1. Agenda 1/10 Water Properties Lab Water Properties Notes (or POGIL)
pH quick talk
Biochem Review
Homework: Water Video and Notes, Study for Biochem Exam

2. Water and the Fitness of the Environment
Biochemistry
Water and the Fitness of the Environment
Water is essential to life on earth. It is important that each property of water be linked to biological processes through illustrative examples.

3. Water: The molecule that supports life on this planet!
Water is the biological medium for all life on Earth
All living organisms require water more than any other substance
Emphasize that the polar covalent bonds are actual chemical bonds where a single pair of electrons are shared between two atomic nuclei. BUT a H-bond is not an actual chemical bond, but rather an intermolecular force (IMF) that is not to be ignored. H-bonds can form only between the H attached to a highly electronegative element such as F, O or N (think “phone”) on one molecule and an unshared pair of electrons on a highly electronegative element on an adjacent molecule (F, O or N again).

4. Water: The molecule that supports life on this planet!
Most cells are surrounded by water, and cells themselves are about 70–95% water
The abundance of water is the main reason the Earth is habitable
We’re still trying to confirm the existence of water on other heavenly bodies!

5. FOUR Emergent Properties of water contribute to Earth’s suitability for life
Water’s cohesive & adhesive behavior
Water’s ability to moderate temperature
Water’s expansion upon freezing
Water’s versatility as a solvent.
This in most definitely the main emphasis of this lesson! Refer to these properties over and over throughout the course. Sorry about the pun on “polar”, just couldn’t resist it!

6. What causes water molecules to both cohere and adhere?
Yep! Our good buddy the intermolecular force (IMF) named hydrogen bonding
Cohesion is when water molecules stick to each other.
Adhesion is when water molecules stick to some other type of substance like plant cell walls.
Also a good time to discuss capillary action with glass tubes. The thinner the tube, the more surface area vs. volume, the higher the water will “climb” since water’s adhesive force for glass is greater than the cohesive forces it has for other water molecules.
This animation should be embedded into your ppt if you are using PowerPoint Scroll over the bottom of the picture and you should see a bar that allows you to press “play”—the blue “PLAY” button is pretty, but useless, the control is hidden until you scroll over the picture.
Animation by scrolling over image

7. What causes water molecules to both cohere and adhere?
Surface tension is a measure of how hard it is to break the surface of a liquid.
Surface tension is a consequence of cohesion. What causes it? Go ahead, guess!
Hydrogen bonding, yet again!
This may well be “prior learning” from middle school. You can always do the “how many drops fit on a penny” demo/activity to reinforce water’s adhesive vs. cohesive forces. Students predict how many drops will fit on a penny and are surprised to see how high the “bubble” of water will get before the balance of cohesive forces are overcome and water spills off the penny. If you have an evil streak, you can use your own pennies, some of which you’ve covertly coated with a bit of bar soap to act as a surfactant! Great inquiry moment even if you’ve stacked the deck so to speak.

8. How does water moderate temperature?
Water has a high heat capacity.
Heat capacity is the amount of heat required to raise 1 gram of water by 1o Celsius (1 calorie = J)
What does that mean? It means water can absorb large quantities of heat without much change in its own temperature, thus it’s a good thermo regulator.
Again, probably prior learning from physical science or chemistry I.

9. It’s a 2-way street!
Water’s high specific heat can be traced to hydrogen bonding (IMFs)
Heat is absorbed when hydrogen bonds break
Heat is released when hydrogen bonds form
The high specific heat of water minimizes temperature fluctuations to within limits that permit life
Recite over and over to students: “It takes energy to break bonds, but energy is released when bonds form.” You’ll be glad you did by the time we reach cell respiration and photosynthesis!

