Class 10 Science
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. In this section, you will read about sodium hydroxide, bleaching powder and sodium bicarbonate.
Sodium Hydroxide (NaOH): Sodium hydroxide is a strong base. It is also known as caustic soda or Lye. It is obtained by 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
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.
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.
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.
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.
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.
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 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.
| Caustic Soda | NaOH | Made by electrolytic decomposition of brine. |
|---|---|---|
| Bleaching Powder | CaOCl2 | Made by reaction between slaked lime and chlorine gas. |
| Baking Soda | NaHCO3 | Made by reaction of brine with carbon dioxide and ammonia. This reaction is called Solvay Process. |
| Baking Powder | A mixture of baking soda and edible acid | Edible acid prevents bitterness in cooked dish. |
| Washing Soda | NaCO3 | Obtained by thermal decomposition of sodium bi-carbonate. |
| Plaster of Paris | CaSO4.0.5H2O | Made by heating gypsum CaSO4.2H2O |
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