Why Water is Weird
Learning Target
I can relate the molecular structure of simple substances to their
properties
Water is the molecule of life
• https://www.youtube.com/watch?v=HVT3Y3_gHGg
Water is a polar molecule
• Oxygen is a very electronegative atom (you can
think of it as being a stronger electron “magnet”
than most other elements)
• Because oxygen is electronegative, it has a
partial negative charge
• Because the hydrogens are weaker electron
“magnets” than oxygen, they have a partial
positive charge
• When molecules have an unequal distribution of
electron density, we say that they are polar
Water has hydrogen bonds
• Because they are polar, water molecules
attract each other
• A positive hydrogen from one water
molecule will attract the negative oxygen
from another water molecule
• This hydrogen to oxygen attraction is
called a hydrogen bond
• Hydrogen bonds are one type of
intermolecular force (acts between
molecules)
• On the other hand, covalent bonds are a
type of intramolecular force (acts within
molecules).
Cohesion
• The hydrogen bonding between water
molecules causes water molecules to stick
together. This sticking together between water
molecules is called cohesion
• The cohesion among water molecules causes
water to have a high surface tension. In other
words, it is elastic and sticky, and tends to clump
together in drops rather than spread out in a
thin film
• High surface tension allows insects (and one
lizard) to walk along the surface of the water.
https://www.youtube.com/watch?v=45yabrnryXk
start at :50
Adhesion & capillary action
• Adhesion is the attraction of
water molecules to other
surfaces
• Adhesion can be stronger than
cohesion, resulting in capillary
action
Water is an excellent solvent
• Many molecules are polar because the
atoms that make it up have different
electronegativities
• Water can dissolve other polar
substances but cannot dissolve nonpolar substances.
• Fats/oils are examples of non-polar
substances
Ice floats in liquid water
• Water expands as it freezes
because the polar molecules
arrange themselves in a crystal
structure held together by
hydrogen bonds
• Water molecules are less dense
in solid form than in liquid
which is why the solid form of
water (ice) floats
• Molecules of substances
without hydrogen bonding pack
more tightly as they freeze,
causing the frozen form to sink.
Water has a high heat capacity
• It requires a great deal of energy to
increase the temperature of water.
• When the temperatures of a substance
increases, it means that the average
speed of the particles in the substance
increases
• To increase the speed of the particles,
the attractive forces between the water
molecules must be overcome
• Breaking the hydrogen bonds between
water molecules requires a lot of energy
(heat)
• On the other hand, when water cools, a
great deal of energy is released.
• The cooling properties of water are
apparent during perspiration (sweating)