Transcript Chapter 2-2 Properties of Water

Chapter 2-2
Properties of Water
Why is Earth called the blue planet
• Covered by water
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.
Water Facts
• Water exists mainly as a liquid over much
of Earth’s surface
Water Facts
• As water freezes it expands; this makes
ice less dense than water causing it to
float
Water
molecules
as a liquid
As water
freezes the
molecules
arrange
themselves in
a very specific
pattern (ice is
classified as a
crystalline
solid)
A Molecule of Water
One Atom of
Oxygen; O
This gives us
Two Atoms of
Hydrogen; H
Water is a neutral molecule. It
has 10 proton and 10
electrons
H2 0
2 p+
8 p+
2 e-
8 e-
Although water is an electrically neutral
molecule it is does exhibit polarity.
Polarity refers to the unequal sharing of
electrons.
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
_______________
The oxygen end
gets a partial
negative charge
Hydrogen end
gets a partial
positive charge
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
Hydrogen bonding
Why are the charges in parentheses?
they are partial charges
(-)
O
(+)
H
(+)
(-)
H
(-)
O
O
H (+)
(+)
H
(+) (-)
H
(-)
H (+)
(-)
O
O
O
H
(+) H
H
(+)
H
(+)
H
(+)
(+)
H
(+)
Water can form multiple hydrogen
bonds between molecules
Cohesion
• Cohesion is an attraction between
molecules of the same substance.
• Water is very cohesive because of
hydrogen bonding.
Cohesion creates surface tension
The weight of the paper clip isn’t
enough to break the weak
hydrogen bonds between the water
molecules
Water striders rely on
cohesion between
water molecules
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.
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
Polarity also affects Solubility
Solubility refers to the ability of one
substance (solute) to dissolve in another
(solvent)
Solubility
• Polar substances can dissolve other polar
substances.
• Non-polar substances dissolve other nonpolar substances.
• Polar substances and non-polar
substances do not mix.
Urea dissolves in water because
both are polar
(-)
(+)
(+)
(+)
(-)
(-)
(+)
(-)
(+)
Weak Hydrogen bonds
(+)
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
(-)
(-)
(+)
(+)
(+)
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
(+)
C
C
C
C
• Water is a polar molecule
• Oil is a non-polar molecule
• Oil and water do not mix for this reason
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.
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!
• 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
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”
Other suspensions:
Blood, which is mostly
water contains many
dissolved and
undissolved particles
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.
Water can form ions, but remains
neutral.
H2O
Water
H+
+
OH-
Hydrogen
+
Hydroxide
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)
Lets draw our scale…
• Acids have a low [OH-]; high [H+]
• Bases have high [OH-]; low [H+]
• What are examples of each?
Acids
Have extra H+ ions
Have a pH less than 7
Hydrochloric Acid and
Vinegar
Bases
• Have more OH- ions
• Have a pH greater
than 7
• Also called Alkalines
• Soap and Ammonia
Neutral
Neutral solutions have exactly the same
number of H+ as OH- ions
Pure Water….. Has one H+ for every OHTap water usually has a pH just above 7
Myth: All acids are harmful and will burn
your skin.
Busted: Not all acids cause burns…some
examples are lemon juice and vinegar
Myth: Bases are safe to handle
Busted: Lye or Sodium Hydroxide is very
dangerous
• 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.
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.
Buffers
• Why are buffers important in the body?
– to help maintain homeostasis.
– The pH in the body needs to be between 6.57.5 for chemical reactions to occur properly.