The Chemical Basis for Life

1
The Chemical Basis for Life

• Unique Properties of Water

2
Lesson Objectives

• Describe the distribution of Earths water and
  outline the water cycle.
• Identify waters unique properties that support
  life on Earth.
• Identify the chemical structure of water, and
  explain how it relates to waters unique
  properties.
• Define solution, and describe waters role as a
  solvent.
• State how water is used to define acids and
• bases, and identify the pH ranges of acids
  and
• bases.
• Explain why water is essential for life processes.

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Introduction

• Water is biologically important
• H2O is a simple molecule, but it has some very
  unique properties

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WATER, WATER EVERYWHERE

• Common chemical substance on Earth
• Water refers to it in a liquid state
• Also can ba a solid (ice) or gas (water vapor)

5
Where Is All the Water?

• Water covers about 75 of Earths surface
• 97 of this water is salt water
• Only 3 is freshwater
• Most of the freshwater is frozen in glaciers and
  the polar ice caps
• About 2 of water is stored in underground spaces

6
How Water Recycles

• Water is continuously recycled
• Cycle keeps repeating

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CHEMICAL STRUCTURE AND PROPERTIES OF WATER

• Water is tasteless, odorless, and transparent
• The transparency of water is important for
  organisms that live in water.
• sunlight can pass through it
• sunlight is needed by water plants and other
  water organisms for photosynthesis

8
Chemical Structure of Water

• Chemical formula H2O
• Nucleus of the oxygen atom (8 positively charged
  protons) attracts electrons much more strongly
  than do the hydrogen nuclei (1 positively charged
  proton). This results in a negative electrical
  charge near the oxygen atom (due to the pull of
  the negatively charged electrons toward the
  oxygen nucleus) and a positive electrical charge
  near the hydrogen atoms.
• A difference in electrical charge between
  different parts of a molecule is called polarity.

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SOLUTIONS

• Homogeneous mixture composed of two or more
  substances
• One substance is dissolved (solute) in another
  substance (solvent), forming a mixture that has
  the same proportion of substances throughout
• The ability of a solute to dissolve in a
  particular solvent is called solubility.
• Many chemical substances are soluble in water
  thus water is known as the universal solvent.

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Sticky, Wet Water

• Water has unusual properties due to its hydrogen
  bonds water molecules tend to stick together.
• If you drop a tiny amount of water onto a very
  smooth surface, the water molecules will stick
  together and form a droplet, rather than spread
  out over the surface this is called cohesion.
• Water also sticks to other substances because of
  its unique bonding properties this is called
  adhesion.

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Cohesion

• Cohesion is the attraction of one molecule to a
  similar molecule. Water molecules form droplets
  because of this principle.

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Adhesion

• When a molecule is attracted to a different
  molecule, it is called adhesion. Water sticks to
  other substances.

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Surface Tension

• Water's surface, molecules pulled from side to
  side and down only
• Result skin of water at the surface in which the
  molecules are held tightly together.
• Surface tension measurement of the amount of
  force required to break this skin on the surface
  of water.
• http//www.youtube.com/watch?v45yabrnryXk

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Capillarity

• Check out the demonstration of capillarity!

Use cohesion and adhesion to explain what is
happening WHY!
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Quick Review of Info

• Water cohesion, adhesion, polarity, solvency
• https//www.youtube.com/watch?vDAilC0sjvy0
• Surface Tension Capillary Action
• https//www.youtube.com/watch?vwOOY1szbcX4

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Specific Heat

• Water has a high specific heat takes a lot of
  energy to raise or lower its temperature
• Specific heat is a measure in joules of how much
  energy it takes to raise the temperature of 1
  gram of a substance by 1C.
• Every substance has its own specific heat
  capacity, the specific heat capacity of water is
  much higher than that of other common liquid
  substances.

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Density of Ice and Water

• Water in a solid state has a lower density than
  water in a liquid state water expands when it
  freezes.
• Hydrogen bonds cause water molecules to line up
  less efficiently in ice than in liquid water. As
  a result, water molecules are spaced farther
  apart in ice, giving ice a lower density than
  liquid water.
• A substance with lower density floats on a
  substance with higher density. This explains why
  ice floats on liquid water, whereas many other
  solids sink to the bottom of liquid water.
• In a large body of water, such as a lake or the
  ocean, the water with the greatest density always
  sinks to the bottom. Water is most dense at about
  4 C (39.2F).
• In climates with cold winters, this layer of 4 C
  water insulates the bottom of a lake from
  freezing temperatures.
• Lake organisms such as fish can survive the
  winter by staying in this cold, but unfrozen,
  water at the bottom of the lake.

