Chemistry of Life: Water and Solutions

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1 Chemistry of Life: Water and Solutions

2 Unit Objective I can describe the role of organic and inorganic chemicals important to living things.

3 During this unit, we will answer the following very important questions:

4 #1: What is the most important inorganic chemical to living things, and why is it important?

5 #2: What are the most important organic chemicals to living things, and how are they important to the function of living things?

6 #3: What are the monomers of these organic chemicals, and how are they important in the function of living things?

7 We will continue our study in this unit by focusing on water and solutions. To do this, we will learn how to analyze the properties of water describe how water dissolves substances distinguish between acids and bases

8 One of the properties of water is its ability to store energy.

9 Water absorbs heat more slowly,

10 and retains this energy longer than most other substances.

11 Many organisms release excess heat from their bodies through evaporation of water from their bodies.

12 Because water contains absorbed energy, the evaporation of water from the body carries heat away from the body.

13 This ability by cells to control the temperature of an organism helps them maintain a constant internal temperature when the external temperature changes drastically.

14 This ability of water to store energy thus helps cells maintain homeostasis in an organism.

15 Another property of water that is important to life processes involves the processes of cohesion and adhesion.

16 The hydrogen bonds between water molecules cause the cohesion of liquid water, the attraction between substances of the same type (in this case, water molecules).

17 Because of cohesion, water and other liquids form thin films and droplets.

18 This attraction of water molecules for one another causes a phenomenon known as surface tension, the prevention of stretching and breaking of the surface of water.

19 Water is attracted not only to itself, however, but also to many other substances.

20 Adhesion is the attraction between different substances.

21 This attraction of water molecules to other different substances powers the process of capillary action, the process by which water spontaneously moves upward through narrow tubes, like the stems of plants.

22 The attraction of water molecules to the molecules inside of a plant stem cause the water to be pulled up the inside of the stem more strongly than the force of gravity on the water molecules can pull them down.

23 Water is thus moved upward through a plant from the roots of the plant to the leaves of a plant through a combination of capillary action, cohesion, and other factors.

24 Hydrogen bonds in ice, unlike those in liquid water molecules, form a crystalline structure.

25 The water molecules in an ice crystal are spaced farther apart, whereas in liquid water, they are very close together.

26 A fixed volume of ice, then, contains fewer water molecules than does the same volume of liquid water.

27 Because ice contains fewer water molecules than liquid water, ice is less dense than water.

28 REVIEW: PROPERTIES OF WATER 1. Water is capable of storing energy. 2. Water is capable of cohesion and adhesion.

29 Another important property of water is that is a universal solvent.

30 Because water is a polar molecule, many substances are capable of dissolving in water.

31 Ionic compounds and polar molecules dissolve best in water.

32 When ionic compounds are dissolved in water, for example, the ions become surrounded by polar water molecules.

33 The ions with their positive and negative electrical charges are attracted to the ends of water molecules with the opposite charge.

34 The resulting solution is a mixture of water molecules and ions.

35 A similar attraction results when polar molecules are dissolved in water.

36 In both cases, the ions or molecules become evenly distributed throughout the water.

37 And now, for some new vocabulary!

38 A solvent is a substance in which another substance is dissolved. If salt is dissolved in water, for example, the water would be the solvent.

39 A solute is a substance dissolved in a solvent. In this case, salt is the solute.

40 The resulting combination of a solute dissolved in a solvent is a solution, a mixture in which one or more substances (solutes) is evenly distributed throughout another substance (the solvent).

41 An aqueous solution is a solution in which water is the solvent.

42 Many important substances in the body are dissolved in blood and other aqueous solutions.

43 Because ions and polar molecules are easily dissolved in water, they are better able to move into, out of, and between cells.

44 Nonpolar molecules do not dissolve well in water.

45 When nonpolar molecules are added to water, the polar water molecules are more strongly attracted to one another than to the nonpolar molecules.

46 As a result, the nonpolar molecules are forced out of the water, isolating them from the water.

47 This inability of nonpolar molecules to be dissolved in water is important to organisms.

48 The shape and function of cell membranes, for example, depends on the interaction of water molecules and the nonpolar molecules in cell membranes.

$fraction of those bonds might break, forming hydrogen ions (H + ) and hydroxide ions$

49 While the chemical bonds in water molecules are strong, at any given time, a tiny fraction of those bonds might break, forming hydrogen ions (H + ) and hydroxide ions (OH - ).

50 As a result, pure water always has a low concentration of hydrogen and hydroxide ions, present in equal numbers.

51 Compounds that form hydrogen ions when dissolved in water (thus adding to the hydrogen ions already present in water) are called acids.

52 When acids themselves are added to water, the concentration of hydrogen ions in the solution is increased above that already present in water.

53 On the other hand...compounds that reduce the concentration of hydrogen ions in solution are called bases.

54 Many bases form hydroxide ions when dissolved in water.

55 Such bases effectively lower the concentration of hydrogen ions in solution because hydroxide ions combine with hydrogen ions in solution to form water molecules, reducing the number of hydrogen ions in solution and increasing the number of water molecules.

56 Scientists use a tool called the ph scale to determine how acidic or basic a solution is.

57 This scale is based on the concentration of hydrogen ions in a solution.

58 All solutions have a ph of between 0 and 14.

59 Pure water (with an equal number of hydrogen and hydroxide ions in solution) has a ph of exactly 7.0.

60 Solutions containing more hydrogen than hydroxide ions are said to be acidic, and have ph values less than 7.

61 The more hydrogen ions a solution has, the more strongly acidic it is, and the lower its ph value (i.e., the strongest acids have very low ph values).

62 Solutions containing less hydrogen than hydroxide ions are said to be basic or alkaline, and have ph values greater than 7.

63 The more hydroxide ions a solution has, the more strongly alkaline it is, and the higher its ph value (i.e., the strongest bases have very high ph values).

64 The ph scale is a logarithmic scale; i.e., each whole number on the scale represents a power of 10.

65 This means that a solution with a ph of 5 has 100 times less hydrogen ions than a solution with a ph of 3,

whereas a solution with a ph of 9 has 10 000 times less hydrogen

66 whereas a solution with a ph of 9 has times less hydrogen ions (and times more hydroxide ions) than the solution of ph 5.

Atoms, Molecules, and Life

1 Atoms, Molecules, and Life The Nature of Matter: Atoms and Molecules Elements All matter is composed of chemical elements, substances that cannot be broken down to other substances by ordinary chemical