TAKS Objective 4 TEKS 9A: Properties of Water

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TAKS Objective 4TEKS 9A Properties of Water
Topic Review
The student knows how solution chemistry is a
part of everyday life. The student should be able
to relate the structure of water to its function
as the universal solvent.
Number of expected TAKS questions 1 to 2
Presented by Alexander Fedorov Composite
Science Teacher Round Rock ISD Mr. Fedorovs web
Address https//www.myscienceparty.org
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Filling the outer shell

• When we talked about bonding, we discussed how
  some atoms want to gain an electron to fill their
  outer shell. Which atoms on the periodic table
  want to do this?
• We also talked about how some atoms want to lose
  their outer electron so that the shell below is
  filled. Which atoms on the periodic table do
  this?

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Electronegativity

• Fluorine, which is on the top right of the
  periodic table, wants electrons. Fluorine will
  pull or tug on an electron to try and take
  that electron from other atoms.
• In chemistry, we call this pulling or tugging
  on electrons electronegativity.
• The higher the electronegative value, the
  stronger the pull or affinity for the electron.
• Atoms which have strong electronegativity
  Fluorine (F), Oxygen (O), Chlorine (Cl).
• Atoms which have low electronegativity Hydrogen
  (H), Lithium (Li), Sodium.

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Electronegativity

• Every element on the periodic table has some form
  of electronegativity. Each element is different.
  The atoms on the far right are the most
  electronegative and the atoms on the far left are
  the least electronegative.
• In chemistry, we assign numerical values to each
  atom, or an electronegative value, to show how
  strongly an atom will pull on an electron.

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Even Electron Disbursement

• When two identical elements combine, as in the
  case of oxygen (O2), both oxygen atoms pull on
  the electron with the same strength. As a
  result, the electrons are at rest in the middle
  between the two atoms and the molecule is neutral.

Electrons
O
O
Midpoint between Two atoms
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The Tug-of-War for electrons

• When atoms of high electronegativity combine with
  atoms of low electronegativity, the electrons do
  no disperse evenly. The electrons will remain
  closer to the atom with high electronegativity
  and stay further away from the atoms with low
  electronegativity.

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Uneven electron disbursementHydrogen Oxygen
Bond

• When hydrogen combines with oxygen to form a
  bond, electrons are shared. The electrons stay
  closer to Oxygen during bonding because oxygen is
  more electronegative.

Electrons
H
O
Midpoint between Two atoms
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Electrons
H
O
d-
d
Midpoint between Two atoms

• Because oxygen holds the electrons closer to
  itself, oxygen is said to have a partial negative
  charge. (remember, electrons are negative and
  since the electrons are close to oxygen, the
  electrons give the oxygen atom a partial negative
  charge).
• The symbol for a partial charge is d. The symbol
  is always followed by a () or (-) representing a
  positive or negative.

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Electrons
H
O
d-
d
Midpoint between Two atoms

• Since oxygen is partially negative, electrons
  have been moved away from hydrogen. Hydrogen is
  now said to be partially positive. Note the
  partial symbols shown above.
• This separation of charges is very important.
  This makes one side of the molecule positive and
  the other side of the molecule negative. This
  separation of charges actually turns the molecule
  into a weak magnet.

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Electrons
H
O
d-
d
Midpoint between Two atoms
Above picture similar to a magnet

-

• Since we have a separation of charges, where one
  side is positive and the other side is negative,
  the molecule is said to be polar (like a magnet).
• Polarity refers to this separation of charges.

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Lets take a look at water

• Water is composed of 2 hydrogen atoms linked to 1
  oxygen atom
• H2O

O
H
H
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Electrons
H
O
d-
d
Midpoint between Two atoms
If we remember the polarity of between the
hydrogen and oxygen bond, then we can assign this
polarity to the water molecule.
d-
O
H
H
d
d
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So water is essentially a magnet!
d-

O
H
H
-
d
d
This core concept is extremely importantthat
water is polar (has separate opposite charges at
each end) and essentially acts like a magnet. We
will now explore why this is so important!
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Water holds itself together

• Have you ever noticed that water will bead up.
  When rain falls, it fall in clumps (called water
  drops) and not as vapor. When it hits, it bead
  up on your cars windshield.
• Why does it bead up? What holds these
  molecules together?
• Molecules of water are held together due to the
  magnetic properties just discussed.