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WATER AND LIFE

• Humans are composed of about 70 water
• Waters ability to dissolve most biologically
  significant compoundsfrom inorganic salts to
  large organic moleculesmakes it a vital solvent
  inside organisms and cells.
• As a result, virtually all life processes depend
  on water.
• Without water, life as we know it could not exist

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WATER ACTS LIKE A BUFFER

• Defined substance that helps to moderate any
  changes in pH that result from the addition of
  acids or bases.
• Importance most chemical processes that occur in
  living organisms are highly sensitive to pH, and
  drastic changes in pH can cause some serious
  trouble.
• Water, can act like a buffer if there is a sudden
  change in pH.
• HOW you ask????

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• Water disassociates into hydrogen (H)and
  hydroxide (OH-) ions constantly.
• dissociated water molecules are what give water
  its buffering ability.
• If we add an acid to solution, some of the free
  OH- ions will bind to the newly added H ions,
  which will moderate the decrease in pH.
• If we add a whole bunch of base to the solution,
  some of the added base will bind to the free H
  ions in solution, which will moderate the
  increase in pH.
• HOWEVER, WATER CANNOT BUFFER EXTREMELY DRASTIC
  CHANGES IN pH

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• Think of it as a mathematical equation with
  positive and negative integers
• For example
• -13 2 -11
• So if you take a base with a pH of 13 (filled
  with OH- ions) and increase the amount of
  positive H ions in the solution the bases pH
  will drop down to 11 making it less alkaline

22
Acids and Bases

• Pure water is the solvent in solutions called
  acids and bases.
• In pure water (such as distilled water), a tiny
  fraction of water molecules naturally breaks
  down, or dissociates, to form ions.
• An ion is an electrically charged atom or
  molecule. The dissociation of pure water into
  ions is represented by the chemical equation
• 2 H2O ? H3O OH-

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Acids, Bases and pH

• The hydronium ions in pure water are balanced by
  hydroxide ions, so pure water is neutral (neither
  an acid nor a base). This gives pure water a pH
  of 7.
• If a solution has a higher concentration of
  hydronium ions and lower pH than pure water, it
  is called an acid.
• If a solution has a lower concentration of
  hydronium ions and higher pH than pure water, it
  is called a base.

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The pH Scale

• Ranges from 0 to 14
• The pH scale is a negative logarithmic scale as
  the ion concentration increases, the pH value
  decreases. In other words, the more acidic the
  solution, the lower the pH value.
• Each one-point change in pH reflects a ten-fold
  change in the hydronium ion concentration and
  acidity.
• For example, a solution with a pH of 6 is ten
  times as acidic as pure water with a pH of 7.
• Acids have a pH less than 7, and bases have a pH
  greater than 7.

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Acids

• Defined as a hydrogen ion donor
• Acids have a sour taste and may sting or burn the
  skin.
• Testing solutions with litmus paper is an easy
  way to identify acids. Acids turn blue litmus
  paper red.

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Bases

• Defined as a hydrogen ion acceptor
• Bases have a bitter taste and feel slimy to the
  touch. They can also burn the skin.
• Bases, like acids, can be identified with litmus
  paper. Bases turn red litmus paper blue.

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Quick Review of How Acids and Bases Form

• https//www.youtube.com/watch?v6zGHCyFhZk0

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Acids and Bases in Organisms

• Enzymes do their job within a specific range of
  pH and are needed to speed up biochemical
  reactions.
• For example, the enzyme pepsin, which helps break
  down proteins in the human stomach, requires a
  very acidic environment in order to function.
• Strong acid is secreted into the stomach,
  allowing pepsin to work.
• Once the contents of the stomach enter the small
  intestine, where most digestion occurs, the acid
  must be neutralized. Enzymes that work in the
  small intestine need a basic environment.
• An organ near the small intestine, called the
  pancreas, secretes bicarbonate ions (HCO3-) into
  the small intestine to neutralize the stomach
  acid.

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Neutralization

• What do you think would happen if you mixed an
  acid and a base?
• Cancel each other out, causing a
  neutralization reaction
• For example, when the base sodium hydroxide
  (NaOH) and hydrochloric acid (HCl) react, they
  form a neutral solution of water and the salt
  sodium chloride (NaCl). This reaction is
  represented by the chemical equation
• NaOH HCl ? NaCl H2O.
• In this reaction, hydroxide ions (OH-) from the
  base combine with hydrogen ions (H) from the
  acid to form water. The other ions in the
  solution (Na) and (Cl-) combine to form sodium
  chloride.

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Lesson Summary

• Most of Earths water is salt water located on
  the planets surface. Water is constantly
  recycled through the water cycle.
• Water molecules are polar, so they form hydrogen
  bonds. This gives water unique properties, such
  as a relatively high boiling point.
• A solution is a homogeneous mixture in which a
  solute dissolves in a solvent. Water is a very
  common solvent, especially in organisms.
• The ion concentration of neutral, pure water
  gives water a pH of 7 and sets the standard for
  defining acids and bases. Acids have a pH lower
  than 7, and bases have a pH higher than 7.
• Water is essential for most life processes,
  including photosynthesis, cellular respiration,
  and other important chemical reactions that occur
  in organisms.