AP Biology
Properties of Water- 2.5
-All organisms are made mostly of water and life in an environment dominated by water
-Most cells are about 70-95% water
-A water molecule is held in a V-shape by polar covalent bonds
-Water is polar. The oxygen region of the molecule has a partial negative charge (δ-) and each
hydrogen has a partial positive charge (δ+)
-Hydrogen bond: the slightly positive hydrogen of one molecule is attracted to the slightly negative
oxygen of a nearby molecule
-Hydrogen bonds are fragile (1/20th the strength of a covalent bond)
-At any instant, a substantial percentage of water molecules are hydrogen bonded to their
neighbors
-One water molecule can form multiple hydrogen bonds at the same time
-Hydrogen bonding gives water properties that help it make life possible on earth. Properties include:
1. Cohesion
2. Good temperature regulator
3. Loses density as it freezes
4. Good solvent
1. Cohesion: the collective hydrogen bonds that hold a substance together
-Cohesion contributes to the transport of water from roots to leaves (against gravity) in plants
-When water molecules evaporate from leaves, their hydrogen bonds tug on the
molecules farther down in the veins
-This upward pull is transmitted through the water conducting cells all the way to the
roots
-Also involved in this process is adhesion
-Adhesion: the clinging of one substance to another
-Surface tension: a measure of how difficult it is to stretch of break the surface of a liquid
2. Temperature regulation
-Water moderates air temperature by absorbing and releasing large amounts of heat with only a
slight change to its own temperature
-Kinetic energy: the energy of motion
-Thermal energy: the kinetic energy of the random movement of atoms/molecules
-Thermal energy depends on volume
-Temperature: the average kinetic energy of molecules, regardless of volume
-Heat: thermal energy in transfer from one body of matter to another. Units of
heat/energy include:
-Calorie (cal): the amount of heat it takes to raise the temperatue of 1g of water
by 1o C
-Kilocalerie (kcal): 1,000 cal
-Joule (J): .239 cal
-Water has high specific heat, meaning that it resists changing its temperature when it absorbs
or loses heat. This gives water the ability to stabilize temperature.
-Specific heat: the amount of heat that must be absorbed or lost for 1g of that
substance to change its temperature by 1o C
-Specific heat is due to H-bonding. Energy is required to break H-bonds, so, as a
body of water gains heat energy, most of it has to be used to break H-bonds first
before it can be used to increase the movement of the water molecules and
increase temperature.
-Water’s high specific heat gives it the ability to regulate temperature in an ecosystem,
stabilize ocean temperatures, and protect living cells from extreme changes in
temperature
-Evaporative cooling: as liquid evaporates, the surface of the liquid left behind is cooled
-Evaporation: when molecules move fast enough to overcome the bonds holding them
together and enter the air as a gas (liquid  gas)
-Evaporation occurs at any temperature but increases as temperature increases
-Heat of vaporization: the quantity of heat a liquid must absorb for 1g of it to be
converted from liquid to gas
-Water has a high heat of vaporization- a lot of heat must be absorbed for water
to be vaporized
-Due to the hydrogen bonds that must be broken
-Water’s high heat of vaporization makes it a good regulator of earth’s climate.
It’s also what causes steam burns to be so severe compared to that of boiling
water- steam contains so much more heat energy.
3. Water loses density as it freezes
-While other materials contract and become more dense when they solidify, water expands due
because the hydrogen bonds keep water molecules a certain distance apart in the solid state
-This causes ice to be about 10% less dense than liquid water
-This property of water protects aquatic environments from completely freezing over in the
winter, allowing life to exist below the surface ice
4. Water is a good solvent
-Solution: a liquid that is a completely homogenous mixture of two or more substances
-Solvent: the dissolving agent in a solution; whatever there’s “more of”
-Solute: the substance that is dissolved into a solution; whatever there’s “less of”
-Aqueous solution: water is the solvent
-Water is a versatile solvent because of its polar nature
-Water dissolves substances by forming hydration shells around molecules based on
their charges
-Hydration shell: the sphere of water molecules around each dissolved ion
-A substance must be hydrophilic to be water-soluble
-Hydrophilic: has an affinity for water
Ex. Ions, polar molecules, any molecule with ionic or polar regions on its surface
-Hydrophobic: substances that repel water
Ex. Nonionic, nonpolar, or otherwise cannot form H-bonds
-Calculating solute concentration in aqueous solutions
-Molecular mass: the sum of the masses of all the atoms in a molecule
-Mole (mol): the number of grams of a substance that equals its molecular weight in
daltons and contains Avogadro’s number of molecules
-A mole of one substance has exactly the same number of molecules as a mole
of any other substance
-Molarity: the number of moles of solute per liter of solution
-A 1M solution of sucrose=
molecular mass of sucrose (342 daltons)
= 342g sucrose + enough water to bring the total volume to 1L