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pH scale

02 Acid, bases and salt

  • The pH scale is used to determine the concentration of hydronium ions on a logarithmic scale. By definition, pH of acids and bases is the negative logarithm (with base 10) of activity of hydrogen ion (H+).
  • pH scale was devised by Sorenson in 1909.
  • The strength of acid solutions as well as basic solutions could be represented by hydrogen ion concentrations in the pH scale.
  • The numeric value of the degree of acidity or basicity of a substance can be determined using a pH scale.
  • pH stands for ‘potential of hydrogen’.
  • The pH of a solution is inversely proportional to the concentration of hydrogen ions in it.
  • A pH scale ranges from 0 to 14. 0 indicates the most acidic and 14 indicates the most basic nature of a substance.
  • So, we can say that
    • Neutral substances have a pH of exactly 7.
    • Acids have a pH of less than 7.
    • Bases have a pH of more than 7.
  • Solutions that have pH value of 7 are known as neutral solutions.
  • Solutions having the value of pH equal to 0 are called strongly acidic solutions.
  • Solutions with the value of pH equal to 14 are termed as strongly basic solutions.
  • Acidity decreases as pH value increases from 0 to 7 and basicity decreases as pH value decreases from 14 to 7.


  • Some substances do not have both acidic and basic characteristics and have a pH value of 7. Such substances are called neutral substances.
  • They do not have any effect on either type of litmus paper
  • For example: Water, cooking oil, common salt.


  • Universal indicator is a mixture of many different indicators (or dyes) which gives different colours at different pH values of the entire pH scale.


  • Digestive System:-
    • Our stomach produces hydrochloric acid of pH about 1.4, which helps in digestion. However, the excess acid in the stomach causes indigestion which produces pain and irritation. Bases called ‘Antacids’ (‘antacid’ means ‘anti-acid’) are used to cure indigestion.
    • Common antacids are magnesium hydroxide (milk of magnesia) and Sodium hydrogen carbonate.
  • Tooth Decay:-
    • The bacteria present in our mouth break down the sugar to form acids (Such as lactic acid). Thus, the formation of acid takes place after the take in of sugary food.
    • This acid lowers the pH in the mouth making it acidic.
    • Tooth decay starts when the pH in the mouth falls below 5.5.
    • The best way to prevent tooth decay is to clean the mouth thoroughly after eating food.
    • So, using the kinds of toothpaste, which are basic for cleaning the teeth can neutralise the excess acid in mouth and prevent tooth decay.
  • Plants and Animals are Sensitive to pH Changes:-
    • Soil pH and Plant Growth: The optimum pH value for most of the plants is close to 7.
    • If the soil is too acidic or too basic (too alkaline), the plant’s growth gets affected.
    • Chemicals can be added to soil to adjust its pH and make it suitable for growing plants (such as crops).
    • If the soil is too acidic i.e. have low pH, then it is treated with materials like quicklime (calcium oxide) or slaked lime (calcium hydroxide) or chalk (calcium carbonate).
    • If the soil is too basic (or too alkaline) i.e. having a high pH. Then it is treated with decaying organic matter (manure or compost) which contains acidic materials.
    • pH Change and Survival of Animals:- Our body works well within a narrow pH range of 7.0 to 7.8.
    • Disturbance of pH range in the body of a person, can result in many ailments. The aquatic animals like fishes can survive in lake or river water within a narrow range of pH change.
    • In case of Acidic rain this has a pH value of 5.6, results in bringing down the pH value of lake. In turn it becomes difficult for aquatic animals to survive.
  • Self Defence by Animals and Plants through Chemical Warfare:-
    • When a honey-bee stings a person, it injects an acidic liquid into the skin which causes  immense pain and irritation. The rubbing of a mild base like baking soda solution neutralizes the acid to give relief to the affected area.

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