The Chemical Basis for Life

1. The Chemical Basis for Life Unique Properties of Water

2. Lesson Objectives • Describe the distribution of Earth’s water and outline the water cycle. • Identify water’s unique properties that support life on Earth. • Identify the chemical structure of water, and explain how it relates to water’s unique properties. • Define solution, and describe water’s role as a solvent. • State how water is used to define acids and bases, and identify the pH ranges of acids and bases. • Explain why water is essential for life processes.

3. Introduction • Water is biologically important… • H2O is a simple molecule, but it has some very unique properties…

4. WATER, WATER EVERYWHERE • Common chemical substance on Earth • ‘Water’ refers to it in a liquid state • Also can ba a solid (ice) or gas (water vapor)

5. Where Is All the Water? • Water covers about 75% of Earth’s surface • 97% of this water is salt water • Only 3% is freshwater • Most of the freshwater is frozen in glaciers and the polar ice caps • About 2% of water is stored in underground spaces

6. How Water Recycles • Water is continuously recycled • Cycle keeps repeating

7. CHEMICAL STRUCTURE AND PROPERTIES OF WATER • Water is tasteless, odorless, and transparent • The transparency of water is important for organisms that live in water. • sunlight can pass through it • sunlight is needed by water plants and other water organisms for photosynthesis

8. Chemical Structure of Water • Chemical formula H2O • Nucleus of the oxygen atom (8 positively charged protons) attracts electrons much more strongly than do the hydrogen nuclei (1 positively charged proton). This results in a negative electrical charge near the oxygen atom (due to the ”pull” of the negatively charged electrons toward the oxygen nucleus) and a positive electrical charge near the hydrogen atoms. • A difference in electrical charge between different parts of a molecule is called polarity.

9. SOLUTIONS • Homogeneous mixture composed of two or more substances • One substance is dissolved (solute) in another substance (solvent), forming a mixture that has the same proportion of substances throughout • The ability of a solute to dissolve in a particular solvent is called solubility. • Many chemical substances are soluble in water; thus water is known as the universal solvent.

10. Sticky, Wet Water • Water has unusual properties due to its hydrogen bonds; water molecules tend to stick together. • If you drop a tiny amount of water onto a very smooth surface, the water molecules will stick together and form a droplet, rather than spread out over the surface; this is called cohesion. • Water also sticks to other substances because of its unique bonding properties; this is called adhesion.

11. Cohesion • Cohesion is the attraction of one molecule to a similar molecule. Water molecules form droplets because of this principle.

12. Adhesion • When a molecule is attracted to a different molecule, it is called adhesion. Water sticks to other substances.

13. Surface Tension • Water's surface, molecules pulled from side to side and down only • Result: skin of water at the surface in which the molecules are held tightly together. • Surface tension: measurement of the amount of force required to break this skin on the surface of water. http://www.youtube.com/watch?v=45yabrnryXk

14. Capillarity • Check out the demonstration of capillarity! Use cohesion and adhesion to explain what is happening & WHY!

15. Specific Heat • Water has a high specific heat; takes a lot of energy to raise or lower its temperature • Specific heat is a measure in joules of how much energy it takes to raise the temperature of 1 gram of a substance by 1°C. • Every substance has its own specific heat capacity, the specific heat capacity of water is much higher than that of other common liquid substances.

16. Density of Ice and Water • Water in a solid state has a lower density than water in a liquid state; water expands when it freezes. • Hydrogen bonds cause water molecules to line up less efficiently in ice than in liquid water. As a result, water molecules are spaced farther apart in ice, giving ice a lower density than liquid water. • A substance with lower density floats on a substance with higher density. This explains why ice floats on liquid water, whereas many other solids sink to the bottom of liquid water. • In a large body of water, such as a lake or the ocean, the water with the greatest density always sinks to the bottom. Water is most dense at about 4° C (39.2°F). • In climates with cold winters, this layer of 4° C water insulates the bottom of a lake from freezing temperatures. • Lake organisms such as fish can survive the winter by staying in this cold, but unfrozen, water at the bottom of the lake.

17. WATER AND LIFE • Humans are composed of about 70% water • Water’s ability to dissolve most biologically significant compounds—from inorganic salts to large organic molecules—makes it a vital solvent inside organisms and cells. • As a result, virtually all life processes depend on water. • Without water, life as we know it could not exist

18. WATER ACTS LIKE A BUFFER • Defined: substance that helps to moderate any changes in pH that result from the addition of acids or bases. • Importance: most chemical processes that occur in living organisms are highly sensitive to pH, and drastic changes in pH can cause some serious trouble. • Water, can act like a buffer if there is a sudden change in pH. • HOW you ask????

