Chapter 3 Water

Why is Water Important to Living Things? Water is the substance that makes possible life as we know it here on Earth Covers ¾ of the Earth’s Surface Makes up 70-95% of cells; most cells are surrounded by water Only common substance to exist in all three physical states of matter – Solid, Liquid, and Gas – in the natural environment.

Polarity and Hydrogen Bonding The O and H in a water molecules are held together by polar covalent bond Polarity – Unequal charge distribution due to differences in electronegativity. O is more electronegative than H. Hydrogen Bonding – Weak, intermolecular, transient attraction Each water molecule can make up to 4 bonds with other water molecules

Cohesion, Adhesion Cohesion – Hydrogen bonds hold water molecules together. One water molecule can bond with up to 4 other water molecules at one time. Adhesion – Allow water molecules to adhere to hydrophilic surfaces. Benefit to Life Transport of water through the xylem of plants against gravity.

Water Cohesion/Adhesion Allows water to move as continuous column upward through stems of plants

Surface Tension Benefit to Life Surface Tension – a measure of how difficult it is to stretch or break the surface of a liquid Water has a greater surface tension than most other liquids At the interface between water and air is an ordered arrangement of water molecules, hydrogen-bonded to one another and to the water below Benefit to Life Some animals can stand, walk, or run on water without breaking the surface

Heat and Temperature Anything that moves has kinetic energy, the energy of motion The faster a molecule moves, the greater its kinetic energy. Heat is a measure of the total quantity of kinetic energy due to molecular motion Temperature measures the intensity of heat due to the average kinetic energy of the molecules – when the average speed of the molecules increases, a thermometer records this as a rise in temperature

High Specific Heat of Water Amount of heat that must be absorbed or lost for 1g of substance to change its temp. by 1oC For water, the specific heat is 1cal/g/oC Water resists changes in its temp. Heat is absorbed to break H-bonds; released when bonds form.

High Specific Heat Benefit to Life Large bodies of water (i.e. oceans) can absorb and store a huge amount of heat. In winter, heat lost to the air can warm it and make coastal areas milder in climate than inland regions. Stabilizes temperatures – favorable environment for life – keeps land/water temperatures within limits that permit life. Living things are mostly water – resists change in temperature and maintains somewhat constant temp.

High Heat of Vaporization Energy required to change 1g of substance from a liquid to a gas. Water has a high heat of vap. – it takes a lot of energy to vaporize it. (580 cal at 25oC for 1 g of water) An increase in heat breaks H-bonds releasing molecules to a gaseous state. Benefit to Life The solar heat absorbed by tropical seas causes evaporation of water. As the moist air travels towards the poles it condenses to form rain

Evaporative Cooling Benefit to Life Water molecules with high kinetic energy (the “hottest” ones) evaporate (change from a liquid to a gas); remaining molecules are cooler. Benefit to Life Stabilizes temperatures in lake and ponds. Prevent terrestrial organisms from overheating. High humidity prevents evaporation and sweat and makes us more uncomfortable.

Ice Floats Benefit to Life Water expands as it freezes; less dense as a solid (fewer molecules for same volume) Hydrogen bonds in ice keep the molecules far enough apart to make ice less dense than liquid water at 4oC or above. Benefit to Life Floating ice insulates bodies of water so they don’t freeze solid – keeps the water below the ice from the colder air

Ice Floats

Versatile Solvent Polar water molecules cause ions and other polar solutes to dissolve by ionization.

Keeping Ions Separate = Dissolving Power Spheres of Hydration Na+ Cl– – + Keeping Ions Separate = Dissolving Power

Versatile Solvent Benefit to Life Solution – a liquid that is a completely homogeneous mixture of two or more substances Solvent – the dissolving agent of a solution Solute – the substance that is dissolved Aqueous solution- solution where water is the solvent Benefit to Life Most chemical reactions in living things involve solutes dissolved in water

To Dissolve or Not to Dissolve? Hydrophilic: A substance with an affinity/able to dissolve in water. What kind of substances? Hydrophobic: A substance that repels water/don’t dissolve.

Dissociation/Ionization 2H2O H3O + OH- H2O H+ + OH- In pure water, one water molecule in every 554 million is dissociated. The pH scale is based on the concentrations of H+ and OH- in solution

Acids and Bases Acid – when dissolved in water, add H+ ions to the solution. HCl H+ + Cl- pH< 7 Base – reduces the hydrogen ion concentration of a solution; work by either accepting hydrogen ions or dissociating to form hydroxide ions. NaOH Na+ + OH- pH>7

An Important Biological Buffer Buffer – minimize changes in concentration of H+ and OH- ions in a solution Buffers normally maintain the pH of human blood very close to 7.4 Work by accepting H+ ions from the solution when they are in excess and donating H+ ions to a solution when they have been depleted H2CO3 HCO3- + H+ Carbonic Acid Bicarbonate ion Hydrogen ion H+ Donor (acid) H+ Acceptor (base) Response to a rise in pH Response to a drop in pH

Carbonic Acid-Bicarbonate Buffer System When blood pH rises, carbonic acid dissociates to form bicarbonate and H+ H2C03 -----> HC03- + H+ When blood pH drops, bicarbonate binds H+ to form carbonic acid HC03- + H+ -----> H2C03

Acid Precipitation Mostly sulfur oxides and nitrogen oxides dissolved in rainwater. Come from fossil fuels burned in factories and automobiles Aquatic animal’s egg and young are vulnerable to low pH levels – can alter the structure of biochemical molecules and prevent them from carrying out essential chemical processes of life.

Acid Precipitation