AP Biology
Unit 1 Practice Essay – Scoring Guide
Question:
1. The unique properties of water make life possible on Earth. Select three properties of water and:
a) for each property, identify and define the property and explain it in terms of the physical/chemical nature of water.
b) for each property, describe one example of how the property affects the functioning of living organisms.
This question was composed of two parts. Part A asked the student to identify, define, and explain (in terms of the physical/chemical nature of water) three
properties of water. Part B asked the student to describe one example of how the selected property affects the functioning of living organisms. For
convenience of grading, the scoring guide was divided into four parts: identification, definition, elaboration, and example. Each part was awarded 1 point. The
essay was worth 10 points, but notice there were 12 possible points, or 12 possible "ways" to get those 10 points.
Identification (1 pt)
polarity
Definition (1 pt)
unequal charge distribution
Elaboration (1 pt)
electronegativity, tetrahedron, Hd+,
Od-, unequal sharing of electrons
Example (1 pt)
water readily dissolves biologically
important ions
hydrogen bonding
weak, intermolecular, transient,
electrostatic attraction
attraction between Hδ+ and Oδ-
various (hydrogen bonding is the basis
for most of the other unique properties)
cohesion
attraction of water molecules to each
other
hydrogen bonding explained
transpiration, surface tension, e.g.,
water striders
high surface tension
measure of how hard it is to break the
surface of a liquid (resistance to being
stretched)
attraction of water molecules to other
molecules
hydrogen bonding explained
water walking; water held by soil
particles
hydrogen bonding explained
capillary action, transpiration
many hydrogen bonds, high boiling
point, water absorbs/releases heat
slowly, heat buffering
Breaking hydrogen bonds, good coolant,
high boiling point
moderates temperature effects on
organisms and environmental
temperature
sweating, evaporation from body
surfaces results in cooling, water cycle
and transpiration
any valid biological example involving
salts, minerals, or gases, acid rain
effects on organisms
ice insulates bodies of water, snow
effects on organisms, ocean level
fluctuates with amount of ice/snow
availability of water varies with
seasonal/climatic temperature changes
adhesion
high specific heat
heat needed for 1g to change 1ºC (1
calorie)
high heat of vaporization
energy required for liquid to become a
gas (540 cal/g/ºC)
versatile solvent
water dissolves many compounds
ice floats
solid water is less dense than liquid
water
all three phases on earth
water can exist as a solid, liquid, or
gas under normal earth temperatures
dissolves by ionization (hydration
shells), dissolves large molecules by
attraction to hydrophilic regions
maximum # of hydrogen bonds forms
crystal lattice with molecules farther
apart, water is most dense at 4ºC
phase changes involve greater energy
absorbed/released due to hydrogen
bonds, water cycle