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WATER’S LIFE-SUPPORTING PROPERTIES
Water’s polarity leads to hydrogen bonding and other unusual properties
• The charged regions on water molecules are attracted to the oppositely charged regions on nearby molecules– This attraction forms
weak bonds called hydrogen bonds
Like no other common substance, water exists in nature in all three physical states:
– as a solid– as a liquid– as a gas
2.11 Hydrogen bonds make liquid water cohesive
• Due to hydrogen bonding, water molecules can move from a plant’s roots to its leaves
• Insects can walk on water due to surface tension created by cohesive water molecules
2.12 Water’s hydrogen bonds moderate temperature
• It takes a lot of energy to disrupt hydrogen bonds– Therefore water is able to absorb a great deal of
heat energy without a large increase in temperature
– As water cools, a slight drop in temperature releases a large amount of heat
Water moderates temperatures
• A water molecule takes a large amount of energy with it when it evaporates– This leads to evaporative cooling
2.13 Ice is less dense than liquid water
• Molecules in ice are farther apart than those in liquid water – Ice is therefore less dense than liquid water, which causes
it to float
2.14 Water is a versatile solvent
• Solutes whose charges or polarity allow them to stick to water molecules dissolve in water
• They form aqueous solutions
Mixtures and Solutions• A mixture is a combination of substances in which the
individual components retain their own properties• Neither component of the mixture changes.
• A solution is a mixture in which one or more substances (solutes) are distributed evenly in another substance (solvent).
2.15 The chemistry of life is sensitive to acidic and basic conditions
• A few water molecules can break apart into ions• this is called Dissociation of Water
– Some are hydrogen ions (H+)– Some are hydroxide ions (OH–)
– Both are extremely reactive– A balance between the two is critical for chemical
processes to occur in a living organism
Copyright © 2009 Pearson Education, Inc.
Dissociation of Water
• At equilibrium the concentration of water molecules greatly exceeds that of H+ and OH-.
• Adding certain solutes, called acids and bases, disrupts the equilibrium and modifies the concentrations of hydrogen and hydroxide ions
2.15 The chemistry of life is sensitive to acidic and basic conditions
• Chemicals other than water can contribute H+ to a solution– They are called acids– An example is hydrochloric acid (HCl)
– This is the acid in your stomach that aids in digestion
• An acidic solution has a higher concentration of H+ than OH–
Copyright © 2009 Pearson Education, Inc.
2.15 The chemistry of life is sensitive to acidic and basic conditions
• Some chemicals accept hydrogen ions and remove them from solution– These chemicals are called bases– For example, sodium hydroxide (NaOH) provides OH–
that combines with H+ to produce H2O (water)– This reduces the H+ concentration
Copyright © 2009 Pearson Education, Inc.
2.15 The chemistry of life is sensitive to acidic and basic conditions
• A pH scale (pH = potential of hydrogen) is used to describe whether a solution is acidic or basic– pH ranges from 0
(most acidic) to 14 (most basic)
– A solution that is neither acidic or basic is neutral (pH = 7)
pH scale• A compound that releases
H+ ions in solution is an acid, and one that accepts H+ ions in solution is a base thus increasing OH- ions
• The pH scale is used to describe how acidic or basic a solution is..– 0-7 is acidic – 8-14 is basic – Pure water and solutions that
are neither basic nor acidic are neutral, with a pH of 7
Neutralization
• Neutralization occurs when an acid is mixed with a base (correct amount) producing a neutral solution– H+ + OH- - H2OAnother product ,a salt is also formed
HCl + NaOH -> H2O + NaCl
Buffers
• pH can affect chemical reactions in an organism
• Cells are kept close to pH 7 by buffers• Buffers are substances that resist pH change
– They accept H+ ions when they are in excess and donate H+ ions when they are depleted
– Buffers are not foolproof
2.16 CONNECTION: Acid precipitation and ocean acidification threaten the environment
When we burn fossil fuels (gasoline and heating oil), air-polluting compounds and CO2 are released into the atmosphere
– Sulfur and nitrous oxides react with water in the air to form acids
– These fall to Earth as acid precipitation, which is rain, snow, or fog with a pH lower than 5.6
– Additional CO2 in the atmosphere contributes to the “greenhouse” effect and alters ocean chemistry
Copyright © 2009 Pearson Education, Inc.
2.17 EVOLUTION CONNECTION: The search for extraterrestrial life centers on the search for water
An important question is, has life evolved elsewhere?
– Water is necessary for life as we know it
The National Aeronautics and Space Administration (NASA) has evidence that water was once abundant on Mars
– Scientists have proposed that reservoirs of water beneath the surface of Mars could harbor microbial life
Copyright © 2009 Pearson Education, Inc.
2.18 Chemical reactions make and break bonds, changing the composition of matter
You learned that the structure of atoms and molecules determines the way they behave
– Remember that atoms combine to form molecules
– Hydrogen and oxygen can react to form water
2H2 + O2 2H2O
Copyright © 2009 Pearson Education, Inc.
2.18 Chemical reactions make and break bonds, changing the composition of matter
The formation of water from hydrogen and oxygen is an example of a chemical reaction
The reactants (H2 and O2) are converted to H2O, the product
– Organisms do not make water, but they do carry out a large number of chemical reactions that rearrange matter
– Photosynthesis is an example where plants drive a sequence of chemical reactions that produce glucose
Copyright © 2009 Pearson Education, Inc.