Water

1. Water The Biological Medium on Earth

2. These are basic notes only!!!! • 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. The Molecule that Supports All Life • 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. Polarity and Hydrogen Bonding • H2O: 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. Structure of a Water Molecule • The oxygen end of the molecule has a partial negative charge, and the hydrogens have a partial positive charge

6. Hydrogen Bonding • 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. Four Properties of Water • 1. Cohesion • 2. Moderation of Temperature • 3. Insulation of Bodies of Water by Floating Ice • 4. The Solvent of Life

8. 1. Cohesion • 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. 2. Moderation of Temperature • 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. Moderation of Temperature • 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 1oC

11. Specific Heat Capacity of Water • 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 1oC • The specific heat of water is 1cal/g/oC • 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. Evaporative Cooling • 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. Evaporative Cooling Cont’d • 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. 3. Insulation of Bodies of Water by Floating Ice • 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. 4. The Solvent of Life • 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. Dissociation leads to Acidic and Basic conditions • 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. Molarity and Molecular Mass • Molecular mass: the sum of the masses of all the atoms in a molecule. For example, in sucrose: C12H22O11, 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. Acid Precipitation • 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