2.9 Water is a polar molecule THE PROPERTIES OF WATER 2.9 Water is a polar molecule Atoms in a covalently bonded molecule may share electrons equally, creating a nonpolar molecule If electrons are shared unequally, a polar molecule is created

In a water molecule, oxygen exerts a stronger pull on the shared electrons than hydrogen This makes the oxygen end of the molecule slightly negatively charged The hydrogen end of the molecule is slightly positively charged Water is therefore a polar molecule (–) (–) O H H (+) (+) Figure 2.9

2.10 Overview: Water’s polarity leads to hydrogen bonding and other unusual properties The charged regions on water molecules are attracted to the oppositely charged regions on nearby molecules This attraction forms weak bonds called hydrogen bonds Hydrogen bond Figure 2.10A

Like no other common substance, water exists in nature in all three physical states: as a solid as a liquid as a gas The Hydrogen bonds between water molecules gives water many important properties Figure 2.10B

2.11 Hydrogen bonds make liquid water cohesive Due to hydrogen bonding, water molecules can move from a plant’s roots to its leaves Insects can walk on water due to surface tension created by cohesive water molecules Figure 2.11

2.12 Water’s hydrogen bonds moderate temperature It takes a lot of energy to disrupt hydrogen bonds Therefore water is able to absorb a great deal of heat energy without a large increase in temperature As water cools, a slight drop in temperature releases a large amount of heat

This leads to evaporative cooling A water molecule takes a large amount of energy with it when it evaporates This leads to evaporative cooling Figure 2.12

2.13 Ice is less dense than liquid water Molecules in ice are farther apart than those in liquid water Hydrogen bond ICE Hydrogen bonds are stable LIQUID WATER Hydrogen bonds constantly break and re-form Figure 2.13

Ice is therefore less dense than liquid water, which causes it to float If ice sank, it would seldom have a chance to thaw Ponds, lakes, and oceans would eventually freeze solid

2.14 Water is a versatile solvent Solutes whose charges or polarity allow them to stick to water molecules dissolve in water They form aqueous solutions Na+ – – Na+ + + Cl– Cl– – – + + – Ions in solution Salt crystal Figure 2.14

2.15 The chemistry of life is sensitive to acidic and basic conditions A compound that releases H+ ions in solution is an acid, and one that accepts H+ ions in solution is a base Acidity is measured on the pH scale: 0-7 is acidic 8-14 is basic Pure water and solutions that are neither basic nor acidic are neutral, with a pH of 7

(Higher concentration of H+) (Lower concentration of H+) pH scale The pH scale H+ OH– Lemon juice; gastric juice (Higher concentration of H+) Increasingly ACIDIC Grapefruit juice Acidic solution Tomato juice Urine NEUTRAL [H+] = [OH–] PURE WATER Human blood Seawater Neutral solution (Lower concentration of H+) Increasingly BASIC Milk of magnesia Household ammonia Household bleach Oven cleaner Basic solution Figure 2.15

Cells are kept close to pH 7 by buffers Buffers are substances that resist pH change They accept H+ ions when they are in excess and donate H+ ions when they are depleted Buffers are not foolproof

2.16 Connection: Acid precipitation threatens the environment Some ecosystems are threatened by acid precipitation Acid precipitation is formed when air pollutants from burning fossil fuels combine with water vapor in the air to form sulfuric and nitric acids Figure 2.16A

These acids can kill fish, damage buildings, and injure trees Regulations, new technology, and energy conservation may help us reduce acid precipitation Figure 2.16B

2.17 Chemical reactions rearrange matter REARRANGEMENTS OF ATOMS 2.17 Chemical reactions rearrange matter In a chemical reaction: reactants interact atoms rearrange products result Figure 2.17A 2 H2 + O2  2 H2O

Living cells carry out thousands of chemical reactions that rearrange matter in significant ways Beta-carotene Vitamin A (2 molecules) Figure 2.17B