I.
Water’s Life-Supporting Properties
Module 2.11 Hydrogen bonds make liquid water cohesive.
A. Cohesion is the tendency of water molecules to stick together.
1. In a baby born premature, a lipid compound in the alveolar sacs
has yet to be formed.
2. As a result, the water in the sacs forms droplets and the sacs
collapse upon birth.
B. Cohesion between water molecules allows them to stick to other things(ADHESION) and
be transported through the xylem tissue of a tree against gravity.
C. Surface tension results from the cohesion of water molecules to each other so that a small
aquatic insect such as a water strider can walk across the top of a pond without sinking.
Preview: Transpiration (Module 32.3) is an example of how living systems take advantage
of these characteristics of water.
Module 2.12
Water’s hydrogen bonds moderate temperature.
A. Breaking hydrogen bonds requires a large amount of energy.
1. Therefore, as water is heated, it takes a large amount of energy to observe an increase
in the
temperature of the water.
2. The temperature of water rises more slowly when heated than does the temperature
of nonpolar liquids because water has so many hydrogen bonds.
B. The opposite is true of water as it cools.
1. Formation of hydrogen bonds causes the temperature of water to lower more slowly
when cooled.
2. This is because heat energy is released as the hydrogen bonds are formed.
a. This accounts for warmer temperatures in coastal regions.
b. Florida orange growers place tubs of water under orange trees to ward off a late
frost.
Module 2.13 Ice is less dense than liquid water (Figure 2.13).
A. Water is unique in that it exists on this planet as a solid, liquid and gas.
B. Hydrogen bonds in ice result in an extremely stable, three-dimensional structure.
1. As water freezes, each molecule forms stable hydrogen bonds with four other
water molecules.
2. These molecules are held at ”arm’s length” creating a 3D crystal.
C. A given volume of ice has fewer water molecules than an equal volume of liquid water
and is therefore less dense.
1. If ice were more dense, then eventually all ponds, lakes, and oceans would freeze over.
2. This is extremely important for aquatic organisms as the floating ice insulates
the water below allowing life to persist under the frozen surface.
Module 2.14 Water is the solvent of life.
A. A solution is a homogeneous mixture of a liquid solvent and one
or more solutes (solid or liquid compounds that dissolve in the solvent).
1. The solvent is the dissolving agent.
2. The solute is the substance which is dissolved.
B. Because water is a polar molecule,
1. it readily forms solutions with a wide variety of other polar compounds (for
example, sugar)
2. and with the charged ionic compounds such as sodium chloride.
Module 2.15 The chemistry of life is sensitive to acidic and basic conditions (Figure 2.15).
A. Another property of aqueous solutions important to living things is pH.
1. pH is the potential hydrogen of the solution.
2. pH is a measure of acidity or basicity (alkalinity).
B. pH expresses the tendency of water to ionize.
1. In aqueous form, water dissociates into ions.
2. In pure water, there are equal amounts of OH- (hydroxide ions) and H+ (hydrogen
ions- actually, hydronium H3O+ ions).
C. Biological pH ranges from 1 (stomach acid, high concentration of H _) to 9 (seawater, low
concentration of H _).
D. The pH scale ranges from 0-14.
1. Each pH unit represents a tenfold change in the concentration of H+.
2. 7.0 represents a neutral solution where the OH- equals the H+ ions.
3. A pH below 7.0 is an acid; above 7.0, a base.
4. The further from 7.0 the stronger the acid or base.
a. For example, a pH of 3.0 is a stronger acid than one with a pH of 5.0, and has
100X more H+.
b. A base with a pH of 10.0 has 100 times fewer H+ than a base at 8.0.
E. The slightest change in pH can be harmful to organisms.
1. Biological fluids contain buffers, substances that resist changes in pH by reacting
with, and neutralizing, H_ or OH_ ions.
2. Human blood has a pH of around 7.4 which is maintained by buffers.
Module 2.16 Connection: Acid precipitation threatens the environment.
A. Compounds of sulfur and nitrogen are part of air pollutants released from the burning of
fossil fuels.
1. These compounds react with atmospheric water to form acidic compounds (sulfuric
and nitric acid).
2. Rain with a pH between 2-3 has been recorded in the eastern US while acid fog
with a pH of 1.7, approaching that of stomach juices, has been recorded in Los Angeles.
B. Low pH associated with acid rain can be harmful to organisms adapted to neutral pH.
1. In the spring, as snow melts, the surface melts first, drains down, and sends much of the
acid into lakes and streams all at once.
2. Early meltwater often has a pH as low as 3 which hits fish and other organisms at a
particularly vulnerable time—mating season; eggs and young are more vulnerable to
changes in pH.
C. Acid rains definitely harm aquatic environments and likely cause various imbalances in
terrestrial environments such as forests.
Preview: Module 36.18 discusses the impact of acid precipitation on a deciduous forest
ecosystem (the Hubbard Brook studies).
D. The capacity to withstand changes in pH ( buffering ) is naturally a characteristic
of some areas (for example, limestone buffers acid rain).
E. In recent decades, in the United States, Canada, and Europe, levels of acid precipitation
have declined.
III.Chemical Reactions
Module 2.17 Chemical reactions change the composition of matter.
A. Behavior of atoms and molecules is determined by the structure of the subatomic particles
, particularly the electron configuration.
B. Chemical terminology:
1. A chemical element is represented by a symbol ; ex: H
for hydrogen and He for helium.
2. A chemical compound is represented by a formula; ex:
__________H2O_____________________________.
a. A covalent compound is composed of units called molecules .
b. An ionic compound forms salts or crystals.
3. A number before a symbol or formula is a coefficient and represents
the number of molecules of that substance.
4. A number after a symbol is a subscript and indicates the number of atoms
of that element in that compound.
B. The general form of a chemical reaction is:
_____is a chemical equation_________________.
1. The reactants are the starting material;
2. The products are the result.
3. For example, in the chemical reaction:
2H2 + O2 à 2H2O
a. _2H2 + O2 _are the reactants and
b. ____2H2O ___is the product.
c. Notice that there are the same numbers of each type of atom on both sides.
C. Chemical reactions can be either:
1. Catabolic reactions: reactions that break things up______or
2. Anabolic reactions____reactions that put things together______.
3. Endothermic________reactions that need an input of energy_______________or
4. Exothermic_______reactions that release energy____________.
The author of the text assumed you knew the following.
The definition of mixture is a substance produced when two substances are combined with no chemical
reaction or bonding occurring__________
There are three types of mixtures: solutions, suspensions, and colloids. These are distinguished by the size
and homogeneity of their particles.
Suspensions:_____particles are too large to stay mixed, settle into layers_____________
Colloids:_______particles are large but will stay suspended homogeneously_______________