STRENGTH OF ACID OR BASE SOLUTIONS:
A scale for measuring hydrogen ion concentration in a solution, called pH scale has been developed. The p in pH stands for ‘potenz’ in German, meaning power.
p = potential or Power            H = Hydrogen

pH of a solution: pH of a solution is the negative logarithm to the base 10 of the hydrogen ion concentration expressed in mole per litre.  pH = –log10 (H+)

pH = 7
Neutral Solution
H3O+ = OH –
pH > 7
Basic Solution
H3O+ < OH –
pH< 7
Acidic Solution
H3O+ > OH –
Range of pH is from 1 to 14
                   


  Ø  Detecting an Acid or a Base: We use litmus solution, phenolphthalein or methyl orange for this purpose. The colour change that they show in acidic and basic solutions are:

Neutral
Acid
Base
Litmus solution
Purple
Red
Blue
Phenolphthalein
Colourless
No change
Pink
Methyl orange
Orange
Red
Yellow

We can also measure strength of an acid or a base using a pH scale, which looks something like this:

  Ø  pH Sensitivity of Plants & Animals:
·       Human body works in a narrow range of pH 7 to 7.8. Acidity can be lethal for plants and animals.
·       pH of Digestive System: Stomach secretes HCl to kill bacteria in the food. The inner lining of stomach protects vital cells from this acidic pH.
·       pH and Tooth Decay: Lower pH because of sour food and sweet food can cause tooth decay. The pH of mouth should always be more than 5.5.
·       pH as self Defence Mechanism in Plants & Animals: Certain animals like bee and plants like nettle secrete highly acidic substance for self-defence.
·       Acid Rain:  If the pH of rain is lower than 5.6 we term it ‘acid rain’. This water, when flows in water bodies, makes the water acidic and difficult for aquatic organisms to survive. It also causes a harm to monuments like Taj Mahal.