• Acids are sour to taste. 
  • They change the color of methyl orange from orange to red, Phenolphthalein remains colorless in acidic solutions.
  • Aqueous solution of acid conduct electricity.

On the basis of origin, acids are classified as :

  • Organic acids: Acids derived from living organisms like plants and animals.

Natural sources of acids are:

Natural source


Natural source



Acetic acid

Sour milk (Curd)

Lactic acid


Citric acid


Citric acid


Tartaric acid

Ant sting

Methanoic acid


Oxalic acid

Nettle sting

Methanoic acid


  • Mineral acids: They are also called inorganic acids. They are dangerous Example sulphuric acid (H2SO4), hydrochloric acid (HCl) etc.

On the basis of their strength, acids are classified as :

Strong Acids

Weak acids

Acids that give rise to more H+ ions are said to be strong acids.

Acids that give less H+ ions are said to be weak acids.







 On the basis of their concentration, acids are classified as :

  • Dilute acids: Have a low concentration of acids in aqueous solutions.
  • Concentrated acids: Have a high concentration of acids in aqueous solutions.

On the basis of number of hydrogen ion, acids can be classified as :

  • Monoprotic acid – Such type of acid produces one mole of H+ ions per mole of acid, e.g., HCl , HNO3
  • Diprotic acid – They can produce two moles of H+ ions per mole of acid, e.g., H2SO4.
  • Triprotic acid – They produce three moles of H+ ions per mole of acid, e.g., H3PO4.
  • Polyprotic – They can produce more than three H+ ions per mole of acid.

Chemical properties of acids:

  • Acids react with active metals to give hydrogen gas.

Zn + H2SO3 →ZnSO4 + H2

  • Acids react with metal carbonate and metal hydrogen carbonate to give carbon dioxide.

NaHCO3 +HCl → NaCl + H2O + CO2

  • Acids react with bases to give salt and water. This reaction is called as neutralization reaction.

NaOH + HCl → NaCl +H2O

  • Acids react with metals oxides to give salt and water.

CuO + H2SO4 → CuSO4 + H2O

Uses of acids-

  • Hydrochloric acid is used in digestion.
  • Nitric acid  is used in the purification of gold and silver and in the explosives (TNT, TNB, picric acid, etc)
  • Oxalic acid is used to remove rust spot.
  • Boric acid is a constituent of eyes wash.
  • Formic acid is present in red ants.
  • Uric acid is present in urine of mammals.
  • Sulphuric acid is used in lead storage battery and in the manufacture of fertilizers, drugs, alum, etc.
  • Acetic acid and citric acid – flavouring agent and food preservatives.
  • Tartaric acid – souring agents for pickles and it is a component of baking powder.

Addition of Acids or Bases to Water

  • The process of dissolving an acid, specially nitric acid or sulfuric acid or a base in water is a highly exothermic one.
  • As a rule: Always add acid to water and never the other way!
  • The acid must be added slowly to water with constant stirring. If one mixes the other way by adding water to a concentrated acid, the heat generated causes the mixture to splash out and cause burns.