Chapter 14: Substances in Common Use

This chapter explores various chemical substances that we encounter and use in our daily lives, understanding their chemical properties, preparation methods, and diverse applications.

1. Common Salt (Sodium Chloride) - NaCl

Common salt, also known as table salt, is chemically Sodium Chloride.

• Chemical Name: Sodium Chloride
• Chemical Formula: NaCl
• Occurrence:
  • Found in sea water (major component).
  • Deposits of solid salt (rock salt) are found in various parts of the world.
• Properties:
  • White crystalline solid.
  • Highly soluble in water.
  • Has a high melting point.
• Uses:
  • Essential component of our diet (cooking).
  • Used as a food preservative (e.g., curing meat, pickling).
  • Raw material for the manufacture of many important chemicals like sodium hydroxide (NaOH), washing soda (Na₂CO₃·10H₂O), baking soda (NaHCO₃), chlorine gas (Cl₂), and hydrogen gas (H₂).

2. Sodium Hydroxide (Caustic Soda) - NaOH

Sodium hydroxide is a strong base commonly known as caustic soda.

• Preparation: Chlor-alkali Process
  • It is produced by the electrolysis of an aqueous solution of sodium chloride (brine).
  • The process is called the 'chlor-alkali' process because of the products formed: 'chlor' for chlorine and 'alkali' for sodium hydroxide.
  • Reaction: $2NaCl(aq) + 2H_2O(l) \xrightarrow{\text{Electrolysis}} 2NaOH(aq) + Cl_2(g) + H_2(g)$
  • Products and their uses:

    Product	Produced At	Uses
    Sodium Hydroxide (NaOH)	Near Cathode	Soaps and detergents, paper making, artificial fibers, petroleum refining, degreasing metals.
    Chlorine (Cl₂)	At Anode	Water treatment, PVC manufacturing, disinfectants, CFCs, pesticides.
    Hydrogen (H₂)	At Cathode	Fuels, ammonia for fertilizers, margarine, HCl.
    Hydrochloric Acid (HCl)	(From H₂ and Cl₂)	Cleaning steel, medicines, cosmetics.
    Sodium Hypochlorite (NaClO)	(From NaOH and Cl₂)	Bleach for domestic use, bleaching fabrics.

3. Bleaching Powder (Calcium Oxychloride) - CaOCl₂

Bleaching powder is a yellowish-white powder with a strong smell of chlorine.

• Chemical Name: Calcium Oxychloride
• Chemical Formula: CaOCl₂
• Preparation:
  • It is produced by the action of chlorine gas on dry slaked lime (calcium hydroxide).
  • Reaction: $Ca(OH)_2(s) + Cl_2(g) \rightarrow CaOCl_2(s) + H_2O(l)$
• Uses:
  • For bleaching cotton and linen in the textile industry.
  • For bleaching wood pulp in paper factories.
  • As an oxidizing agent in many chemical industries.
  • For disinfecting drinking water to make it free from germs.

4. Baking Soda (Sodium Hydrogen Carbonate) - NaHCO₃

Baking soda is a common household chemical used in cooking.

• Chemical Name: Sodium Hydrogen Carbonate (or Sodium Bicarbonate)
• Chemical Formula: NaHCO₃
• Preparation:
  • It is prepared using sodium chloride as one of the raw materials (Solvay process).
  • Reaction: $NaCl(aq) + H_2O(l) + CO_2(g) + NH_3(g) \rightarrow NH_4Cl(aq) + NaHCO_3(s)$
• Properties:
  • It is a mild non-corrosive basic salt.
  • When heated during cooking, it decomposes to produce carbon dioxide, which causes dough to rise.
  • Reaction on heating: $2NaHCO_3(s) \xrightarrow{\text{Heat}} Na_2CO_3(s) + H_2O(l) + CO_2(g)$
• Uses:
  • As a component of baking powder (baking soda + a mild edible acid like tartaric acid). When baking powder is heated or mixed with water, CO₂ is produced, making cakes and bread soft and spongy.
  • As an antacid to neutralize excess acid in the stomach and provide relief from indigestion.
  • In soda-acid fire extinguishers.

5. Washing Soda (Sodium Carbonate Decahydrate) - Na₂CO₃·10H₂O

Washing soda is a common cleaning agent and industrial chemical.

