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.

International-mindedness:

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) • 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.

Understandings: Theory of knowledge:

• 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.

• 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?

Applications and skills:

Aim 6: Probes can be used to determine the effect of different factors likely influence cooling with water.

• 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.

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) Cohesion/Adhesion 2) Thermal Properties 3) Water’s Expansion Upon Freezing 4) 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=V cXzZt2iVk&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).* H 2 O has a high specific heat of 1 cal/g/⁰C. Why? Properties of Water: http://www.sumanasinc.com/webcontent/animations/content/propertiesofwater/water.html

Compare H 2 O’s specific heat to Methane (CH 4 ), which is 0.55 cal/g/ ⁰C. Compare it to Iron (Fe) 0.1 cal/g/⁰C.**

•

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. 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 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.

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