Life depends on hydrogen bonds in water. Water is the most abundant compound in living things. Some of water’s properties that facilitate an environment for life are: Cohesive and adhesive behavior Ability to moderate temperature (high specific heat) Expansion upon freezing Versatility as a solvent http://www.sumanasinc.com/webcontent/animations/content/propertiesofwater/water.html

Polarity & Hydrogen Bonding in Water The water molecule is a polar molecule: the opposite ends have opposite charges. Slightly negative and slightly positive charge Water is polar because the oxygen atom has a slightly negative charge and the hydrogen atoms have slightly positive charges. Water shares electrons unequally! Polarity allows water molecules to form hydrogen bonds with each other. These are weak covalent bonds. Ionic > Covalent > Hydrogen 3

Cohesion & Adhesion Cohesion Adhesion Hydrogen bonds hold water molecules together, cohesion: Cohesion is the attraction of molecules of like substance. Water molecules attracted to other water molecules Examples: Water forming beads on a car, puddles of water, tear drops The clinging of one substance to a different substance Water molecules stick to other things. Examples: Water sticking to glass mirror, water sticking to your skin 4

Cohesion + Adhesion = Capillary Action Cohesion and adhesion work together to give capillarity – the ability of water to spread through fine pores or to move upward through narrow tubes against the force of gravity. Cohesion due to hydrogen bonding contributes to the transport of water and dissolved nutrients against gravity in plants. Adhesion of water to cell walls by hydrogen bonds helps to counter the downward pull of gravity on the liquids passing through plants Significance: The process of water moving through plants against the force of gravity.

Adhesion Water-conducting cells Cohesion 150 µm Direction of water movement Cohesion Water transport in plants - transport of water against gravity in plants 150 µm Cohesion and adhesion work together to give capillarity – the ability of water to spread through fine pores or to move upward through narrow tubes against the force of gravity. 6

Surface Tension Surface tension is directly related to the cohesive property of water It is a measurement of how difficult it is to stretch or break the surface of a liquid. Example: Water Strider Significance: allows the water strider to walk on the surface of the pond to escape a predator or find food 7

High Specific Heat Moderation of Temperature Water can absorb or release a large amount of heat with only a slight change in its own temperature. The ability of water to stabilize temperature results from its relatively high specific heat: Water’s high specific heat can be traced to hydrogen bonding. H-Bonds require a lot of energy to break the bonds Significance: Helps minimize temperature fluctuations by regulating the rates of water and air temperatures causing temperatures to gradually change rather than suddenly (Season Changes) A greater input of energy is required to raise the temperature of water than the temperature of air! This minimizes temperature fluctuations in aquatic environments to within limits that permit life. 8

The Density Anomaly Ice floats in liquid water because hydrogen bonds in ice are more “ordered,” making ice less dense. Significance: If ice sank, all bodies of water would eventually freeze solid, making life impossible on Earth. Water reaches its greatest density at 4°C. Due to geometry of water molecule, they must move slightly apart to maintain the max number of H bonds in a stable structure. So at Zero degrees Celsius, an open latticework is formed, allowing air in – thus ice becomes less dense than liquid water floats on top of the water. 9

The Solvent of Life Universal Solvent Water provides living systems with excellent dissolving capabilities. Can dissolve several substances because of its electronegative charge A solution is a liquid that is a homogeneous mixture of substances Solvent (dissolving agent) Greater Amount Solute (substance that is dissolved) Lesser Amount Significance: Breakdown food, Plasma (liquid part of blood) is 95% water allowing sugar and nutrients to dissolve in the water of blood plasma because they are polar 10

Acids and Bases Neutral = pH of 7 neither acidic or basic An acid is any substance that increases the H+ concentration of a solution. Acid pH= below 7 ( 0-6) Neutral = pH of 7 neither acidic or basic A base is any substance that reduces the H+ concentration of a solution. Base pH = above 7 ( 8 – 14) Solution with pH between 0 & 7 is acidic (high H+ concentration). Solution with pH between 7 & 14 is basic (low H+ concentration). Solution with pH of 7 is neutral – neither acidic nor basic. Most living organisms need to keep their pH within a very narrow range around neutral (pH 7). In the human body, both the respiratory system and the excretory system help regulate pH. 11

Figure 3.9 The pH scale and pH values of some aqueous solutions 1 Battery acid Gastric juice, lemon juice 2 H+ H+ OH– H+ H+ 3 Vinegar, beer, wine, cola OH– H+ H+ Increasingly Acidic [H+] > [OH–] H+ H+ 4 Acidic solution Tomato juice Black coffee 5 Rainwater 6 Urine OH– Saliva OH– Neutral [H+] = [OH–] H+ H+ OH– 7 Pure water OH– OH– H+ Human blood, tears H+ H+ 8 Seawater Neutral solution 9 Figure 3.9 The pH scale and pH values of some aqueous solutions 10 Increasingly Basic [H+] < [OH–] Milk of magnesia OH– OH– 11 OH– H+ OH– Household ammonia OH– OH– H+ OH– 12 Basic solution Household bleach 13 Oven cleaner 14 12

Water Summary Video http://vimeo.com/37064053