chapter 2-2 properties of water
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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. - PowerPoint PPT PresentationTRANSCRIPT
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Chapter 2-2
Properties 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 Earth’s 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 WaterOne Atom of Oxygen; O
Two Atoms of Hydrogen; H
This gives us
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Water is a neutral molecule. It has 10 proton and 10 electrons
H2
2 p+
2 e- 08 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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POLARITYThe 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
H
H
HH
H
H
HH
H HH H
O
O
O
O
OO
(-)
(-)
(-)
(-)
(-)
(-)
(+)(+)
(+)
(+)
(+)
(+)
(+)
(+)
(+)(+)(+)
(+)
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 tensionThe weight of the paper clip isn’t 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
H H
O
(-)
(+)(+)(-)
H H
O(+)(+)
(-)
(-)(+)
(+)
Weak Hydrogen bonds
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H H
O(+)(+)
(-)C
CC
C
C
C
C
CC
C
C
C
CC
C
C
C
C
CC
CC
CC
C
C
C
C
CC
C
Molecules such as Naphthalene (White Tar) will not dissolve in water because Naphthalene is a non-polar molecule
H H
O(+)(+)
(-)
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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-aidOH OH
YEAHYEAH!!
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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 burns…some 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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