Class 10 Science

Washing Soda (Sodium carbonate)

Washing soda and plaster of Paris are important chemicals. They are used for various household and industrial purposes. On this page, you will learn about preparation, characteristics and usages of washing soda and plaster of Paris. You will also learn about water of crystallisation. These notes are based on the chapter Acids Bases and Salts from NCERT book based on CBSE syllabus.


Sodium carbonate is manufactured by the thermal decomposition of sodium hydrogen carbonate obtained by Solvay process.

NaCl + CO2 + NH3 + H2O ⇨ NH4Cl + NaHCO3

NaHCO3 + C4H6O6 ⇨ CO2 + H2O + Na2C4H4O6

The sodium carbonate obtained in this process is dry. It is called soda ash or anhydrous sodium carbonate. Washing soda is obtained by rehydration of anhydrous sodium carbonate.

Na2CO3 + 10H2O ⇨ Na2CO3.10H2O

Since there are 10 water molecules in washing soda, hence it is known as Sodium bicarbonate decahydrate.

Sodium carbonate is a crystalline solid and it is soluble in water when most of the carbonates are insoluble in water.

Use of sodium carbonate

  • It is used in cleaning of cloths; especially in rural areas.
  • In making of detergent cake and powder.
  • In removing permanent hardness of water.
  • It is used in glass and paper industries.

Plaster of Paris


Plaster of Paris is obtained by heating of gypsum, a hydrated salt of calcium.

CaSO4.2H2O + Heat ⇨ CaSO4.(0.5)H2O + (1.5)H2O

After addition of water Plaster of Paris is again converted into gypsum.

CaSO4.(0.5)H2O + (1.5)H2O ⇨ CaSO4.2H2O

Uses of Plaster of Paris

Plaster of Paris is used in making of toys, designer false ceiling, etc. Doctors use Plaster of Paris to set the fractured bone.

Water of crystallization

Many salts contain water molecule and are known as hydrated salts. The water molecule present in salt is known as water of crystallization. Characteristic colour of a salt is because of presence of water of crystallisation in the salt. For example: copper sulphate appears blue because of presence of water of crystallisation. Ferrous sulphate (iron sulphate) appears green because of presence of water of crystallisation.


Copper sulphate

Copper sulphate pentahydrate (CuSO4.5H2O): Blue colour of copper sulphate is due to presence of 5 molecules of water. When copper sulphate is heated, it loses water molecules and turns into grey-white colour, which is known as anhydrous copper sulphate. After adding water; anhydrous copper sulphate becomes blue again.

CuSO4.5H2O + heat ⇨ CuSO4

Ferrous Sulphate heptahydrate (FeSO4.7H2O): The green colour of Ferrous sulphate heptahydrate; commonly known as ferrous sulphate; is due to the presence of 7 molecules of water in it.

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