Chapter 2-2 Properties of Water

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Chapter 2-2Properties of Water
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Why is Earth called the blue planet

• Covered by water

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Water is unique because

• 1. Liquid at temperature found on most of the
  Earth
• 2. Expands as it freezes, unlike most substances
• Because ice is less dense than water, it floats.

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Water Facts

• Water exists mainly as a liquid over much of
  Earths surface

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Water Facts

• As water freezes it expands this makes ice less
  dense than water causing it to float

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Water molecules as a liquid
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As water freezes the molecules arrange themselves
in a very specific pattern (ice is classified as
a crystalline solid)
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A Molecule of Water
One Atom of Oxygen O
This gives us
Two Atoms of Hydrogen H
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• Water is a neutral molecule. It has 10 proton
  and 10 electrons

0
H2
2 p 2 e-
8 p 8 e-
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• Although water is an electrically neutral
  molecule it is does exhibit polarity.
• Polarity refers to the unequal sharing of
  electrons.

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POLARITY

• The shared electrons between oxygen and hydrogen
  tend to spend more time orbiting the oxygen atom
  giving it a unequal charge distribution

e
In a water molecule, are the electrons more
likely to be near the oxygen nucleus or the
hydrogen nucleus? _______________
Oxygen nucleus
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The oxygen end gets a partial negative
charge Hydrogen end gets a partial positive
charge
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Negative charges are attracted to positive
charges. In a water molecule the negative
oxygen end is attracted to the positive hydrogen
end of another molecule This creates a weak
Hydrogen Bond between water molecules
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Hydrogen bonding
Why are the charges in parentheses?
they are partial charges
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Water can form multiple hydrogen bonds between
molecules
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Cohesion

• Cohesion is an attraction between molecules of
  the same substance.
• Water is very cohesive because of hydrogen
  bonding.

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Cohesion creates surface tension
The weight of the paper clip isnt enough to
break the weak hydrogen bonds between the water
molecules
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Water striders rely on cohesion between water
molecules
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Adhesion

• Adhesion is an attraction between molecules of
  different substances.

• Water also sticks to, or adheres, to other
  surfaces well. This is why a meniscus forms when
  you are measuring liquid.

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Capillary action

• Cohesion and adhesion combined allows water to
  move up certain materials.
• This is how plants get water from the ground up
  through their stems

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Polarity also affects Solubility

• Solubility refers to the ability of one substance
  (solute) to dissolve in another (solvent)

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Solubility

• Polar substances can dissolve other polar
  substances.
• Non-polar substances dissolve other non-polar
  substances.
• Polar substances and non-polar substances do not
  mix.

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Urea dissolves in water because both are polar
(-)
O
()
()
H
H
(-)
()
(-)
()
(-)
O
()
()
H
H
Weak Hydrogen bonds
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Molecules such as Naphthalene (White Tar) will
not dissolve in water because Naphthalene is a
non-polar molecule
C
C
C
C
C
C
C
C
C
C
C
(-)
O
(-)
()
()
O
H
H
()
()
H
H
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
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• Water is a polar molecule
• Oil is a non-polar molecule
• Oil and water do not mix for this reason

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Mixtures

• A mixture is two or more elements or compounds
  that are physically mixed together but not
  chemically combined.
• Example Salt and pepper mixed together
• Mixtures made with water include solutions and
  suspensions.

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Mixtures made with Water

• Solutions a mixture of two or more substances in
  which the substances are evenly distributed.
• A solution has both a solvent and a solute.
• Kool-aid

OH YEAH!
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• A solvent is a substance in which a solute is
  dissolved to form a solution.
• Ex water
• WATER is the UNIVERSAL SOLVENT because of its
  polarity
• A solute is a substance that is dissolved in a
  solvent to make a solution.
• Ex kool-aid, salt, sugar

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Mixtures made with Water

• Suspensions a mixture of water and a
  non-dissolved material.
• Mud is a suspension, the dirt is not fully
  dissolved in the water so the particles are
  suspended

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Other suspensions

• Blood, which is mostly water contains many
  dissolved and undissolved particles

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So is blood a solution or suspension?

• Both!
• Blood is mostly water, many substances are
  dissolved in it. Here blood is a solution.
• Cells in the blood remain in suspension.

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Water can form ions, but remains neutral.

• H2O H OH-
• Water Hydrogen Hydroxide

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Acid or Base

• The pH scale represents how many H and are in a
  solution. The pOH scale represents how many OH
  ions
• pH scale ranges from 0 (strong acid) to 14
  (strong base)

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Lets draw our scale

• Acids have a low OH- high H
• Bases have high OH- low H
• What are examples of each?

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Acids

• Have extra H ions
• Have a pH less than 7
• Hydrochloric Acid and Vinegar

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Bases

• Have more OH- ions
• Have a pH greater than 7
• Also called Alkalines
• Soap and Ammonia

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Neutral

• Neutral solutions have exactly the same number of
  H as OH- ions
• Pure Water.. Has one H for every OH-
• Tap water usually has a pH just above 7

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• Myth All acids are harmful and will burn your
  skin.
• Busted Not all acids cause burnssome examples
  are lemon juice and vinegar

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• Myth Bases are safe to handle
• Busted Lye or Sodium Hydroxide is very
  dangerous

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• Ph scale is logarithmic- each number increase is
  10X more.
• pH of 1 is 10x more acidic than a pH of 2 and
  100x more acidic than a pH of 3.

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Buffers

• Resist changes in pH
• A buffer is a weak acid or base that reacts with
  strong acids or bases to prevent sudden changes
  in pH.

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Buffers

• Why are buffers important in the body?
• to help maintain homeostasis.
• The pH in the body needs to be between 6.5-7.5
  for chemical reactions to occur properly.

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