Ch 3: The Polarity of Water and Its Properties

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

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Surface Tension Droplets: http://www.youtube.com/watch?v=ynk4vJa-VaQ&feature=player_embedded

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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.

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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.

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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.

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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