water’s life-supporting properties. water’s polarity leads to hydrogen bonding and other unusual...
TRANSCRIPT
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WATER’S LIFE-SUPPORTING PROPERTIES
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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
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Like no other common substance, water exists in nature in all three physical states:
– as a solid– as a liquid– as a gas
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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
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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
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Water moderates temperatures
• A water molecule takes a large amount of energy with it when it evaporates– This leads to evaporative cooling
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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
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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
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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).
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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.
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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
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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.
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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.
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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)
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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
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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
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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
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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.
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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.
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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
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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.