Water, Acid & Base, and pH Nestor T. Hilvano, M.D., M.P.H. (Images Copyright Discover Biology, 5 th ed., Singh-Cundy and Cain, Textbook, 2012.)

Learning Objectives 1.Describe the special properties of water important to life related to H bonding. 2.Explain why ice is less dense than liquid water. 3.Define pH and buffer. 4.Explain the basis for the pH scale.

Water Word stems: hydro- water; phobic- fear; phallic- loving Essential for life- 70% to 95% found in cells;75% covered earth’s surface Low molecular weight; Oxygen has a greater electronegativity than the hydrogen (polarity) Special (Emergent)properties of water: - cohesion (H 2 O hold together) - adhesion (H 2 O hold to other molecules) - surface tension (cohesion + adhesion) - ability to moderate temperature - expansion upon freezing - versatility as solvent

Cohesion b/c of H+ bonds, H 2 0 has a HIGH surface tension –water striders

Adhesion b/c of H+ bonds, H 2 0 has a HIGH surface tension –meniscus in glass containers

Water’s H bonds moderate temperature Water can absorb heat, has better ability to resist temperature change Breaking H bonds requires a large amount of energy (i.e. heating H 2 O) b/c water molecules are strongly attracted Evaporative cooling

States of Matter solid: electrons moving slowly liquid: electrons moving faster gas: electrons moving FAST!! (usually when a substance solidifies, it shrinks: the atoms move more closely together); (NOT H 2 O: expands b/c H bonds form rigid structure holding each molecule apart) Water is the solvent of life - solvent + solute = solution - polar compounds and ionic compounds are dissolve in water

Why ice ( solid H 2 O) is less dense than liquid water? ________________

pH Scale In any aqueous solution at 25°C the product of H + and OH – is constant and can be written as [H + ][OH – ] = 10 –14 The pH of a solution is defined by the negative logarithm of H + concentration, written as pH = –log [H + ] For a neutral aqueous solution [H + ] is 10 –7 = –(–7) = 7

Acids and Bases pH – a measure of acidity or basicity [H+]; [OH-] Types of solutions a) neutral = pH 7 b) acids = pH between 1-7 (release H+); strong acids (dissolve completely in water); weak acids (dissolve partially in water) c) bases = pH between 7-14 (accept H+); strong bases (dissolve completely); weak bases (dissolve partially) pure H 2 O has [H+] = = 7 HCl has [H+] = 1 NaOH has [H+] = = 14 Buffer = substance that resist (minimize) changes in pH in a solution (accept/donate H+) Ex. HCO -3 + H +  H 2 CO 3 bicarbonate carbonic acid

Figure 2.14_3 Acidic solution Neutral solution Basic solution

Homework 1.Define the following: cohesion, adhesion, surface tension, pH, and buffer. 2.Differentiate acid, base, and neutral solutions as to pH scale. 3.Discuss why ice is less dense than water. 4.Describe the 3 forms (state of matter) of water as to movement of atoms (electrons). 5.Describe the special (emergent) properties of water important to life.