Ch 3: The Polarity of Water and Its Properties

Report
Ch 3: The Polarity of Water and
Its Properties
2016
Chapter 3: Water
From Topic 2.2
Essential idea: Water is the medium of life.
Nature of science: Use theories to explain natural
phenomena—the theory that hydrogen bonds form
between water molecules explains the properties of
water. (2.2)
Understandings:
• Water molecules are polar and hydrogen bonds form
between them.
• Hydrogen bonding and dipolarity explain the cohesive,
adhesive, thermal and solvent properties of water.
• Substances can be hydrophilic or hydrophobic.
Applications and skills:
• Application: Comparison of the thermal properties of
water with those of methane.
• Application: Use of water as a coolant in sweat.
• Application: Modes of transport of glucose, amino acids,
cholesterol, fats, oxygen and sodium chloride in blood in
relation to their solubility in water.
Guidance:
• Students should know at least one example of a benefit
to living organisms of each property of water.
• Transparency of water and maximum density at 4°C do
not need to be included.
• Comparison of the thermal properties of water and
methane assists in the understanding of the significance
of hydrogen bonding in water.
International-mindedness:
• There are challenges for the increasing human
population in sharing water resources equitably for
drinking and irrigation, electricity generation and a
range of industrial and domestic processes.
Theory of knowledge:
• Claims about the “memory of water” have been
categorized as pseudoscientific. What are the criteria
that can be used to distinguish scientific claims from
pseudoscientific claims?
Aim 6: Probes can be used to determine the effect of
different factors likely influence cooling with water.
From Topic 9.1
Understandings:
• The cohesive property of water and the structure of
the xylem vessels allow transport under tension.
• The adhesive property of water and evaporation
generate tension forces in leaf cell walls.
Applications and skills:
Application: Models of water transport in xylem using
simple apparatus including blotting or filter paper,
porous pots and capillary tubing.
Why is water polar?
• CrashCourse: Water http://www.youtube.com/watch?v=HVT3Y3_gHGg*
• In a water molecule, two hydrogen atoms form single polar
covalent bonds with an oxygen atom.
• What do you notice about the charges?
Hydrogen Bonding of Water Molecules
Nature of science: Use theories to explain natural phenomena—the theory that hydrogen bonds form between
water molecules explains the properties of water (2.2).
Understandings:
• Water molecules are polar and hydrogen bonds form between them.*
• H2O has a variety of unusual properties because of the
attractions between these polar molecules.
• Notice how individual H2O molecules orient themselves to
form an H-Bond.
4 Emergent Properties of Water
Understandings:
Hydrogen bonding and dipolarity explain the cohesive, adhesive, thermal and solvent properties of water.
Guidance:
• Students should know at least one example of a benefit to living organisms of each property of water.
1)
2)
3)
4)
Cohesion/Adhesion
Thermal Properties
Water’s Expansion Upon Freezing
Solvent Properties
1) Cohesion/Adhesion
Understandings:
• The cohesive property of water and the structure of the
xylem vessels allow transport under tension.
• The adhesive property of water and evaporation generate
tension forces in leaf cell walls.
• Cohesion: Co---“like with like”,
water molecules attracted to
each other via hydrogen bonds
• Adhesion: Ad---“like with
opposite,” water molecules
attracted to different
molecules/surface
Application of Cohesion/Adhesion
• Cohesion among water
molecules plays a key role in
the transport of water
against gravity in plants,
called transpiration.
• Transpiration:
http://www.youtube.com/watch?v=mc9gUm1mMzc&feature=player_e
mbedded *
Application of Cohesion/Adhesion
• Surface tension, a measure
of the force necessary to
stretch or break the surface
of a liquid, is related to
cohesion.*
Video links: **
• Water Striders: http://www.youtube.com/watch?v=VcXzZt2iVk&feature=player_embedded
•
Surface Tension Droplets:
http://www.youtube.com/watch?v=ynk4vJa-VaQ&feature=player_embedded
•
Jesus Christ Lizard:
http://www.youtube.com/watch?feature=player_embedded&v=45yabrnryXk
2) Thermal Properties
• Application: Comparison of the thermal properties of water with those of methane.
• Application: Use of water as a coolant in sweat.
Guidance: Comparison of the thermal properties of water and methane assists in the understanding of the
significance of hydrogen bonding in water.
Aim 6: Probes can be used to determine the effect of different factors likely to influence cooling with water.
• Water stabilizes air temperatures by absorbing heat from
warmer air and releasing heat to cooler air.
