water and pH

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H 2 O: The Molecule That Supports All of Life © 2011 Pearson Education, Inc. All living organisms require water more than any other substance Most cells are surrounded by and made of 70–95% water Water is the main reason Earth is habitable

Times New Roman:

4 properties of water contribute to Earth’s suitability for life © 2011 Pearson Education, Inc. Cohesion (and adhesion) Ability to moderate temperature Expansion upon freezing Versatility as a solvent

Wingdings:

H H H 2 O + + – O Polar molecule - the opposite ends have opposite charges

Symbol:

Hydrogen bond Polar covalent bonds    +  +      +  +   hydrogen bonds cause water molecules to “stick” together - cohesion Polarity allows water molecules to form hydrogen bonds with each other

Times:

Figure 3.3 Adhesion Cohesion Direction of water movement Adhesion – opposite charge attraction between different substances Cohesion and Adhesion help the transport of water against gravity in plants

ヒラギノ角ゴ ProN W3:

Figure 3.4 Surface tension - measure of how hard it is to break the surface of a liquid

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Moderation of Temperature by Water Water absorbs heat from warmer air Water releases stored heat to cooler air Water can absorb or release a large amount of heat with only a slight change in its own temperature © 2011 Pearson Education, Inc. Earth’s giant water supply causes temperatures to stay within limits that permit life.

H2O: The Molecule That Supports All of Life:

Heat and Temperature Kinetic energy - the energy of motion (“work”) Heat - a measure of the total amount of kinetic energy due to molecular motion Temperature - measures the intensity of heat due to the average kinetic energy of molecules © 2011 Pearson Education, Inc. specific heat of a substance = the amount of heat that must be absorbed or lost for 1 g of that substance to change its temperature by 1ºC

4 properties of water contribute to Earth’s suitability for life:

Water has High Specific Heat Water resists changing its temperature because of it’s high specific heat © 2011 Pearson Education, Inc. Water’s high specific heat is due to hydrogen bonding Heat is absorbed when hydrogen bonds break Heat is released when hydrogen bonds form specific heat of water = 1 cal/g/ºC

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© 2011 Pearson Education, Inc. Ice floats on top of liquid water because ice is less dense Water reaches its greatest density at 4°C

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Hydrogen bond Ice: Hydrogen bonds are stable Liquid water: Hydrogen bonds break and re-form When water molecules get cold enough -- they move apart….. forming ice. A chunk of ice has fewer molecules than an equal volume of liquid water. Ice floats on water because hydrogen bonds in ice are more “ordered,” making ice less dense

Figure 3.3:

© 2011 Pearson Education, Inc. Ice insulates water below – provides a “warm” environment for living organisms

Figure 3.4:

“Water is the Solvent of Life” solution - a liquid that is a homogeneous mixture of substances solvent - the dissolving agent of a solution solute - the substance that is dissolved aqueous solution - water is the solvent © 2011 Pearson Education, Inc.

Moderation of Temperature by Water:

Cl  Cl  Na                  Na     When an ionic compound is dissolved in water, each ion is surrounded by a sphere of water molecules called a hydration shell Water is a versatile solvent due to its polarity……... It forms hydrogen bonds easily

Heat and Temperature:

   +  +   Even large polar molecules (ex. Proteins) can dissolve in water if they have ionic and/or polar regions

Water has High Specific Heat:

Hydrophilic and Hydrophobic Substances hydrophilic – substance has an affinity for water (“water-loving”) hydrophobic – substance does not have an affinity for water (“water-hating”) Oil molecules are hydrophobic because they have relatively nonpolar bonds © 2011 Pearson Education, Inc.

