1. The Biological Medium on Earth

2.  To get the most out of these notes, you MUST read along with pages 47-56 in the textbook  Not everything in these pages are covered in the powerpoint, only main points.  To understand these points, you should read along with the textbook as it elaborates on, and explains the main points presented in this powerpoint presentation.

3.  All living organisms require water more than any other substance  Water molecules participate in many chemical reactions needed to sustain life  Most cells are surrounded by water, and are composed of 70-95% water themselves  ¾ of the Earth’s surface is submerged in water

4.  H 2 O: two hydrogen atoms connected to an oxygen atom by single covalent bonds  Oxygen is more electronegative than hydrogen (has a stronger attraction for the electrons of a covalent bond), so the electrons of the polar bonds spend more time closer to the oxygen atom  In other words, the bonds that hold together the atoms in a water molecule are polar covalent bonds  The water molecule is a polar molecule, meaning the ends of the molecule have opposite charges

5.  The oxygen end of the molecule has a partial negative charge, and the hydrogens have a partial positive charge

6.  The attraction is electrical; The slightly positive hydrogen of one water molecule is attracted to the slightly negative oxygen of a nearby water molecule  Why is it unlikely that two neighbouring water molecules would be arranged like this? H H / \ 0 0 \ / H H

7.  1. Cohesion  2. Moderation of Temperature  3. Insulation of Bodies of Water by Floating Ice  4. The Solvent of Life

8.  In liquid form, hydrogen bonds are fragile  Collectively, the hydrogen bonds hold the substance together, a phenomenon called cohesion  Adhesion: the clinging of one substance to another  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

9.  Atoms and molecules have kinetic energy (the energy of motion) because they are always moving  Heat is a measure of the total amount of kinetic energy due to molecular motion in a body of matter  Temperature measures the intensity of heat due to the average kinetic energy of the molecules

10.  When two objects of different temperature come into contact, heat passes from the warmer to the cooler object until they are the same temperature  Molecules in the cooler object speed up (at the expense of the kinetic energy of the warmer object)  Calorie: a unit of heat. The amount of heat it takes to raise the temp. of 1g of water by 1 o C

11.  Specific heat: the amount of heat that must be absorbed or lost for 1g of that substance (in this case, water) to change its temperature by 1 o C  The specific heat of water is 1cal/g/ o C  Compared to other substances, water has a high specific heat capacity  Large bodies of water can absorb and store a huge amount of heat from the sun

12.  Heat of vaporization: the amount of heat a liquid must absorb for 1g of it to be converted from the liquid to gaseous state  Water has a high heat of vaporization compared to other liquids, caused by hydrogen bonds (which must be broken before the molecules can make their exodus from the liquid)  Water’s high heat of vaporization helps moderate Earth’s climate

13.  As a liquid evaporates, the surface of the liquid that remains behind cools down  This evaporative cooling occurs because the “hottest” molecules (those with the greatest kinetic energy) are the most likely to leave as gas  Evaporative cooling contributes to the stability of temperature in lakes and ponds

14.  Water is less dense as a solid than as a liquid  So, ice floats in water  Water expands when it solidifies, again, a result of hydrogen bonding  If ice sank, all water bodies would freeze, killing all the life inside them

15.  Solution: a liquid that is a completely homogeneous mixture of two substances  Solvent: the dissolving agent of a solution  Solute: the substance that is dissolved  Aqueous solution: water is the solvent  Water is the most versatile solvent, due to its polarity  Hydrogen shell: the sphere of water molecules around each dissolved ion

16.  Hydrophilic: a substance that has an affinity for water (“water-loving”)  Some substances are hydrophilic, but do not dissolve. They remain suspended in the aqueous liquid (this is called a “colloid”  a stable suspension of fine particles in a liquid)  Hydrophobic: a substance that repels water due to being nonionic and nonpolar (“water- fearing”). An example is vegetable oil.

17.  Molecular mass: the sum of the masses of all the atoms in a molecule. For example, in sucrose: C 12 H 22 O 11, the mass of a carbon atom is 12, hydrogen is 1, and oxygen is 16. So, sucrose has a molecular weight of: 12(12) + 1(22) + 11(16) = 342 daltons (atom masses given in daltons rounded to whole numbers)  Substances are often measured in units called Moles (mol), which represents and exact number of objects  Molarity: the number of moles of solute per litre of solution

18.  An acid: a substance increases the hydrogen ion concentration of a solution  A base: a substance that reduces the hydrogen ion concentration of a solution  pH scale: the pH of a solution is defined as the negative logarithm (base 10) of the hydrogen ion concentration: pH = -log[H + ]  Buffers are substances that minimize changes in ion concentration (H + and OH - )  Acid precipitation has a pH lower or more acidic than pH=5.6