Very basic presentation on pH targeting students of class 6

1. Understanding pH and pH indicators
Acid Alkali/Base
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2. What is pH?
• pH is a number indicating acidity or alkalinity of an aqueous
solution
– Introduced by Danish Chemist Søren Peder Lauritz Sørensen in 1909
– He worked in Carlsberg Laboratory, Denmark
– The ‘H’ in pH stands for ‘Hydrogen’
– What about ‘p’?
• German chemists think ‘p’ for ‘Potenz (power)’
• French chemists think ‘p’ for ‘Puissance (power)’
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Carlsberg Foundation believes pH stands for ‘Power of Hydrogen’

3. pH of some solutions in our daily life
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4. • pH plays important role in everyday life
– pH (7.0 – 7.8) is essential for ideal growth/development of human
body
– pH of 1 – 3 is essential in stomach to activate an enzyme pepsin
responsible for digestion
– pH of 6.5 – 7.3 is essential for growth of plants and crops
– pH in mouth should be around 7 to prevent tooth decay
– Insects use chemical warfare for self-defence
• Ants and honey bees inject powerful and skin irritating acids for self-defence
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Why pH is important?

5. How to test pH of a solution?
• Litmus paper test
– Litmus is a dye obtained from
lichen (a slow growing plant)
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Rocella Tinctoria Lecanora tartarea

6. How litmus test work?
• Litmus is a Dye
– Dyes are colored compounds
– The color of the dye depend on its structure
• Structure?
– Arrangement of objects in a particular way
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7. • Structure of molecule/compound
– Arrangement of different atoms in a molecule/compound
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O HH
H2O (water)
C C
O
O H
H
H
H
CH3COOH (Acetic acid)
Chlorophyll
(Green)
Notice so many are molecules made with different
arrangement of atoms of ‘C’, ‘H’ & ‘O’

8. 8
Litmus dye
Purple color
Acid
RED Litmus
H
+
-
BLUE Litmus
So..how do you
make BLUE litmus
paper and RED
litmus paper?

9. • Acid-Base indicator solutions
– They are compounds which show different color with acids and bases
Examples:
• Phenolphthalein Acid (colorless) Base (pink)
• Methyl orange Acid (Red)  Base (yellow)
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Color change of the indicator in acidic and basic
solution is due to change in structure of the indicator
compound

10. • Phenolphthalein
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Acid (colorless) Base(Pink)
Acid (RED) Base(Yellow)
• Methyl orange

11. Calculating the pH
• pH is a number indicating acidity or alkalinity of a solution
• pH of a solution is related to the concentration of hydrogen ions
• Concentration of a substance
– It is a measure of amount of that substance in solution
11800 gram of sugar in 400 mL of water 80 gram of sugar in 80 mL of water
800 g/400 mL
2 g/mL
2000 g/L
2 kg/L
80 g/80 mL
1 g/mL
1000 g/L
1 kg/L

12. 12
1 kg 1 kg1 kg
Will the number of fruits be the same in all baskets?
NO

13. 10 g
H2SO4
13
10 g
Sugar (C12H22O11)
10 g
Salt (NaCl)
10 g
NaOH
10 g
Water
Will the number of molecules be the same in each case?
NO
10 g
HCl

14. Amount in terms of number
• Dozen
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12 numbers 12 numbers12 numbers
Will a dozen apples, oranges and mangos have the same weight?
NO

15. Mole
• Mole is another unit for amount in terms of number
• Mole is similar to the dozen unit
• 1 dozen = 12 numbers
•BUT
• 1 Mole = 6.023 x 100000000000000000000000 items
=6.023 x 1023 items
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The number 6.023 x 1023 is called Avagadro’s
number

16. • 1 Mole of H2O  6.023 x 1023 molecules of H2O
• 1 Mole of He (gas)  6.023 x 1023 atoms of He
• 1 Mole of Sugar  6.023 x 1023 molecules sugar (C12H22O11)
• 1 Mole of HCl  6.023 x 1023 molecules of HCl
• 1 Mole of HCl  atoms of H
• 1 Mole of HCl  atoms of Cl
• 1 Mole of H2SO4  Mole of H atoms, Mole of S atoms, Moles of O atoms
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6.023 x 1023
6.023 x 1023
2 1 4

17. Concentration of solutions
• Concentration
– Number of moles of a substance in one litre of solution
• If you dissolved 1 Mole of HCl in 1 L of water you will get HCl
solution of concentration 1 mole/L
• So how many HCl molecules will be there in 1 litre of HCl
solution having concentration 1 mole/L?
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6.023 x 1023 molecules of HCl

18. Calculating the pH
• pH is a number indicating acidity or alkalinity of a solution
• pH of a solution is related to the concentration of hydrogen ions
• Ion
– An atom or a molecule with charge
• Dissociation
– Splitting of molecules into ions
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H+ Cl -
H+
Cl -
H+
Cl -
1 mole HCl
1 mole H+
1 mole Cl -

19. HCl  H+ + Cl-
H2SO4  2H+ + SO4
2-
KOH  K+ + OH-
NaOH  Na+ + OH-
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H+ OH -
H+
OH -
1 Litre of water
• concentration of H+ ions = 0.0000001 moles/L
= 10-7 moles/L
• In water, concentration of H+ ions and
concentration of OH- ions are equal
In acidic solution we have more of H+ ions
In basic solution we have more of OH- ions

20. 20
Concentration of H+
(moles/L)
Concentration of OH-
(moles/L)
pH
Acidic 1 - 0
Acidic 0.1 (10-1) - 1
Acidic 0.01(10-2) - 2
Acidic 0.001 (10-3) - 3
Acidic 0.0001 (10-4) - 4
Acidic 0.00001 (10-5) - 5
Acidic 0.000001 (10-6) - 6
Water (Neutral pH) 0.0000001 (10-7) 0.0000001 (10-7) 7
Basic - 0.000001 (10-6) 8
Basic - 0.00001 (10-5) 9
Basic - 0.0001 (10-4) 10
Basic - 0.001 (10-3) 11
Basic - 0.01 (10-2) 12
Basic - 0.1 (10-1) 13
Basic - 1 14
HCl concentration = 0.1 moles/L
pH = 1
HCl concentration = 0.5 moles/L
pH between 0 and 1
NaOH concentration = 1 moles/L
pH = 14

21. Universal indicator
• A combination of several indicators which can cover pH from 0 to 14
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22. Make your own pH indicator at home
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Red/Violet cabbage
chop cabbage
boil cabbage Separate the juice