Reece et al 9 Edition
Water and the Fitness of the Environment
Water is essential for all living things. It is one of the most abundance substances on earth.
Why is water so important for life on earth?
Water has a number of emergent properties that make it universally important to life.
Chemical structure of Water (Figure 3.2)
1* Polar covalent bonds between oxygen and hydrogen result in up to 4 hydrogen bonds
between neighboring water molecules.
2* The hydrogen bonds of water are about 0.05 times as strong as its covalent bonds
I. Water Emergent Properties that Contribute to the Habitability of Earth
1. Cohesion of water molecules
Cohesion - the binding together of like molecules
- The continuous forming and breaking of hydrogen bonds between water
molecules make water a very structured liquid despite the fact that these
bonds last only several trillionths of a second.
Surface tension - a measure of how difficult it is to stretch or break the surface of a
Liquid (e.g., water striders walking on water)
Adhesion - clinging of one substance to another
e.g., water adheres to cell walls of xylem cells in trees
Strong Chemical bonds
- Covalent: non polar (ex: dihydrogen gas, equal sharing) and polar(ex: water, unequal)
- Comparing electronegativity: F>O>N>(C=S)>(P=H)
- Ionic: ex: NaCl, chlorine takes electron from sodium, completes outer shell for both but
now a difference of charge. There relationship is due to charged interactions
- Hydrogen bond:
- Ionic aqueous solution
2. Moderation of temperature
Water absorbs heat from air that is warmer and releases it to air that is cooler. Water can
absorb or release a relatively large amount of heat with only a slight change in its temperature
Kinetic energy - energy of motion 2
Heat - measure of the total kinetic energy of a material due to molecular motion
• 1 calorie (cal) = heat units = heat energy required to raise the temperature of 1 g of water by
1▯C = 4.184 joule (j)
Temperature - measure of heat intensity that represents the average kinetic energy of the
molecules in a system
Specific heat - amount of heat that must be absorbed or lost for 1 g of the substance to change
its temperature by 1▯C
Water has a high specific heat = 1 cal/g/ ▯C
•Because of the high specific heat of water relative to other substances, water
temperature will change less when it absorbs a given amount of heat.
•Water’s high specific heat can be attributed to its hydrogen bonding capacity.
•Heat must be absorbed to break hydrogen bonds. Heat is absorbed to break the
hydrogen bonds before the water molecules can move faster
•Heat must be given off to make hydrogen bonds. When water cools slightly, many
additional hydrogen bonds form thereby releasing heat.
•This property results in large bodies of water warming up by a few degrees in the day
and absorbing and storing large amounts of heat that is released at night.
•Results in moderating coastal and ocean, and organism temperatures (Recall: most
living things are 70 to 95 % water).
Vaporization - phase change from liquid to gas.
Condensation - phase change from gas to liquid
Heat of vaporization = amount of heat required for 1 g of a substance to go from liquid state
to gaseous state.
•Water has a high heat of vaporization (at 25▯C = 580 cal/g) - because of the hydrogen
bonds that must be broken before molecules can leave the liquid state
•The most energetic molecules leave resulting in a decline in temperature (Remember:
heat is total kinetic energy; temperature is average kinetic energy of a substance)
• When the energetic molecules leave, the surface of the liquid cools down
Examples Reece et al 9 Edition
3. Ice floats
As a chemical changes from solid ▯ liquid ▯ gas the density decreases
This is not so for water.
• Water freezes at 0˚C yet it’s density is greatest at 4˚C.
•As substances cool they contract. As molecules lose energy they move closer together
(The opposite occurs as they get warmer ▯ they expand). Contraction of molecules allows
hydrogen bonds to form.
•As water freezes the molecules are not moving vigorously enough to break the hydrogen
bonds and molecules become locked in a crystal lattice with the formation of four
• This results in ice being 10% less