Water, Acid & Base, and pH

Nestor T. Hilvano, M.D., M.P.H.(Images Copyright Discover Biology, 5th 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 (H2O hold together)

- adhesion (H2O hold to other molecules)

- surface tension (cohesion + adhesion)

- ability to moderate temperature

- expansion upon freezing

- versatility as solvent

Cohesion

• b/c of H+ bonds, H20 has a HIGH surface tension– water striders

Adhesion• b/c of H+ bonds, H20 has a HIGH surface

tension– meniscus in glass containers

http://www.chemistryland.com/CHM151W/04-Solutions/liquids/meniscus.jpg

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 H2O) 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 H2O: 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 H2O) 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 7b) 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 H2O has [H+] = 10-7 = 7

HCl has [H+] = 1

NaOH has [H+] = 10-14 = 14

• Buffer = substance that resist (minimize) changes in pH in a solution (accept/donate H+)

Ex. HCO-3 + H+ H2CO3 bicarbonate carbonic acid

Figure 2.14_3

Acidicsolution

Neutralsolution

Basicsolution

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.