1. Water and the Fitness of the Environment
Chapter 3
Water and the Fitness of the Environment

2. Concept 3.2: Four emergent properties of water contribute to Earth’s fitness for life
Four of water’s properties that facilitate an environment for life are:
Cohesive behavior
Ability to moderate temperature
Expansion upon freezing
Versatility as a solvent
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3. Concept 3.1: The polarity of water molecules results in hydrogen bonding
The water molecule is a polar molecule: The opposite ends have opposite charges
Polarity allows water molecules to form hydrogen bonds with each other
Water is polar because the oxygen atom has a stronger electronegative pull on shared electrons in the molecule than do the hydrogen atoms
Figure 3.2 Hydrogen bonds between water molecules
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4. Cohesion & Adhesion
Collectively, hydrogen bonds hold water molecules together, a phenomenon called cohesion
the attraction of water molecules to other water molecules as a result of hydrogen bonding
Cohesion due to hydrogen bonding contributes to the transport of water and dissolved nutrients against gravity in plants
Adhesion is the clinging of one substance to another
Adhesion of water to cell walls by hydrogen bonds helps to counter the downward pull of gravity on the liquids passing through plants
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5. Fig. 3-3
Adhesion
Water-conducting
cells
Direction
of water
movement
Cohesion
Figure 3.3 Water transport in plants - transport of water against gravity in plants
150 µm
Cohesion and adhesion work together to give capillarity – the ability of water to spread through fine pores or to move upward through narrow tubes against the force of gravity.

6. Surface Tension
The high surface tension of water, resulting from the collective strength of its hydrogen bonds, allows the water strider to walk on the surface of the pond.
Surface tension is directly related to the cohesive property of water – it is a measurement of how difficult it is to stretch or break the surface of a liquid.
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7. Moderation of Temperature
Water moderates air temperature by absorbing heat from air that is warmer and releasing the stored heat to air that is cooler
Water can absorb or release a large amount of heat with only a slight change in its own temperature
The ability of water to stabilize temperature stems from its relatively high specific heat
This is the amount of heat that must be absorbed or lost for 1g of a substance to change its temperature by 1°C
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8. Water’s High Specific Heat
Water’s high specific heat can be traced to hydrogen bonding
Heat is absorbed when hydrogen bonds break
Heat is released when hydrogen bonds form
High specific heat of water is due to hydrogen bonding – H-bonds tend to restrict molecular movement, so when we add heat energy to water, it must break bonds first rather than increase molecular motion.
A greater input of energy is required to raise the temperature of water than the temperature of air!
Minimizes temperature fluctuations to within limits that permit life
A large body of water can absorb and store a huge amount of heat from the sun in the daytime and during summer while warming up only a few degrees.
And at night and during winter, gradually cooling water can warm the air.
THE HIGH SPECIFIC HEAT OF WATER TENDS TO STABILIZE OCEAN TEMPERATURES – CREATING A FAVORABLE ENVIRONMENT FOR LIFE.
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9. Water’s High Specific Heat
The world is covered mostly by water.
Large bodies of water can absorb and store a huge amount of heat from the sun in the daytime and during summer while warming up only a few degrees.
Gradually cooling water warms air at night.
Organisms are made primarily of water, so these properties help to maintain body temperatures.
This is vital because many biologically important chemical reactions take place in a very narrow temperature range.
A large body of water can absorb and store a huge amount of heat from the sun in the daytime and during summer while warming up only a few degrees.
And at night and during winter, gradually cooling water can warm the air.
THE HIGH SPECIFIC HEAT OF WATER TENDS TO STABILIZE OCEAN TEMPERATURES – CREATING A FAVORABLE ENVIRONMENT FOR LIFE.
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10. Evaporation is transformation of a substance from liquid to gas
Evaporative Cooling
Evaporation is transformation of a substance from liquid to gas
Heat of vaporization is the heat a liquid must absorb for 1 g to be converted to gas
As a liquid evaporates, its remaining surface cools, a process called evaporative cooling
The high amount of energy required to vaporize water has a wide range of effects:
Helps stabilize temperatures in organisms and bodies of water
Evaporation of sweat from human skin dissipates body heat and helps prevent overheating on a hot day or when excess heat is generated by strenuous activity.
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11. Insulation of Bodies of Water by Floating Ice
Ice floats in liquid water because hydrogen bonds in ice are more “ordered,” making ice less dense
Water reaches its greatest density at 4°C
If ice sank, all bodies of water would eventually freeze solid, making life impossible on Earth
Due to geometry of water molecule, they must move slightly apart to maintain the max number of H bonds in a stable structure.
So at Zero degrees Celsius, an open latticework is formed, allowing air in – thus ice becomes less dense than liquid water floats on top of the water.
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12. Hydrogen bonds are stable Hydrogen bonds break and re-form
Fig. 3-6
Hydrogen
bond
Ice
Hydrogen bonds are stable
Figure 3.6 Ice: crystalline structure and floating barrier
Liquid water
Hydrogen bonds break and re-form

