1. Lesson Overview Lesson Overview Properties of Water Lesson Overview 2.2 Properties of Water

2. Lesson Overview Lesson Overview Properties of Water THINK ABOUT IT Looking back at Earth from space, an astronaut called it “the blue planet,” referring to the oceans of water that cover nearly three fourths of Earth’s surface. The very presence of liquid water tells a scientist that life may also be present on such a planet. Why should life itself be connected so strongly to something so ordinary that we often take it for granted? There is something very special about water and the role it plays in living things.

3. Lesson Overview Lesson Overview Properties of Water The Water Molecule Water is one of the few compounds found in a liquid state over most of Earth’s surface. Like other molecules, the water molecule (H 2 O) is neutral. The positive charges on its 10 protons (8 in oxygen, 1 in each hydrogen) balance out the negative charges of its 10 electrons (8 in oxygen, 1 in each hydrogen).

4. Lesson Overview Lesson Overview Properties of Water Polarity Because of the angles of its bonds, the oxygen atom is on one end of the molecule and the hydrogen atoms are on the other. With 8 protons (8+) in its nucleus, an oxygen atom has a much stronger attraction for electrons (-) than does a hydrogen atom with its single proton (1+). This means that, as the electrons move around in space, they move around the oxygen atom a greater percentage of the time than they do the hydrogen atoms.

5. Lesson Overview Lesson Overview Properties of Water Polarity There is a greater probability, then, of finding the electrons shared by oxygen and hydrogen closer to the oxygen atom than either hydrogen atom. As a result, the oxygen end of the molecule develops a slight negative charge and the hydrogen ends of the molecule develops a slight positive charges.

6. Lesson Overview Lesson Overview Properties of Water Polarity A molecule in which the charges are unevenly distributed is said to be “polar,” because the molecule is a bit like a magnet with two poles. The charges on a polar molecule are written in parentheses, (–) or (+), to show that they are weaker than the charges on ions such as Na + and Cl –.

7. Lesson Overview Lesson Overview Properties of Water Hydrogen Bonding Because of their partial positive and negative charges, polar molecules such as water can attract each other. The attraction between a hydrogen atom on one water molecule and the oxygen atom on another is known as a hydrogen bond.

8. Lesson Overview Lesson Overview Properties of Water Hydrogen Bonding Because water is a polar molecule, it is able to form multiple hydrogen bonds, which account for many of water’s special properties. Hydrogen bonds are not as strong as covalent or ionic bonds, and they can form in other compounds besides water.

9. Lesson Overview Lesson Overview Properties of Water Cohesion Cohesion is an attraction between molecules of the same substance. Because a single water molecule may be involved in as many as four hydrogen bonds at the same time, water is extremely cohesive.

10. Lesson Overview Lesson Overview Properties of Water Cohesion Cohesion is the reason water has a high surface tension. Water molecules at the surface of water have a greater attraction to each other than the molecules below. They create a kind of “skin” at the water’s surface that is tough to break through..

11. Lesson Overview Lesson Overview Properties of Water Cohesion The high surface tension of water explains why some insects and spiders can walk on the surface of water. It is also the reason why water will form beads on a smooth surface.

12. Lesson Overview Lesson Overview Properties of Water Adhesion Adhesion is an attraction between molecules of different substances. The surface of water in a graduated cylinder dips slightly in the center, forming a curve called a meniscus, because the adhesion between water molecules and glass molecules is stronger than the cohesion between water molecules.

13. Lesson Overview Lesson Overview Properties of Water Adhesion Adhesion between water and glass also causes water to rise in a narrow tube against the force of gravity. This effect is called capillary action. Capillary action is one of the forces that draws water out of the roots of a plant and up into its stems and leaves. Cohesion holds the column of water together as it rises.

14. Lesson Overview Lesson Overview Properties of Water Heat Capacity Because of the multiple hydrogen bonds between water molecules, it takes a large amount of heat energy to cause those molecules to move fast enough to raise the temperature of the water. Water’s heat capacity, the amount of heat energy required to increase its temperature, is, therefore, relatively high. Large bodies of water, such as oceans and lakes, can absorb large amounts of heat with only small changes in temperature. This protects organisms living within from drastic changes in temperature. At the cellular level, water absorbs the heat produced by cell processes, regulating the temperature of the cell.

15. Lesson Overview Lesson Overview Properties of Water Solutions and Suspensions Water is not always pure; it is often found as part of a mixture. A mixture is a material composed of two or more elements or compounds that are physically mixed together but not chemically combined. Living things are in part composed of mixtures involving water. Two types of mixtures that can be made with water are solutions and suspensions.