10. Ever sweat?
Evaporation is transformation of a substance from liquid to vapor
Heat of vaporization is the heat a liquid must absorb for 1 g to be converted to vapor
As a liquid evaporates, its remaining surface cools, a process called evaporative cooling
As the sweat leaves your skin, it carries a large amount of heat with it!
If you can get a container of rubbing alcohol , 2 thermometers and a pipet, you can do a quick demo. Have a thermometer in the container that is clearly labeled alcohol. Your second thermometer is for establishing room temperature. Fill a pipet with alcohol, then have each student make a fist, and put a drop of alcohol on the “top side” of their fist. The alcohol will feel “cold” to them. They’ll be quite surprised that it is at room temperature. Ask them: “Where did the energy for the evaporation of the alcohol come from?” Ans: the heat in the environment and their own body temp. “Which has more thermal energy for transfer, you or the room?” Ans: Them! “Ask a student that answers correctly, to explain how they know “them”. Ans: Body temp = 98.6 F while room is about 75 F. “Why did you believe the alcohol was colder than the room temp?” Ans: The heat from our “body” was absorbed by the alcohol during its evaporation and our body’s thermo receptors perceive an absence of heat as “cold”.

11. Water is a freak! It expands upon freezing!
Very few substances on the planet expand upon freezing. Among them are antimony, bismuth, gallium, germanium and silicon…just in case you are asked! There are a few others, but expanding upon freezing is definitely not the norm! A good analogy to use with student involves filling a room with people. If all the people have their arms out, not as many people can fit in the room, so fewer “molecules” fit in the same defined space, thus some of the space is “wasted” or empty.
Ice: stable hydrogen bonds
Liquid water: transient
hydrogen bonds

12. Water is a freak! It expands upon freezing!
Ice floats in liquid water because hydrogen bonds in ice are more “ordered” forming a hexagonal shape with a hole in the middle, making ice less dense.
If ice sank, all bodies of water would eventually freeze solid, making life impossible on Earth
You can also explain the pond or lake “turning over” in the late fall and early spring. As the surface water is cooled (fall) or warmed (spring) to 4o C, that layer of water sinks, displacing the water that was on bottom. That will continue in the fall until all the water is at 4o C. Now, the top layer cools to 3 (and floats on top of the 4o water layer) , 2, 1 and 0oC and freezes solid. Fish don’t freeze unless the entire body of water freezes solid!

13. Water is a freak! It expands upon freezing!
Hydrogen bond
Liquid water:
Hydrogen bonds
break and re-form
Ice floats!
Ice:
Hydrogen bonds
are stable

14. Water’s Versatility as a Solvent
Water is a versatile solvent due to its polarity and ability to form hydrogen bonds
When an ionic compound is dissolved in water, each ion is surrounded by a sphere of water molecules called a hydration shell
Avoid the use of the phrase “universal solvent” which would imply the container would also be dissolved! Emphasize that ionic and polar molecules are more attracted to water than to each other if dissolving occurs.

15. Water’s Versatility as a Solvent
Water can also dissolve compounds made of nonionic polar molecules
Even large polar molecules such as proteins can dissolve in water if they have ionic and polar regions
It’s all about electrostatic attractions! (Opposite charges attract.)

16. The pH Scale
An acid is any substance that increases the H+ concentration of a solution
A base is any substance that reduces the H+ concentration of a solution
The scale was designed to compare WEAK acids and bases.
If asked, the graphic is using the Arrhenius and Brönsted-Lowry definitions of acids and bases.

17. The pH Scale
In any aqueous solution at 25°C the product of H+ and OH– is constant and can be written as the autoionization constant of water
The pH of a solution is defined by the negative logarithm of H+ concentration, written as
For a neutral aqueous solution, [H+] is 10–7, so
[H+][OH–] = 10–14
Don’t be frightened! It’s just that students should have done these calculations in Chemistry I class.
pH = –log [H+]
pH = –(–7) = 7

18. The pH Scale Acidic solutions have pH values less than 7
Basic solutions have pH values greater than 7
Most biological fluids have pH values in the range of 6 to 8
This is the stuff they should know well!