve.
 Unsaturated
solution: Contains less solute
than a saturated solution.
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You could still add more sugar to your koolaid if you wanted to.
 Solubility:
the amount of substance that
will dissolve in a given quantity of solvent
at a given temperature to produce a
saturated solution.
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How much sugar can you possibly put in your
kool-aid?
 Miscible:
two liquids that can dissolve in
each other.
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Ex: oil & gas
 Immiscible:
two liquids that are insoluble
with each other (won’t dissolve)
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Ex: oil & water