2.2 Properties of Water WATER, H20 • The “blue planet” • There is something very special about water and the role it plays in living things. • Imagine our world without it!
You should know: • How does the structure of water contribute to its unique properties? • How does water’s polarity influence its properties as a solvent? • Why is it important for cells to buffer solutions against rapid changes in pH • Discuss the unique properties of water • Differentiate between solutions and suspensions • Explain what acidic solutions and basic solutions are
The Water Molecule • Water is one of the few compounds found in a liquid state over most of Earth’s surface. • Like other molecules, water (H2O) is neutral. The positive charges on its 10 protons balance out the negative charges on its 10 electrons. • 1 Oxygen: _______ Protons, _________ Electrons • 2 Hydrogen: _______ Protons, _________ Electrons • TOTAL: ________Protons • ________ Electrons water is neutral, no charge
Polarity • O wants 10 e- to be happy , so if it joins with hydrogen, they can share! • Because of the angles of its chemical bonds: (covalent bonds) • -oxygen atom- on one end of the molecule • -hydrogen atoms-on the other end 8 protons ( ) ) ( ( ) 1 proton 1 proton
Polarity • 8 protons in O more attractive to electrons than a single proton in H…so shared electrons hang out more often near the O nucleus. • As a result, the oxygen end of the molecule has a slight negative charge and the hydrogen end of the molecule has a slight positive charge. ( ) ( ) ( )
Polarity I am water and I am polar! • 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–. ( ) ) ( ( )
Hydrogen Bonding (+) (+) • Because of their partial positive and negative charges, polar molecules, such as water, 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. • Every molecule of water connected to another molecule of water by a hydrogen bond! (-) (+) (-) (-) (+) (+) (-) (+) (+)
Hydrogen Bonding • Water is able to form multiple hydrogen bonds, which account for many of its special properties. • Hydrogen bonds are not as strong as covalent or ionic bonds. • Can form in other compounds besides water.
The Water Molecule • How does the structure of water contribute to its unique properties? • Answer: • Because water is a polar molecule, it is able to form multiple hydrogen bonds with other water molecules, which account for many of water’s special properties.
Cohesion • Cohesionis 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. • Water “sticks” together, until its hydrogen bonds are broken.
Cohesion • Causes water molecules to be drawn together, which is why drops of water form beads on a smooth surface. • Cohesion also produces surface tension, explaining why some insects and spiders can walk on a pond’s surface. • surface tension is a characteristic of water = a “skin-like” layer on top of the water. • Ex: Belly flop hurts so bad b/c have to break surface tension • -Spiders can “walk” on water, distribute their weight • -Paper clip can float on water, even though more dense
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.
Adhesion • Adhesion between water and glass also causes water to rise in a narrow tube against the force of gravity = 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.
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 faster and raise the temperature of the water. • Water’s heat capacity, the amount of heat energy required to increase its temperature, is 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.
Solutions and Suspensions • Water is not always pure; it is often found as part of a mixture. • A mixture = 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: • 1) solutions • 2) suspensions
Solutions • Sodium and chloride ions in salt crystal are attracted to the polar water molecules. • Ions break away from crystal, surrounded by water molecules. • The ions gradually become dispersed in the water, forming a type of mixture called a solution. • Solution = type of mixture in which ALL the components are evenly distributed
Solutions • All the components of a solution are evenly distributed throughout the solution. • In a saltwater solution: • Table salt is the solute— • the substance that is dissolved • Water is the solvent— • the substance in which the solute dissolves • does the dissolving
Solutions • Water’s polarity gives it the ability to dissolve both ionic compounds and other polar molecules. • Water easily dissolves salts, sugars, minerals, gases, and even other solvents such as alcohol.
Suspensions • Some materials do not dissolve when placed in water • --separate into pieces so small that they do not settle out • -- movement of water • Such mixtures of water and nondissolved material are known as suspensions. • Some of the most important biological fluids are both solutions and suspensions. • Ex: Blood is mostly water. It contains many dissolved compounds, but also cells and other undissolved particles that remain in suspension as the blood moves through the body.
Solutions and Suspensions • How does water’s polarity influence its properties as a solvent? • Water’s polarity gives it the ability to dissolve both ionic compounds and other polar molecules. • http://programs.northlandcollege.edu/biology/Biology1111/animations/dissolve.html
Acids, Bases, and pH • Special circumstances when you put something in water and it causes the solution to: • Taste sour • Turn Litmus paper Red • Litmus paper---a special paper made from lichens. = AN ACID • Other times, when you put something in water, it causes the solution to: • Taste bitter • Turn Litmus paper Blue = A BASE
Another test: The pH Scale • Another type of treated paper is pH paper. • Can tell you if a substance is acidic or basic, ac acid or a base. • pH = Potential Hydrogen • The more H+ released in a solution, the more acidic the solution. • The less H+ released in a solution, the more basic the solution • (H+ means Hydrogen ions, Hydrogen lost its e-)
Another test: The pH Scale • Chemists devised a measurement system called the pH scale to indicate how acidic or how basic a solution. • Essentially, measures the amount of H+ in the solution. • 1-14 scale • pH < 7 = ACID • pH > 7 = BASE
Buffers • The pH of the fluids within most cells in the human body must generally be kept between 6.5 and 7.5 in order to maintain homeostasis. 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 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.
Buffers • Adding acid to an unbuffered solution causes the pH of the unbuffered solution to drop. If the solution contains a buffer, however, adding the acid will cause only a slight change in pH.
Acids, Bases, and pH • Why is it important for cells to buffer solutions against rapid changes in pH? • Buffers dissolved in life’s fluids play an important role in maintaining homeostasis in organisms.