Water
The Universal Solvent
What is Water?
• Water – H2O – 2 hydrogens, 1
oxygen
• Each hydrogen is covalently
bonded to oxygen
• Pure covalent or polar covalent?
• Polar!
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
The Water Cycle
Hydrogen Bonding
•
•
•
•
Leads to a higher boiling point
Higher melting point
High specific heat capacity
Surface Tension
Surface Tension
• liquid water acts like it has a skin.
• Water forms round drops.
• All because water hydrogen bonds.
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
Three different drops of
liquid
Water + dish Soap
– Hydrogen bonds
broken, less surface
tension, flat drops
Pure water – drops are
spherical, surface tension
means water wants minimum
surface area
Water + Green Dye – No real
effect on surface tension,
drops still spherical
Surface Tension
• A water molecule in
the middle of
solution is pulled in
all directions.
Surface Tension
• Not true at the
surface.
• Only pulled down and
to each side.
• Holds the molecules
together.
• Causes surface
tension.
Surface Tension
• Water drops are
round because all
the molecules on
the edge are
pulled to the
middle.
Surface Tension
• Glass has polar
molecules.
• Glass can
hydrogen bond.
• Attracts the water
molecules.
• Some of the pull
is up.
Meniscus
• Water curves up
along the side.
• This makes the
meniscus.
In Glass
Meniscus
In Plastic
Heat Capacity
• Water has a high heat capacity (same
as specific heat).
• It absorbs 4.18 J/g ºC while iron
absorbs only 0.447 J/g ºC.
• Remember SH =
heat
Mass x T
• Calculate the heat need to raise
the temperature of 1g of both iron
and water by 75ºC.
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.
• 226 J/g is the heat of vaporization.
• It takes this much energy to boil water.
• You get this much energy back when it
condenses.
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.
• Heat of fusion is 334 J/g.