Activity #14: pH

Vocabulary pH scale Ions Acid Base

pH Water is one of the most important inorganic substances in living organisms Sometime water molecules will break apart (dissociate) to form ions Ion- a positively or negatively charged atom (set of atoms) due to loss or gain of electrons

Water About 1 water molecule in 550 million splits in this way Because the number of positive H+ ions is equal to the number of negative OH- ions, water is neutral pH scale- measures the concentration of H+ ions in solution [H+]

pH Scale Measured 0-14 Each step represents a factor of 10 7 is neutral (like water- has equal amounts of H+ and OH-) Lower from 7 becomes more acidic (has more H+) Higher from 7 becomes more basic/alkaline (has less H+)

Practice Order these substances in order of increasing acidity: human blood, acid rain, bleach, seawater, stomach acid. Answer: bleach, seawater, human blood, acid rain, stomach acid.

Practice Acid, Base, or Neutral? Strong or Weak? pH 7:_______ pH 13.5:_____ pH 8:_______ pH 2:________ Which one is more acidic pH of 2 -or- pH of 4 How many times more?___________ Which one is more basic? Tomato juice -or- Soap How many times more?________ N B (strong) B (weak) A (strong) 100 1,000,000

Definitions of acids and bases ACID- compound that forms H+ ions in solution Acids have more H+ than OH- BASE- compounds that forms OH- ions in solution Bases have more OH- than H+

Practice Study the reaction below. Is this substance an ACID or BASE? (circle) Dissociation In Water HCl H+ + Cl- Hydrogen Chloride Hydrogen ion Chloride ion (Circle) NaOH Na+ + OH- Sodium Hydroxide Sodium ion hydroxide ion

Acids vs. Bases ACIDS BASES Solutions have a pH_______7 Taste sour Taste bitter Can corrode metals Can denature proteins (feel "slippery") In solution: [H+] _____ [OH-] > < > <

ACID Neutral Base Draw a solution with more H+ than OH- Range of pH ______________ [H+] ______ [OH-] in this type of solution. Draw solution with equal numbers of H+ and OH- Draw solution with more OH- than H+ 0-6.9 7 7.1-14 > = < H+ H+ H+ H+ H+ H+ H+ H+ OH- OH- H+ H+ H+ H+ H+ OH- OH- OH-OH- OH- H+ H+ OH- OH- OH- OH- OH- OH- OH- OH-

Sample acids and bases

BUFFERS Controlling pH is important for maintaining homeostasis in cells The pH of most cells in the human body must be kept between 6.5 and 7.5 Buffers- Weak acids or bases that react with strong acids or bases to prevent sharp, sudden changes of pH Example: Sudden pH changes in blood are prevented by buffers such as bicarbonate and phosphate ions