1. Activity #14: pH

2. Vocabulary
pH scale
Ions
Acid
Base

3. 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

4. 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+]

5. 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+)

6. 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.

7. Practice Acid, Base, or Neutral? Strong or Weak?
pH 7:_______ pH 13.5:_____
pH 8:_______ pH 2:________
Which one is more acidic
pH of 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

8. 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+

9. 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

10. 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-]
>
<
>
<

11. 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-

12. Sample acids and bases

13. 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