Class 10 Chemistry



Acid Base and Salt

Common Salt (Sodium Chloride)

Sodium chloride (NaCl) is also known as common or table salt. It is formed after the reaction between sodium hydroxide and hydrochloric acid. It is a neutral salt. The pH value of sodium chloride is about 7. Sodium chloride is used to enhance the taste of food. Sodium chloride is used in manufacturing of many chemicals.

Important chemicals from sodium chloride:

Sodium Hydroxide (NaOH): Sodium hydroxide is a strong base. It is also known as caustic soda or Iye. It is obtained by the electrolytic decomposition of solution of sodium chloride (brine). In the process of electrolytic decomposition of brine (aqueous solution of sodium chloride), brine decomposes to form sodium hydroxide. In this process, chlorine is obtained at anode and hydrogen gas is obtained at cathode as byproducts. This whole process is known as Chlor-Alkali process.

2NaCl + 2H2O ⇨ 2NaOH + Cl2 + H2


Use of products after the electrolysis of brine:

  • Hydrogen gas is used as fuel, margarine, in making of ammonia for fertilizer, etc.
  • Chlorine gas is used in water treatment, manufacturing of PVC, disinfectants, CFC, pesticides. It is also used in manufacturing of bleaching powder and hydrochloric acid.
  • Sodium hydroxide is used for de-greasing of metals, manufacturing of paper, soap, detergents, artificial fibres, bleach, etc.

Bleaching Powder (CaOCl2 ):

Bleaching powder is also known as chloride of lime. It is a solid and yellowish white in colour. Bleaching powder can be easily identified by the strong smell of chlorine.

When calcium hydroxide (slaked lime) reacts with chlorine, it gives calcium oxychloride (bleaching powder) and water is formed.

Ca(OH)2 + Cl2 ⇨ CaOCl2 + H2O

Aqueous solution of bleaching powder is basic in nature. The term bleach means removal of colour. Bleaching powder is often used as bleaching agent. It works because of oxidation. Chlorine in the bleaching powder is responsible for bleaching effect.

Use of bleaching powder:

  • Bleaching powder is used as disinfectant to clean water, moss remover, weed killers, etc.
  • Bleaching powder is used for bleaching of cotton in textile industry, bleaching of wood pulp in paper industry.
  • Bleaching powder is used as oxidizing agent in many industries, such as textiles industry, paper industry, etc.

Baking Soda (NaHCO3 )

Baking soda is another important product which can be obtained using byproducts of chlor-alkali process. The chemical name of baking soda is sodium hydrogen carbonate (NaHCO3 ) or sodium bicarbonate. Bread soda, cooking soda, bicarbonate of soda, sodium bicarb, bicarb of soda or simply bicarb, etc. are some other names of baking soda.

Baking soda is obtained by the reaction of brine with carbon dioxide and ammonia. This is known as Solvay process.

NaCl + CO2 + NH3 + H2O ⇨ NH4Cl + NaHCO3

In this process, calcium carbonate is used as the source of CO2 and the resultant calcium oxide is used to recover ammonia from ammonium chloride.

Properties of sodium bicarbonate:
  • Sodium bicarbonate is white crystalline solid, but it appears as fine powder.
  • Sodium hydrogen carbonate is amphoteric in nature.
  • Sodium hydrogen carbonate is sparingly soluble in water.
  • Thermal decomposition of sodium hydrogen carbonate (baking soda).
  • When baking soda is heated, it decomposes into sodium carbonate, carbon dioxide and water.

2NaHCO3 + heat ⇨ Na2CO3 + CO2 + H2O

Sodium carbonate formed after thermal decomposition of sodium hydrogen carbonate; decomposes into sodium oxide and carbon dioxide on further heating.

Na2CO3 ⇨ Na2O + CO2

This reaction is known as dehydration reaction.

Use of Baking Soda:

  • Baking soda is used in making of baking powder, which is used in cooking as it produces carbon dioxide which makes the batter soft and spongy.
  • Baking soda is used as antacid.
  • Baking soda is used in toothpaste which makes the teeth white and plaque free.
  • Baking soda is used in cleansing of ornaments made of sliver.
  • Since, sodium hydrogen carbonate gives carbon dioxide and sodium oxide on strong heating, thus it is used as fire extinguisher.

Baking powder:

Baking powder produces carbon dioxide on heating, so it is used in cooking to make the batter spongy. Although baking soda also produces carbon dioxide on heating, but it is not used in cooking because on heating; baking soda produces sodium carbonate along with carbon dioxide. The sodium carbonate; thus produced; makes the taste bitter.

2NaHCO3 + heat ⇨ Na2CO3 + CO2 + H2O

Baking powder is the mixture of baking soda and a mild edible acid. Generally, tartaric acid is mixed with baking soda to make baking powder.

NaHCO3 + C4H6O6 ⇨ CO2 + H2O + Na2C4H4O6

When baking powder (mixture of baking soda and an edible acid) is heated, the sodium carbonate formed because of heating of baking soda neutralizes after reacting with tartaric acid and sodium tartarate salt is formed. The smell of sodium tartarate is pleasant and taste is good. This makes the cake or any other food tasty.


Washing Soda (Sodium carbonate)

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.

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.

Examples:

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.

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

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


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Acid

Base

Common properties

Strength

Salts

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