Water
Key Terms
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Polar
Electronegativity
Hydrogen bonds
Solvent
Specific Heat Capacity
Latent heat of vaporization
Background Information
• All elements have a property called
electronegativity
• Electronegativity is a measure of how strongly
an atom attracts electrons
• If an element has a high electronegativity then
it attracts electrons very strongly
Table of Electronegativities
Electronegativity and Polar Bonds
• As you can see from the previous page,
Oxygen has a high electronegativity (3.5) while
hydrogens is quite low (2.1)
• So when oxygen and hydrogen form a covalent
bond, the electrons are not shared equally
• Oxygen holds the electrons closer to its
nucleus which makes is slightly negative
• Hydrogen has the electrons further away from
its centre which makes it slightly positive
Polar Molecules
• The result is a molecule that has a positive
side (i.e. the hydrogens) and a negative
side ( the oxygen)
Polar Molecules
• When a molecule has a positive side and a
negative side, it is said to be “Polar”
• The positive side of a polar molecule will be
attracted to the negative side of another polar
molecule, forming what is called a hydrogen
bond
Hydrogen Bonds
• Hydrogen bonds are not as strong as covalent
bonds
• They are constantly forming as water
molecules move around
Properties of Water
• Water has several important properties that
make it such a vital part of life. They are:
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It is cohesive and adhesive
It has a very high specific heat capacity
It has a very high latent heat of vaporization
It is an excellent solvent
It has a higher density in its liquid form
Cohesion and Adhesion
• Cohesion – Water sticks together.
– Ever see a glass of water filled above the rim?
Water can do this because it is cohesive
– Why is water cohesive? HYDROGEN BONDS!!
• They cause the water molecules to stick together!
Cohesion/Adhesion
• Adhesion – Water clings to other things.
– Ever see water climb up a thin glass?
– Or climb up a thin tube?
– Again, caused by
Hydrogen bonds being attracted to the glass
This allows water to move up a tree from the roots through the vascular
tissue (tiny tubes)
Specific Heat Capacity
• Water can hold a lot of heat!
• In other words, it takes a long time and a lot
of energy to increase the temperature of
water
• That’s a good thing! Especially if you’re a fish!
You wouldn’t want the oceans to get scalding
hot every time it was hot outside
• Our lakes and oceans are what modifies the
temperature on the planet
Latent heat of Vaporization
• It takes a lot of energy to increase the
temperature of water but even more to make
it boil and turn into a gas
• That’s also a good thing if you’re a fish. It
makes the oceans less likely to vaporize
(change to a gas)
Excellent Solvent
• Water can dissolve many solutes. Specifically,
if they’re also polar. It can form hydrogen
bonds with other polar molecules and pull
them apart to help them dissolve
• However, water will not dissolve non-polar
substances
like oil
Water is More Dense in Liquid Form
• Hmmm… how can we prove that??
Liquid Density
• When water freezes, the hydrogen bonds become
more rigid and it forms a crystalline structure
• See how the molecules are closer together when
water is a liquid? That means it’s more dense.
Density
• Water is not the only substance that is denser as
a liquid but it’s pretty rare
• The vast majority are more dense in the solid
state
• Some people say that this property of water is
“proof that God exists”!!
• Why would they say that??
– Think about what the world would be like
if ice was more dense than water