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Hydrogen Bonding

• When one water molecule comes close to another
  water molecule, a magnetic bond forms on the
  positive end of one side of the molecule with the
  negative end of another water molecule. This is
  similar to a magnetic bond.

d-
_
Magnetic bond
O
Hydrogen Bond

H
H
d
_
_
O
d-
O
d-


H
H
H
H
d
d
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Hydrogen Bonding

• So water is held together because of hydrogen
  bonding (a type of magnetic bond). This makes
  water stick together and this is the reason why
  water beads up.
• Hydrogen bonding is responsible for all of
  waters unique properties

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Physical Properties of water

• Water The Universal Solvent
• High surface tension
• High boiling point
• Solid water (ice) floats on the liquid
• High specific heat capacity (holds heat)

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Water The Universal Solvent

• Water is known as the Universal Solvent because
  water will dissolve many substances. Because of
  the polar properties of water, anything that has
  a charge (ion) will dissolve (disappear) in
  water. Water molecules surround the particle
  giving the illusion that the substance has
  disappeared. Water will only dissolve polar
  molecules or charged atoms

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Water has high surface tension
Water is held together because of the hydrogen
bonds. As a result, water will stick together
in clumps. This is why water bead-up to form
droplets of rain. Remember, gravity has nothing
to do with surface tension (ever seen a water
droplet on a space shipit is perfectly round!)
The beading of water caused by surface tension
created by hydrogen bonds within the water
molecule.
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High boiling point

• Because water sticks together, much energy is
  require to break hydrogen bonds and free water
  molecules. The energy we use is heat. When
  water boils, enough energy is put into the water
  to allow the water molecules to break free of
  their hydrogen bonds and break out into the air
  as vapor.

Liquid water breaking out into the air in the
form of steam and quick converting into water
vapor.
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ICE FLOATS!

• Most solids will sink in their liquid counterpart
  because the solid form is almost always more
  dense than the liquid form. This is not true of
  water. ICE FLOATSwhich makes water very unique.
• This property is very important to marine
  lifeice actually acts as an insulator of heat
  below the surface. Ice protects the surface of
  the water from heat loss and insulates the water
  below (preventing it from freezing). This allows
  marine life to exist below the ice if the water
  froze from the bottom up, marine life would die.

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Why does ice expand in freezing

• When ice crystallizes, it forms a lattice
  structure. Empty space forms between the lattice
  making ice less dense than liquid water and
  causing it to expand and float.

Empty space
O
O
Empty space
O
d-
H
H
H
H
H
H
d
d
d
O
d-
O
d-
O
d-
O
d-
H
H
H
H
H
H
H
H
d
d
d
d
O
d-
O
d-
O
d-
H
H
H
H
H
H
d
d
d
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High Specific Heat Capacity

• Because of the large number of hydrogen bonds,
  water has the unique ability to store heat for a
  long period of time. As a result, a cup of hot
  water will stay hot much longer than the same
  volume of metal. (metal has a low specific heat
  capacity).

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The Water Cycle

• The earth has a limited amount of water.  That
  water keeps going around and around and around
  and around and (well, you get the idea) in what
  we call the "Water Cycle". This cycle is made up
  of a few main parts
• evaporation (and transpiration)
• condensation
• precipitation
• collection

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Evaporation

• Evaporation is when the sun heats up water in
  rivers or lakes or the ocean and turns it into
  vapor or steam. The water vapor or steam leaves
  the river, lake or ocean and goes into the air.
  This is a physical change vaporization.

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Transpiration

• Do plants sweat?
• Well, sort of.... people perspire (sweat) and
  plants transpire.  Transpiration is the process
  by which plants lose water out of their leaves. 
  Transpiration gives evaporation a bit of a hand
  in getting the water vapor back up into the air.
  This is a physical change vaporization.

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Condensation
Water vapor in the air gets cold and changes back
into liquid, forming clouds. This is called
condensation. This is a physical change.
You can see the same sort of thing at home...
pour a glass of cold water on a hot day and watch
what happens.  Water forms on the outside of the
glass.  That water didn't somehow leak through
the glass!  It actually came from the air.  Water
vapor in the warm air, turns back into liquid
when it touches the cold glass.
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Precipitation

• Precipitation occurs when so much water has
  condensed that the air cannot hold it anymore. 
  The clouds get heavy and water falls back to the
  earth in the form of rain, hail, sleet or snow.
  Precipitation is not a phase change.

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Collection
When water falls back to earth as precipitation,
it may fall back in the oceans, lakes or rivers
or it may end up on land.  This is not a phase
change either.

• When it ends up on land, it will either soak into
  the earth and become part of the ground water
  that plants and animals use to drink or it may
  run over the soil and collect in the oceans,
  lakes or rivers where the cycle starts over.

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The End

• We have finished this section for review.
• Do you understand this material?
• Next you will take actual TAKS released questions
  for this section.
• Wait for the teacher to give you instructions.
  If you are self studying, move on to the practice
  test.