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“H 2 O dissociates”………………… © 2011 Pearson Education, Inc. Hydronium ion (H 3 O + ) Hydroxide ion (OH  ) (H + )

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2 H 2 O Hydroxide ion (OH  ) Hydronium ion (H 3 O + ) +  Water is in a state of dynamic equilibrium - water molecules dissociate at the same rate at which they are being reformed Changes in concentrations of H + and OH – can drastically affect the chemistry of a cell

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Basic solution Neutral Acidic solution Concentrations of H + and OH – are equal in pure water ACIDS and BASES - modifies the concentrations of H + and OH – Acidic and Basic conditions significantly affect living organisms

“Water is the Solvent of Life”:

Acids and Bases Acid - any substance that increases the H + concentration of a solution…….. donates protons Base - any substance that reduces the H + concentration of a solution………. accepts protons © 2011 Pearson Education, Inc. The pH scale is used to describe whether a solution is acidic or basic

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The pH Scale In any aqueous solution at 25°C the product of H + and OH – is constant and can be written as…………….. The pH of a solution is defined by the negative logarithm of H + concentration, written as………………… For a neutral aqueous solution ………… © 2011 Pearson Education, Inc. [H + ][OH – ] = 10 –14 pH = –log [H + ] [H + ] is 10 –7 = –(–7) = 7

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Figure 3.UN05 Acidic [H + ] > [OH  ] Neutral [H + ] = [OH  ] Basic [H + ] < [OH  ] Bases donate OH  or accept H + 14 7 Acids donate H + 0 Lower H + concentration Greater H + concentration H + concentration = OH – concentration [H + ][OH  ]= 10 -14

Hydrophilic and Hydrophobic Substances:

Figure 3.10 pH Scale Battery acid Gastric juice, lemon juice Vinegar, wine, cola Beer Tomato juice Black coffee Rainwater Urine Saliva Pure water Human blood, tears Seawater Inside of small intestine Milk of magnesia Household ammonia Household bleach Oven cleaner Basic solution Neutral solution Acidic solution 0 1 2 3 4 5 6 7 8 9 10 Neutral [H + ] = [OH  ] Increasingly Basic [H + ] < [OH  ] Increasingly Acidic [H + ] > [OH  ] H + H + H + H + H + H + H + H + OH  OH  H + OH  H + OH  OH  OH  OH  H + H + H + H + OH  OH  OH  OH  OH  OH  OH  H + 11 12 13 14 Most biological fluids have pH values of 6 to 8 Acidic solutions – pH less than 7 Basic solutions – pH greater than 7

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Buffers living organisms use buffers to maintain a neutral pH 7 Buffers = substances that minimize changes in concentrations of H + and OH – in a solution……… © 2011 Pearson Education, Inc. substances that resist pH change by accepting or donating H + Accept H + ions when they are in excess Donate H + ions when they are depleted

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Figure 3.10c H 2 CO 3 HCO 3 - + H + Carbonic acid bicarbonate ion (H + donor) (H + acceptor) Acid Base Buffer system in blood CO 2 reacts with H 2 0 in blood and forms carbonic acid Response to rise in blood pH Response to drop in blood pH

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Acidification: A Threat to Water Quality CO 2 is the main product of fossil fuel combustion combustion CO 2 is absorbed by the oceans CO 2 dissolved in sea water forms carbonic acid  ocean acidification © 2011 Pearson Education, Inc.

Acids and Bases:

CO 2 CO 2 + H 2 O H 2 CO 3 H + + HCO 3  H + + CO 3 2  HCO 3  CaCO 3 CO 3 2  + Ca 2 + H 2 CO 3 H + ions combine with carbonate ions CO 3 2  to produce more bicarbonate Precious Carbonate CO 3 2  is required for calcification (production of calcium carbonate) by many marine organisms to produce shells, coral reefs and is being removed by the acidification of oceans Carbonic acid dissociates to H + and bicarbonate ions HCO 3  CO 2 and H 2 O combine to form Carbonic acid (H 2 CO 3 )

The pH Scale:

Burning fossil fuels also a major source of sulfur oxides and nitrogen oxides They react with water in the air to form strong acids that fall as rain or snow Acid precipitation - rain, fog, or snow with a pH lower than 5.2 damages life in fresh water and changes soil chemistry © 2011 Pearson Education, Inc.

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