chapter 12 water. water molecules l water’s bent shape and ability to hydrogen bond gives water...
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Chapter 12
Water
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Water Molecules Water’s bent shape and ability to
hydrogen bond gives water many special properties.
Water molecules are attracted to one another.
This gives water high surface tension, low vapor pressure, high specific heat, high heat of vaporization, and high boiling point
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Surface Tension liquid water acts like it has a skin. Water forms round drops. All because water hydrogen bonds.
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Surface Tension One water
molecule H bonds to another.
Can H bond to molecules all around.
H HO
+
+
-
H HO
+
-
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Surface Tension A water molecule in
the middle of solution is pulled in all directions.
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Surface Tension Not true at the
surface. Only pulled down and
to each side. Holds the molecules
together. Causes surface
tension.
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Surface Tension Water drops are
round because all the molecules on the edge are pulled to the middle.
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Surface Tension Glass has polar
molecules. Glass can
hydrogen bond. Attracts the water
molecules. Some of the pull
is up.
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Meniscus Water curves up
along the side. This makes the
meniscus.
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MeniscusIn Glass
In Plastic
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Heat Capacity Water has a high heat capacity (same
as specific heat). It absorbs 1 cal/gºC while iron absorbs
only 0.107 cal/gºC. Remember SH = heat
Mass x T Calculate the heat need to raise the
temperature of both iron and water by 75ºC.
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Heat of vaporization Because of the strong hydrogen bonds
it takes a large amount of energy to change water from a liquid to a solid.
540 cal/g is the heat of vaporization. It takes this much energy to boil water. You get this much energy back when it
condenses. Steam burns, but heats things well.
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Ice Most liquids contract (get smaller) as
they are cooled. They get more dense. When they change to solid they are
more dense than the liquid. Solid metals sink in liquid metal. Ice floats in water. Why?
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Ice Water becomes more dense as it cools
until it reaches 4ºC. Then it becomes less dense. As the molecules slow down they
arrange themselves into honeycomb shaped crystals.
These are held together by H-bonds.
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HHO
H
HO
H
H
O
HH
O
H
H O HH
O
H
HO
HHOH
H
O
H
HO H H
O
HH
O
Liquid
Solid
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Ice 10% less dense than water. Water freezes from the top down. It takes a great deal of energy to turn
solid water to liquid water. Heat of fusion is 80 cal/g.
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Aqueous Solutions Solution - a homogenous mixture mixed
molecule by molecule. Solvent - the stuff that does the
dissolving. Solute -the stuff that is dissolved. Aqueous solution- a solution with water
as the solvent.
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Aqueous Solutions Water dissolves ionic solids and polar
covalent solids best. The rule is “like dissolves like” Polar dissolves polar. Nonpolar dissolves nonpolar. Oil is non polar. Oil and water don’t mix. Salt is ionic- salt water.
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How Ionic solids dissolve Called solvation. Water breaks the + and - charged
pieces apart and surround them.
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How Ionic solids dissolve
H HOH
H OH
HO
H HO
HHO
HH
O
HH
OH
H
O
HH
O
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Solids will dissolve if the attractive force of the water molecules is stronger than the attractive force of the crystal.
If not the solids are insoluble. Water doesn’t dissolve nonpolar
molecules because it the water molecules can’t hold onto them.
The water molecule hold onto each other and separate from the polar molecules.
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Hydration Water molecules chemically bonded to
solid salt molecules. These compounds have fixed amounts
of water. The water can be driven off by heating.
CuSO4.5H2O CuSO4 + 5H2O
Called copper(II)sulfate pentahydrate.- heat
+ heat
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Electrolytes Substances that conduct electricity
when dissolved in water. Must have charged particles that can
move. Ionic compounds break into charged
ions. NaCl Na+ + Cl- These ions can conduct electricity.
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Non electrolytes do not conduct electricity when dissolved in water.
Polar covalent molecules such as methanol CH3OH don’t fall apart into
ions when they dissolve. Weak electrolytes don’t fall completely
apart into ions. Strong electrolytes do ionizes
completely.
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Mixtures that are NOT Solutions
Suspensions are mixtures that slowly settle upon standing.
Particles of a suspension are more than 100 times bigger.
Can be separated by filtering. Colloids particles are between the size of
a solid and that of a liquid. Emulsions are colloids of liquids in liquids.