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The Chemical Basis for Life

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Title: 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.

3
Introduction
  • Water is biologically important
  • H2O is a simple molecule, but it has some very
    unique properties

4
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

7
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.

9
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.

10
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.

11
Cohesion
  • Cohesion is the attraction of one molecule to a
    similar molecule. Water molecules form droplets
    because of this principle.

12
Adhesion
  • When a molecule is attracted to a different
    molecule, it is called adhesion. Water sticks to
    other substances.

13
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

14
Capillarity
  • Check out the demonstration of capillarity!

Use cohesion and adhesion to explain what is
happening WHY!
15
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

16
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.

17
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.

18
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

19
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????

20
  • 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

21
  • 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-

23
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.

24
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.

25
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.

26
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.

27
Quick Review of How Acids and Bases Form
  • https//www.youtube.com/watch?v6zGHCyFhZk0

28
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.

29
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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30
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.
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