19. Water disassociates into hydrogen (H+)and hydroxide (OH-) ions constantly. • dissociated water molecules are what give water its buffering ability. • If we add an acid to solution, some of the free OH- ions will bind to the newly added H+ ions, which will moderate the decrease in pH. • If we add a whole bunch of base to the solution, some of the added base will bind to the free H+ ions in solution, which will moderate the increase in pH. • HOWEVER, WATER CANNOT BUFFER EXTREMELY DRASTIC CHANGES IN pH

20. Think of it as a mathematical equation with positive and negative integers: • For example: -13 + 2 = -11 So if you take a base with a pH of 13 (filled with OH- ions) and increase the amount of positive H+ ions in the solution the base’s pH will drop down to 11 making it less alkaline

21. Acids and Bases • Pure water is the solvent in solutions called acids and bases. • In pure water (such as distilled water), a tiny fraction of water molecules naturally breaks down, or dissociates, to form ions. • An ion is an electrically charged atom or molecule. The dissociation of pure water into ions is represented by the chemical equation: 2 H2O → H3O+ + OH-

22. Acids, Bases and pH • The hydronium ions in pure water are balanced by hydroxide ions, so pure water is neutral (neither an acid nor a base). This gives pure water a pH of 7. • If a solution has a higher concentration of hydronium ions and lower pH than pure water, it is called an acid. • If a solution has a lower concentration of hydronium ions and higher pH than pure water, it is called a base.

23. The pH Scale • Ranges from 0 to 14 • The pH scale is a negative logarithmic scale; as the ion concentration increases, the pH value decreases. In other words, the more acidic the solution, the lower the pH value. • Each one-point change in pH reflects a ten-fold change in the hydronium ion concentration and acidity. • For example, a solution with a pH of 6 is ten times as acidic as pure water with a pH of 7. • Acids have a pH less than 7, and bases have a pH greater than 7.

24. Acids • Defined as a hydrogen ion donor • Acids have a sour taste and may sting or burn the skin. • Testing solutions with litmus paper is an easy way to identify acids. Acids turn blue litmus paper red.

25. Bases • Defined as a hydrogen ion acceptor • Bases have a bitter taste and feel slimy to the touch. They can also burn the skin. • Bases, like acids, can be identified with litmus paper. Bases turn red litmus paper blue.

26. Acids and Bases in Organisms • Enzymes do their job within a specific range of pH and are needed to speed up biochemical reactions. • For example, the enzyme pepsin, which helps break down proteins in the human stomach, requires a very acidic environment in order to function. • Strong acid is secreted into the stomach, allowing pepsin to work. • Once the contents of the stomach enter the small intestine, where most digestion occurs, the acid must be neutralized. Enzymes that work in the small intestine need a basic environment. • An organ near the small intestine, called the pancreas, secretes bicarbonate ions (HCO3-) into the small intestine to neutralize the stomach acid.

27. Neutralization • What do you think would happen if you mixed an acid and a base? • ‘‘Cancel each other out”, causing a neutralization reaction • For example, when the base sodium hydroxide (NaOH) and hydrochloric acid (HCl) react, they form a neutral solution of water and the salt sodium chloride (NaCl). This reaction is represented by the chemical equation: NaOH + HCl → NaCl + H2O. • In this reaction, hydroxide ions (OH-) from the base combine with hydrogen ions (H+) from the acid to form water. The other ions in the solution (Na+) and (Cl-) combine to form sodium chloride. XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX

28. Lesson Summary • Most of Earth’s water is salt water located on the planet’s surface. Water is constantly recycled through the water cycle. • Water molecules are polar, so they form hydrogen bonds. This gives water unique properties, such as a relatively high boiling point. • A solution is a homogeneous mixture in which a solute dissolves in a solvent. Water is a very common solvent, especially in organisms. • The ion concentration of neutral, pure water gives water a pH of 7 and sets the standard for defining acids and bases. Acids have a pH lower than 7, and bases have a pH higher than 7. • Water is essential for most life processes, including photosynthesis, cellular respiration, and other important chemical reactions that occur in organisms.