13. A solution is a liquid that is a homogeneous mixture of substances
The Solvent of Life
A solution is a liquid that is a homogeneous mixture of substances
Solvent (dissolving agent)
Solute (substance that is dissolved)
An aqueous solution is one in which water is the solvent
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14. Hydration Shell http://www. sumanasinc
A hydration shell refers to the sphere of water molecules around each dissolved ion in an aqueous solution
Water will work inward from the surface of the solute until it dissolves all of it (provided that the solute is soluble in water)
If a spoonful of salt (or other ionic substance) is placed in water, the ions in the salt and the water molecules have a mutual affinity owing to the attraction between opposite charges.
O is negative and attracts to positive sodium. H is positive and attracts to negative chlorine.
As a result, water will surround the individual sodium and chloride ions, separating and shielding them from one another (called a hydration shell).
WATER IS THE SOLVENT OF LIFE – molecules within a living system must be broken down in order to be used by the system!
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15. Acids and Bases
An acid is any substance that increases the H+ concentration of a solution
A base is any substance that reduces the H+ concentration of a solution
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16. Figure 3.9 The pH scale and pH values of some aqueous solutions1
Battery acid
Gastric juice,
lemon juice 2 H+ H+ H+ H+
OH– 3
Vinegar, beer,
wine, cola
OH– H+ H+
Increasingly Acidic
[H+] > [OH–] H+ H+ 4
Acidic
solution
Tomato juice
Black coffee 5
Rainwater 6
Urine
OH–
Saliva
OH–
Neutral
[H+] = [OH–] H+ H+
OH– 7
Pure water
OH–
OH– H+
Human blood, tears H+ H+ 8
Seawater
Neutral
solution 9
Figure 3.9 The pH scale and pH values of some aqueous solutions 10
Increasingly Basic
[H+] < [OH–]
Milk of magnesia
OH–
OH– 11
OH– H+
OH–
OH–
Household ammonia
OH– H+
OH– 12
Basic
solution
Household
bleach 13
Oven cleaner 14

17. For a neutral aqueous solution [H+] is 10–7 = –(–7) = 7
The pH Scale
In any aqueous solution at 25°C the product of H+ and OH– is constant and can be written as [H+][OH–] = 10–14
The pH of a solution is defined by the negative logarithm of H+ concentration, written as pH = –log [H+]
For a neutral aqueous solution [H+] is 10–7 = –(–7) = 7
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18. CO2 + H2O <= H2CO3 => HCO3- + H+
Buffers
The internal pH of most living cells must remain close to pH 7
Buffers are substances that minimize changes in concentrations of H+ and OH– in a solution
They do so by accepting hydrogen ions from the solution when they are in excess and donating hydrogen ions when they are depleted
Most buffers consist of an acid-base pair that reversibly combines with H+
CO2 + H2O <= H2CO3 => HCO3- + H+
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19. You should now be able to:
List and explain the properties of water and how they result from polarity and hydrogen bonding.
Distinguish between the following sets of terms: hydrophobic and hydrophilic; a solute, a solvent, and a solution.
Define acid, base, and pH; describe the importance of buffers in physiological systems.
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