Simulation Lab:
http://oceanservice.noaa.gov/education/pd/oceans_weather_climate/media/specific
_heat.swf
2) Thermal Properties
• The specific heat of a substance is the amount of heat that
must be absorbed or lost for 1g of that substance to change
its temperature by 1⁰C.
- Or another way of thinking about it is… a measure of how well a substance
resists changing its temperature when it absorbs or releases heat.
• One way to measure heat is calorie (cal).*
• H2O has a high specific heat of 1 cal/g/⁰C. Why?
Properties of Water:
• Compare H2O’s specific heat to Methane (CH4), which is
0.55 cal/g/ ⁰C. Compare it to Iron (Fe) 0.1 cal/g/⁰C.**
http://www.sumanasinc.com/webcontent/animations/content/propertiesofwater/water.html
Application of Thermal Properties
• Heat of vaporization is the
quantity of heat that a liquid
must absorb for 1 g of it to
be converted from the liquid
to the gaseous state.
– 580 cal of heat is to
evaporate 1g of water at
room temperature.
– This is double the heat
required to vaporize the
same quantity of alcohol
or ammonia.
– Why?*
Application of Thermal Properties/Cohesion
Application: Use of water as a coolant in sweat.
Aim 6: Probes can be used to determine the effect
of different factors likely to influence cooling with
water.
• As a liquid evaporates, the
surface of the liquid that remains
behind cools called evaporative
cooling.
• Evaporative cooling moderates
temperature in lakes and ponds
and prevents terrestrial
organisms from overheating.
• Evaporative cooling keeps plants
and animals cool. For humans,
it’s in the form of sweating.
Video Links:*
• Evaporative and Cooling:
https://www.youtube.com/watch?v=HNyoeo
HVnio&feature=player_embedded
• NPR Science: How Much Heat Can You
Take?
https://www.youtube.com/watch?feature=pl
ayer_embedded&v=lqwPS6wJN-c
3) Water Expansion Upon Freezing
Guidance: Transparency of water and maximum density at 4°C do not need to be included.
• Ice floats on liquid water. Why?
http://www.sumanasinc.com/webcontent/animations/content/propertiesofwater/water.html
- This allows oceans and lakes not to freeze.
• Notice the spacing between water molecules in solid and liquid
form.
Application of Water Expansion Upon Freezing
• Since ice is less dense the liquid water, ice floats on top of
the cool water below.
• This oddity has important consequences for life. Why?
4) Solvent Properties
Application: Modes of transport of glucose, amino acids, cholesterol, fats, oxygen and sodium chloride in blood
in relation to their solubility in water.
•
The dissolving agent is the solvent and the substance that is
dissolved is the solute.
- In our example, water is the solvent and sugar the solute.
•
In an aqueous solution, water is the solvent.
- Water is not a universal solvent, but it is very versatile because of the
polarity of water molecules.
4) Solvent Properties
• Water is an effective solvent
because it so readily forms
hydrogen bonds with
charged and polar covalent
molecules.
• Repeat animation?
http://www.sumanasinc.com/webcontent/a
nimations/content/propertiesofwater/water.
html
Hydrophilic vs. Hydrophobic
Understandings: Substances can be hydrophilic or hydrophobic.
•
Hydrophilic: “water loving,” any substance
that has an affinity for water is.
-
Ex: polar or ionic substances
Cotton is hydrophilic because it has numerous polar
covalent bonds in cellulose, which is its major
constituent. Water molecules form hydrogen bonds in
these areas.
• Hydrophobic: “water fearing,” any substance
that is repelled by water.
-
Ex: nonpolar substances
4) Application of Solvent Properties
• Each dissolved ion is
surrounded by a sphere of
water molecules, a
hydration shell.
• Even large molecules,
like proteins, can
dissolve in water if
they have ionic and
polar regions.*
Acids and Bases
• An acid is a substance that
increases the hydrogen ion
concentration in a solution.
• Any substance that reduces the
hydrogen ion concentration in a
solution is a base.
• In a neutral solution [H+] = 10^-7
M, and the pH = 7.
• Buffers resist changes to the
pH of a solution when H+ or
OH- is added to the solution.
Blood Buffering:
https://www.youtube.com/watch?feature=player_embedded&v=NJyAme5GVF8
Ocean Acidification:
https://www.youtube.com/watch?v=RnqJMInH5yM
Buffers: Acid Rain Slayer
https://www.youtube.com/watch?v=8Fdt5WnYn1k
International-mindedness: There are challenges for the increasing
human population in sharing water resources equitably for drinking
and irrigation, electricity generation and a range of industrial and
domestic processes.
Video Links:
BBC America (long): http://www.youtube.com/watch?v=Gg-ac0EaYDQ

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