Chapter 3 Presentation Water and the Fitness of the Environment Essential Idea: Water is the medium of life.

TOK 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? 2

Properties of Water There are 4 main properties of water that make it an important molecule which allows life to exist. 1. Cohesive behavior. 2. Water’s ability to moderate temperature. 3. Expansion upon freezing. 4. Water is a good solvent. 3

Attractive Forces Hydrogen bonds occur from an attractive force between hydrogen and either oxygen, nitrogen, or fluorine. Van der Waals interactions are weak intermolecular forces that occur between the atoms of compounds and molecules. There are two different types: 1. Dispersion (London) forces. 2. Dipole interactions. 4

Attractive Forces Van der Waals forces and H-bonds are very weak individually, but in large number they are very strong. The gecko example from the book and H-bonding that creates surface tension that allows water striders to walk on water provide an example of the collective strength of these forces. 5

Hydrogen Bonds These are a special type of dipole interaction. Hydrogen bonds are the strongest of the dipole interactions. Hydrogen bonds are seen in H-containing compounds that are bonded to very electronegative atoms (O,N,F). H2O 7

1. Dispersion Forces Dispersion forces are the weakest: these forces generally increase as the number of electrons increases. The halogens are an example. Chlorine is a gas, bromine is a liquid, and iodine is a solid. 9 http://www.wikiwand.com/en/Halogen

2. Dipole Interactions Dipole interactions are a little stronger and these result from the interactions of polar molecules. SiO2 is a very polar molecule, they are attracted to other SiO2 molecules. The idea here is that polar bonds can from, and when they are not formed between O,N,F and hydrogen, they are called dipole interactions. 10

Water Structure Water Structure 11

Water Transport Water Transport 12

A comparison of the thermal properties of water compared to methane. https://www.teachengineering.org/activities/view/fiu_models_activity1

1. Cohesion Cohesion is basically the phenomenon of the hydrogen bonds working together. This leads to the idea of surface tension. 15

1. Cohesion--Surface Tension Surface tension is the thing you observe when you see a water bug walking on water or the beads of water forming on a freshly waxed car. The surface tension is created by the hydrogen bonds that form between the water molecules. 16

2. Moderation of Temperature Water has a very high specific heat capacity meaning that it takes a lot of energy to raise and lower its temperature. This high specific heat is due to hydrogen bonding. In order to break the bonds, heat must be absorbed and in order for them to form, heat must be released. 18

2. Moderation of Temperature Thus, adding heat to the water does little more than disrupt H-bonds, it is only when you add a lot of heat do you begin to see a change in temperature. When hydrogen bonds have been disrupted and the H2O molecules begin to move faster only then do we see a change in temperature. 19

2. Moderation of Temperature These characteristics of water enable it to moderate air temperatures in the regions near water. A large amount of heat can be absorbed with only a small change in temperature enabling life in the water to enjoy a relatively stable climate. Likewise, on cool nights “warm” water can give off a lot of energy without itself cooling much. This warms the nearby air again moderating the temperature. 20

3. Expansion Upon Freezing One of the unique properties of water is that it expands when it freezes. Most substance do not do this. It expands because as the water molecules lose energy, they align a certain way according to their hydrogen bonds. This creates a small pocket and makes the ice less dense than the liquid. 21

3. Expansion Upon Freezing When the ice freezes at the top of the body of water it acts to insulate the remaining water preventing it from freezing solid. Because it is less dense and floats, it prevents the body of water from freezing from the bottom up. If this were the case, only a small portion of the ice would ever thaw making life impossible. 22

4. Versatile Solvent Water is a very versatile solvent. Though not universal, water has an amazing ability to dissolve many things that are important to our existence. Much of water’s ability to dissolve things is due to its polarity. The polar regions of the water molecule interact with the positively and negatively charged regions of the molecules with which they are interacting. 24

Hydrophobic Substances These substances do not interact with water. Think oils. Think non-polar. http://caitlynmccartney.wordpress.com/ 26

Hydrophilic Substances These substance mix well with, or have an affinity for water. Think salt water. Think vinegar. http://hewittapbiology.files.wordpress.com/2012/08/salt.png 27 http://www.bishopsnyder.org/blogs/bjsapbiology/2012/08/12/906/hydrophilic-properties-of-matter-water-and-vinegar/

Buffers Buffers are important because they minimize changes in pH by preventing wild swings in H+ and OH- concentration. A common buffer in the human body is carbonic acid. H2O + CO2 <-> H2CO3 <-> HCO3- + H+ 28

Le Chatlier’s Principle When the pH of our blood changes, it can have deadly consequences. So, in accordance with Le Chatelier’s principle, the above buffer system acts to alter these changes in pH. H2O + CO2 <-> H2CO3 <-> HCO3- + H+ 29

Le Chatlier’s Principle When the pH increases, the reaction shifts to the right to lower the pH back to an acceptable level. When the pH decreases, the reaction shifts left in an attempt to normalize the pH. H2O + CO2 <-> H2CO3 <-> HCO3- + H+ 30