Lesson 2: Water Flashcards
Structure
Water is a polar molecule consisting of
two hydrogen atoms covalently
bonded to one oxygen atom.
The high electronegativity of oxygen
pulls the shared electrons towards its
nucleus which leaves the oxygen with
a slight negative charge and the
hydrogens with a slight positive
charge.
Hydrogen Bonding
Due to the polarity of the water
molecule, hydrogen bonds form
between molecules which gives water
many of its characteristic properties.
Properties - Thermal
High specific heat due to the presence of hydrogen bonds.
This allows water to absorb or give off a large amount of heat without
changing very much
This enables living things to maintain a stable temperature which is
important as the range of temperatures over which biological reactions
can occur is very narrow.
Water can remain liquid at most temperatures found on Earth and has a
high boiling point.
Thermal Properties and Life
Water is a very, thermally stable medium
Water helps living organisms resist changes in their environment
To make water change from a liquid to a vapour requires a lot of energy
Evaporation of water on a the surface of a body cools it down significantly
Properties - Solvent
Known as the universal solvent
Its polarity allows it to dissolve other polar compounds.
Inorganic ions (sodium, potassium, chloride) and polar organic
molecules (amino acids and sugars) are soluble in water. These are
known as hydrophilic substances.
Large nonpolar proteins, fats and oils are not soluble in water. These
substances are known as hydrophobic.
Solvent Properties and Life
Water is a very important transport medium for living organisms
Water is an important medium for biochemical reactions
Properties - Adhesion
Adhesive Forces are the forces between liquid molecules and their
container
These forces are strong if the liquid is polar and the surface is polar (or
if the liquid is non-polar and the surface is non-polar)
A liquid will WET a surface if there are strong adhesive forces
CAPILLARY ACTION - the spontaneous rising of a liquid up a tube - is
explained by strong adhesive forces between the liquid and the tube’s
surface, as well as the strong cohesive forces between liquid molecules
Properties - Cohesion
Cohesion gives water many of its biologically important properties
Cohesive Forces are the forces between molecules within the liquid
Strong cohesive forces are the result of strong intermolecular forces
They result in high SURFACE TENSION - the resistance of a liquid to an
increase in its surface area
Properties - Surface
Tension
Water molecules hold together forming a
skin at the surface
This is strong enough for some organisms
to be supported
Properties - Density
Water is densest at 4°C while it is still a
liquid
So ice floats on the surface of water
Organisms which live in water do not
risk freezing solid so easily
Freezing is usually fatal
Water forms a good habitat for living
organisms
Properties -
Transparency
Water is a transparent liquid, light
passes though it
Blue light, with the most energy,
penetrates furthest, red light is the
weakest and penetrates least
Plants can photosynthesize under water
Animals can use their visual systems
