Water

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Water

• Packet 7

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Introduction

• Most abundant liquid in the world
• Makes up at least half of all living organisms
  and up to 95 of some species.
• 75 of earth is submerged in water
• Naturally occurring water consists of 99.76 H2O

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Water--Isotopes

• The remainder consists of various isotopes
• 2H
• Deuterium
• Most often found with a normal hydrogen atom
• Forms HDO
• Occasionally, D2O is formed
• Both are called heavy water and have deleterious
  effect on living organisms.
• 18O

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The Water Molecule

• Recall that H2O is a polar molecule
• This results in the ability to form hydrogen
  bonds and an attraction between individual water
  molecules
• Due to the opposite charges

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The Water Molecule
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Properties of Water I

• Cohesion and Surface tension
• Cohesion
• Tendency of molecules, if in substance, to hold
  together by mutual attraction
• The hydrogen bonding of water results in strong
  cohesion forces.

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Cohesion Surface Tension

• What does this (string cohesion forces) mean?
• One drop of water will assume the smallest
  possible area on a surface
• The water molecule will form a sphere
• Water molecules, at the surface of the drop, will
  be drawn together forming a skin-like layer off
  molecules on the surface.
• The force formed is called surface tension.

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Homework Assignment

• Conduct experiment determining how much water can
  a penny hold
• Must use an old and a new penny
• Difference in composition
• Must complete at least 20 trials for each penny
• Must complete a full lab report with a 5 page
  background research section on cohesion forces
  and surface tension
• PAPER MUST BE SUBMITTED AT TURNITIN.COM
• Due 245pm EDT Friday September 7th 2007

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Surface Tension

• Is a measure of how hard it is to break the
  surface of a liquid
• Insects waling on the surface of water and the
  movement of water up plants are two biological
  processes that can occur as a result of the
  cohesive properties of water molecules.

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Properties of Water I

• Adhesion and capillarity
• Adhesion
• The attraction of molecules of different
  compounds to one another.
• The ability of water to cling readily to other
  molecules is responsible for the upward movement
  of water when a small-bore tube is dipped into
  it.
• This phenomenon is called capillarity
• Xylem vessels of a diameter 0.02mm can, in
  theory, support a column of water of height 1.5m
  by capillary forces.
• Capillary forces has one of its main biological
  effects being the upward movement of water in the
  soil

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PROPERTY 2

• Thermal Capacity/Moderation of Temperature

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Thermal Capacity

• Hydrogen bonding results in the necessity of heat
  to cause increased molecular movement
• Formation of water in the gas form (steam)
• Heat energy first used to break hydrogen bonds
• Resistance to temperature change
• This is why the temperature of water rises only
  very slowly for a given amount of added heat
• Water has a very high specific heat
• And vice versa
• The amount of heat that must be absorbed or lost
  for 1 gram of that substance to change its
  temperature by 1C

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Thermal Capacity II

• Why is the use of heat energy for breaking
  hydrogen bonds important?
• Allows thermal stability so that enzymatic
  reactions(biochemical reactions) in water mediums
  are not subjected to large temperature
  fluctuations and can take place at a more
  constant rate.
• If there were no hydrogen bonds, water would be a
  gas at most temperatures
• Also helps in the cooling affect
• Sweating
• Heat of vaporization

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PROPERTY 3

• Density

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Density

• Water has a maximum density at 4ºC
• This is when water is in a liquid state
• H2O (l) is more dense than H2O (s)
• Water freezes from top downward allowing ice to
  form at the top
• Ice at top can insulate the warmer water below
  that layer from colder temperatures above
• Prevents large bodies of water from freezing
  solid and allowing the survival of aquatic
  organisms

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PROPERTY 4

• Solubility

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Solubility

• Water is a versatile solvent due to its polarity
• Can form an aqueous solution

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Definitions

• Mixture
• A portion of matter that can be separated into
  two or more substances by physical means alone
• Substance composed of 2 or more elements or
  compounds that are mixed together, but are not
  chemically combined
• A composition of two or more substances that are
  not chemically combined with each other and are
  capable of being separated.
• Salt and pepper
• Sugar and sand

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Definitions II

• Solution
• A homogeneous (of uniform structure or
  composition throughout ) mixture of two or more
  substances, which may be solids, liquids, gases,
  or a combination of these.
• Homogenous mixture
• Sugar in water (Sugar-water)
• One component (Major), of a solution, is found in
  a greater amount
• Solvent
• One component (Minor), of a solution, is found in
  a lesser amount
• Solute

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• The different regions of the polar water molecule
  can interact with ionic compounds called solutes
  and dissolve them

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• Water can also interact with polar molecules such
  as proteins

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Solubility

• Likes dissolves likes
• Polar with polar
• Non-polar with non-polar
• Oil and water are dislikes
• Water polar
• Oil non-polar
• We say that oil and water are nearly completely
  immiscible
• The oil is suspended

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Other Important Points II

• Hydrophilic substance
• Water loving
• Has an affinity for water
• Hydrophobic substance
• Water hating
• Does not have an affinity for water

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Other Important Points II

• Remember, most biochemical reactions occur in
  water
• It Is important to calculate the concentration of
  solutes in an aqueous solution
• Mole
• The number of molecules of a substance in a given
  mass
• Molarity
• The number of moles of a solute per liter of
  solution

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DISSOCIATION
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Dissociation (Ionization), pH Buffers

• Water has a slight tendency to dissociate into
  ions according to the equation
• 2H2O ? H2O OH-
• H2O ? H OH-
• Hydronium ions and hydroxide ions
• In 1 liter of water, this dissociation produces
  1/10,000,000 mole of hydrogen ions.
• Equivalent to a pH of 7
• Neutral

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Dissociation II

• If the concentration of hydrogen ions is higher,
  1/1000 (10-3) the pH would be 3 and the solution
  would be acidic
• An acid, therefore, is a substance that donates
  hydrogen ions
• Increases hydrogen ion concentration
• A base is a hydrogen ion acceptor
• Decreases hydrogen ion concentration
• Changes in concentration of ions can have a great
  affect on living organisms
• Particularly enzymes
• Why?

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Dissociation III

• Apart from dissociating itself, water readily
  causes the dissociation of other substances
  placed in it.
• Making it an excellent solvent.

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Buffers

• A buffer solution is one that retains a constant
  pH despite the addition of small quantities of
  acids or bases.
• Buffers contain both hydrogen ion donors and
  acceptors.
• Hydrogen carbonate ions may act as an acceptor or
  a donor
• Hydrogen carbonate salts and phosphate salts are
  responsible for the buffering of blood.
• pH of 7.4

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Osmosis

• More to come later