• Chemical Name: Sodium Carbonate Decahydrate
• Chemical Formula: Na₂CO₃·10H₂O
• Preparation:
  • Sodium carbonate (Na₂CO₃) can be obtained by heating baking soda (NaHCO₃).
  • Recrystallization of sodium carbonate yields washing soda.
  • Reaction: $Na_2CO_3(s) + 10H_2O(l) \rightarrow Na_2CO_3 \cdot 10H_2O(s)$
• Properties:
  • It is a basic salt.
  • It is a crystalline solid containing ten molecules of water of crystallization.
• Uses:
  • In glass, soap, and paper industries.
  • As a cleaning agent for domestic purposes.
  • For removing permanent hardness of water.
  • In the manufacture of sodium compounds like borax.

6. Plaster of Paris (Calcium Sulphate Hemihydrate) - CaSO₄·½H₂O

Plaster of Paris is a quick-setting plaster made from gypsum.

• Chemical Name: Calcium Sulphate Hemihydrate
• Chemical Formula: CaSO₄·½H₂O
• Preparation:
  • It is obtained by heating gypsum (CaSO₄·2H₂O) at 373 K (100°C).
  • Reaction: $CaSO_4 \cdot 2H_2O(s) \xrightarrow{373 K} CaSO_4 \cdot \frac{1}{2}H_2O(s) + 1\frac{1}{2}H_2O(l)$
  • If gypsum is heated above 373 K, it loses all its water of crystallization and forms anhydrous calcium sulphate (dead burnt plaster), which loses its setting property.
• Properties:
  • It is a white powder.
  • On mixing with water, it sets into a hard solid mass due to rehydration to gypsum.
  • Setting Reaction: $CaSO_4 \cdot \frac{1}{2}H_2O(s) + 1\frac{1}{2}H_2O(l) \rightarrow CaSO_4 \cdot 2H_2O(s)$
• Uses:
  • Used by doctors for supporting fractured bones in the correct position (plaster cast).
  • For making toys, decorative materials, and cheap ornaments.
  • For making surfaces smooth (e.g., walls and ceilings).
  • For making casts for statues and molds for pottery.
  • As a fire-proofing material.

7. Soap

Soaps are traditional cleaning agents.

• Definition: Soaps are the sodium or potassium salts of long-chain carboxylic acids (fatty acids).
• Preparation (Saponification):
  • Soaps are formed by the alkaline hydrolysis of esters of higher fatty acids (oils and fats). This process is called saponification.
  • When fats or oils are treated with a strong base (like NaOH or KOH), they hydrolyze to form glycerol and soap.
  • Example: $Fat/Oil + NaOH \rightarrow Glycerol + Sodium\ Salt\ of\ Fatty\ Acid (Soap)$
• Cleaning Action of Soap: (Micelle Formation)
  • A soap molecule has two parts:
    • Hydrophilic part: The ionic (carboxylate) end (-COO⁻Na⁺) which is water-loving and dissolves in water.
    • Hydrophobic part: The long hydrocarbon chain which is water-repelling (oil-loving) and dissolves in oil/grease.
  • When soap is added to water, the hydrophobic parts cluster together to avoid water, and the hydrophilic parts point outwards into the water. This forms a spherical structure called a micelle.
  • Grease or oil particles are trapped in the center of the micelle.
  • These micelles remain suspended in water as a colloid and do not coagulate. The dirt particles are thus removed from the cloth.
  • The micelles repel each other due to the negative charge on their outer surface, preventing them from coming together and settling down.

8. Detergents

Detergents are synthetic cleaning agents that are more effective in hard water.

• Definition: Detergents are generally ammonium or sulphonate salts of long-chain carboxylic acids. More specifically, they are long-chain alkyl hydrogen sulphonates or alkyl benzene sulphonates.
• Advantages of Detergents over Soaps:
  • Effective in Hard Water: Detergents do not form insoluble precipitates (scum) with calcium and magnesium ions present in hard water, unlike soaps. This means they can be used effectively in hard water.
  • Effective in Acidic Water: Detergents work well in acidic water, whereas soaps lose their effectiveness because fatty acids precipitate in acidic conditions.
  • Stronger Cleaning Action: Many detergents have a stronger cleaning action than soaps.
  • Versatile Use: Used in various cleaning products like shampoos, laundry detergents, and dishwashing liquids.
• Disadvantage: Some detergents are non-biodegradable, leading to water pollution.

9. Difference between Soap and Detergent