Water
The Universal Solvent
What is Water?
• Water - compound– H2O – 2
hydrogens, 1 oxygen
 This ratio never changes no
matter what is dissolved in it or
what state water is in (liquid,
solid, or gas).
Water
• Water makes up 66% of the human body by
mass
• We lose it by breathing, perspiration and in
our waste
• If you are inactive you probably lose 25 – 100
ml/hour
• This increases to 3000 ml/hour if you are
exercising!
• Therefore, you need to replenish your water
often when exercising to prevent dehydration
Water
 Water is a molecular molecule
because there is a sharing of
electrons between the 2
hydrogen atoms and the
oxygen atom.
Water
 The hydrogen and oxygen
atoms are held together by
covalent bonds (result of the
sharing of electrons).
Water
 The water molecule has a
positive end (hydrogen) and a
negative end (oxygen). This
makes water polar.
Water
 Because of its molecular
structure, water will dissolve
most any substance. Water will
dissolve polar and ionic
solutes. Water is the
"universal solvent".
Water
 Solvent means that water does
the dissolving.
 Solutes are the substances
that you mix with the water.
Water
 In order for the water to
dissolve something, there
must be an attraction
between the opposite
charges of the water
molecule and the solute
molecule.
Water
 The molecules of the solute are
pulled away into solution and
the solute dissolves.
Water
 The positive end of the water
molecule (H+) is attracted to
the negative end of the solute
molecule. Remember:
Opposites always attract!!!
Water
 For plants and animals to live in
water, they must have oxygen.
If something happens to the
water to cause it not to dissolve
enough oxygen, plants and
animals will die.
Water
 Example: Hot water discharged
into rivers and lakes causing
less oxygen to be dissolved in
the water.
Water
 During the winter, fish
survive frozen lakes and
ponds because the ice is less
dense than the water and
floats on top of the water.
This forms an insulating
layer to protect the fish from
freezing.
Water
 If the ice were more dense and
did not float, no insulating layer
would be formed and the fish
would die.
ELECTROLYTES
 Electrolytes are compounds
that will conduct electricity
when they are dissolved in
water.
ELECTROLYTES
 Water will conduct electricity if
ions or minerals are dissolved
in it. Example: Tap water
(water from your sink or
bathtub) has minerals (ions)
dissolved in it, so it will conduct
electricity.
ELECTROLYTES
 Pure water or distilled water
has no minerals in it, so it will
not conduct electricity.
ELECTROLYTES
 Examples of electrolytes
include: minerals, salts (salt
water and seawater), and acids.
KEYS TO REMEMBER
 The one property of water that
does not change is its formula
(ratio of hydrogen to oxygen)
 Water will dissolve almost anything
because of its molecular structure
(polar) Look for the word "polar"
in the answer.
KEYS TO REMEMBER
 When a solute dissolves in
water, the negative molecules
of the solute are attracted to
the positive hydrogen end of
the water molecule;
KEYS TO REMEMBER
Cont. the positive molecules of
the solute are attracted to the
negative oxygen end of the
water molecule.
KEYS TO REMEMBER
 For questions regarding fish in
ponds during the winter, look
for "ice as an insulating
layer" in the answers.
KEYS TO REMEMBER
 Electrolytes cause water to
conduct electricity.
 Pure or distilled water will not
conduct electricity.
KEYS TO REMEMBER
 Tap water will conduct electricity
because of minerals or ions in it.
Look for "minerals or ions" in the
answer.
 Look for the word "acid" when
asked about specific electrolytes.
The Water Cycle
Surface Tension
• Liquid water acts like it has a
skin.
• Water forms round drops.
• All because water hydrogen
bonds to other water molecules.
Surface Tension
d• One water
molecule H bonds
to another.
• Can H bond to
molecules all
around.
H
d+
d+
d-
H
d+
Surface Tension
• Ever noticed that you can fill a glass so
that the liquid bulges above the rim?
• Insects can walk on water as if it has a
skin, but a needle will break through
water immediately and sink?
• These are examples of surface tension
Surface Tension
• Water drops are
round because all
the molecules on
the edge are
pulled to the
middle.
Meniscus
• Water curves up
along the side.
• This makes the
meniscus.
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?
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.
Liquid
O
Solid
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.