16. Lesson Overview Lesson Overview Properties of Water Solutions If a crystal of table salt (NaCl) is placed in water, sodium and chloride ions on the surface of the crystal are attracted to the polar water molecules. The ions break away from the crystal and are surrounded by water molecules. The “hydrated ions” gradually become dispersed in the water, forming a type of mixture called a solution.

17. Lesson Overview Lesson Overview Properties of Water Solutions In a solution, all of the components are evenly or uniformly distributed throughout the solution. The substance that is dissolved (and is smaller in amount) is called the solute. In salt water, the solute is the salt. Water is the solvent—the substance in which the solute dissolves.

18. Lesson Overview Lesson Overview Properties of Water Solutions Water’s polarity gives it the ability to dissolve both ionic compounds and other polar molecules. Water is often called a universal solvent because it easily dissolves salts, sugars, minerals, gases, and even other solvents such as alcohol. When a given amount of water has dissolved all of the solute it can, the solution is said to be saturated.

19. Lesson Overview Lesson Overview Properties of Water Colloids** ** Please note that the Bio. textbook incorrectly uses the term Suspension. Some materials do not dissolve when placed in water, but separate into pieces so small that they do not settle out. Such mixtures of water and nondissolved material are known as colloids. Some of the most important biological fluids are both a solution and a colloid. Blood plasma is mostly water. It contains many dissolved compounds, but also other undissolved particles that remain in the blood as it moves through the body.

20. Lesson Overview Lesson Overview Properties of Water Acids, Bases, and pH Water exists in nature as a mixture of water molecules, hydrogen ions (H+), and hydroxide ions (OH - ). Some water molecules “ionize” to form this mixture.

21. Lesson Overview Lesson Overview Properties of Water Acids, Bases, and pH In pure water, about 1 water molecule in 550 million splits to form ions in this way. Because the number of positive hydrogen ions produced is always going to equal to the number of negative hydroxide ions produced, pure water is neutral.

22. Lesson Overview Lesson Overview Properties of Water The pH Scale Chemists devised a measurement system called the pH scale to indicate the concentration of H + ions in solution. The pH scale ranges from 0 to 14. At a pH of 7, the concentration of H + ions and OH – ions is equal. Pure water has a pH of 7.

23. Lesson Overview Lesson Overview Properties of Water If a solution is created by adding a compound containing hydrogen ions to water, an acidic solution is formed. Such solutions are called acidic because they have more H + ions than OH – ions. Acidic solutions have a pH below 7. In fact, the lower the pH, the greater the acidity. Acids  ---Acidity Increases-------- -------Basicity Increases-----   

24. Lesson Overview Lesson Overview Properties of Water Bases Solutions made by adding compounds with hydroxide ions to water form basic solutions. Such solutions are called basic because they have more OH – ions than H + ions. Bases have a pH above 7. The higher the pH, the more basic the solution.

25. Lesson Overview Lesson Overview Properties of Water The pH Scale Each step on the pH scale represents a factor of 10. For example, a liter of a solution with a pH of 4 has 10 times as many H + ions as a liter of a solution with a pH of 5.

26. Lesson Overview Lesson Overview Properties of Water Acids In terms of compounds, an acid is any compound that forms H + ions in solution. Recall that acidic solutions contain higher concentrations of H + ions than pure water and have pH values below 7. Strong acids tend to have pH values that range from 1 to 3. Hydrochloric acid (HCl) is a strong acid produced by the stomach to help digest food.

27. Lesson Overview Lesson Overview Properties of Water Bases A base is a compound that produces hydroxide (OH – ) ions in solution. Recall that basic, or alkaline, solutions contain lower concentrations of H + ions than pure water and have pH values above 7. Strong bases, such as the lye (commonly called sodium hydroxide, NaOH) used in soapmaking, tend to have pH values ranging from 11 to 14.

28. Lesson Overview Lesson Overview Properties of Water Buffers The pH of the fluids within most cells in the human body must be kept between 6.5 and 7.5. This is the tolerance range in pH for humans. If the pH is lower or higher, it will affect the chemical reactions that take place within the cells. One of the ways that organisms can control pH is through dissolved compounds called buffers, which are weak acids or bases that can react with strong acids or bases to prevent sharp, sudden changes in pH.

29. Lesson Overview Lesson Overview Properties of Water Buffers Adding acid to an unbuffered solution causes the pH of the unbuffered solution to drop dramatically. If the solution contains a buffer, however, adding the acid will cause only a slight change in pH. Buffers dissolved in life’s fluids play an important role in maintaining homeostasis in organisms.