Chemicals from Common Salt: Sodium Hydroxide and Bleaching Powder for CBSE Class 10

Common salt—chemically known as sodium chloride ($\text{NaCl}$)—is far more than a kitchen seasoning. It is one of the most vital raw materials in the global chemical industry. By subjecting concentrated aqueous sodium chloride (known as brine) to chemical and electrochemical transformations, chemical plants manufacture a vast array of essential products including caustic soda, chlorine, hydrogen, bleaching agents, baking soda, and washing soda.

In CBSE Class 10 Science, the study of chemicals derived from common salt forms a central pillar of Chapter 2 (Acids, Bases and Salts), with questions on the Chlor-Alkali process and Bleaching powder appearing routinely in board examinations.

What You Will Learn

• Classification of salts: acidic, basic, and neutral salts
• Common salt ($\text{NaCl}$) as an industrial chemical feedstock
• The Chlor-Alkali Process: electrolytic production of sodium hydroxide ($\text{NaOH}$)
• Products at the anode ($\text{Cl}_2$), cathode ($\text{H}_2$), and near the cathode ($\text{NaOH}$)
• Bleaching Powder ($\text{CaOCl}_2$): preparation by action of chlorine on dry slaked lime
• Chemical properties and key industrial uses of Bleaching powder
• Board examination questions, reaction conditions, and common mistakes

1. Classification of Salts and Their pH

A salt is formed by the neutralisation reaction between an acid and a base. Its pH in aqueous solution depends on the relative strengths of the parent acid and base:

2. Sodium Hydroxide (NaOH): The Chlor-Alkali Process

Why is it Called the Chlor-Alkali Process?

When electricity is passed through an aqueous solution of sodium chloride (called brine), it decomposes to form sodium hydroxide. The process is called the chlor-alkali process because the products formed are:

• Chlor for chlorine gas ($\text{Cl}_2$).
• Alkali for sodium hydroxide ($\text{NaOH}$).

Overall Chemical Equation

$2\text{NaCl}(aq) + 2\text{H}_2\text{O}(l) \xrightarrow{\quad \text{Electricity} \quad} 2\text{NaOH}(aq) + \text{Cl}_2(g) \uparrow + \text{H}_2(g) \uparrow$

                           The Chlor-Alkali Cell
                                     |
       +-----------------------------+-----------------------------+
       |                             |                             |
At Anode (+)                  At Cathode (-)                 Near Cathode
Chlorine Gas (Cl2)            Hydrogen Gas (H2)              Sodium Hydroxide (NaOH)

Products and Where They Are Formed:

1. Chlorine gas ($\text{Cl}_2$) is given off at the ANODE (positive electrode).
2. Hydrogen gas ($\text{H}_2$) is given off at the CATHODE (negative electrode).
3. Sodium hydroxide ($\text{NaOH}$) solution is formed near the CATHODE.

Important: <u>Memorize electrode allocations: Chlorine at the Anode, Hydrogen at the Cathode, and NaOH near the Cathode. Swapping the electrodes is the single most common student error in board exams!</u>

Industrial Uses of the Chlor-Alkali Products:

1. Hydrogen gas ($\text{H}_2$): Fuel for rockets, manufacture of margarine (hydrogenation of vegetable oils), synthesis of ammonia ($\text{NH}_3$) for fertilizers.
2. Chlorine gas ($\text{Cl}_2$): Water treatment (disinfectant for drinking water and swimming pools), manufacturing PVC (polyvinyl chloride), CFCs, pesticides, and bleaching powder.
3. Sodium Hydroxide ($\text{NaOH}$): De-greasing metals, manufacturing soaps and synthetic detergents, paper making, artificial textile fibres (rayon).
4. Combined Products:
   • $\text{H}_2 + \text{Cl}_2 \longrightarrow 2\text{HCl}$ (Hydrochloric acid: cleaning steel, medicines, cosmetics).
   • $\text{Cl}_2 + \text{NaOH} \longrightarrow$ Household bleach.

3. Bleaching Powder ($ext{CaOCl}_2$)

Chlorine gas produced during the electrolysis of brine is used for the manufacture of bleaching powder.

Chemical Identity

• Chemical Name: Calcium oxychloride
• Chemical Formula: $\text{CaOCl}_2$

Preparation

Bleaching powder is produced by the action of dry chlorine gas on dry slaked lime [$\text{Ca(OH)}_2$]:

$\text{Ca(OH)}_2(s) \text{ [Dry slaked lime]} + \text{Cl}_2(g) \longrightarrow \text{CaOCl}_2(s) \text{ [Bleaching powder]} + \text{H}_2\text{O}(l)$

Remember: Slaked lime must be dry. Chlorine does not react efficiently with wet or liquid milk of lime to form stable bleaching powder.

4. Properties of Bleaching Powder

1. Physical Appearance: It is a pale yellowish-white powder that gives off a strong smell of chlorine.
2. Action of Air: When exposed to air, it reacts slowly with atmospheric carbon dioxide, releasing chlorine gas: $\text{CaOCl}_2(s) + \text{CO}_2(g) \longrightarrow \text{CaCO}_3(s) + \text{Cl}_2(g) \uparrow$
3. Action of Dilute Acids: It reacts with dilute acids (like $\text{H}_2\text{SO}_4$ or $\text{HCl}$) to liberate all its available chlorine: $\text{CaOCl}_2(s) + \text{H}_2\text{SO}_4(aq) \longrightarrow \text{CaSO}_4(aq) + \text{H}_2\text{O}(l) + \text{Cl}_2(g) \uparrow$ $\text{CaOCl}_2(s) + 2\text{HCl}(aq) \longrightarrow \text{CaCl}_2(aq) + \text{H}_2\text{O}(l) + \text{Cl}_2(g) \uparrow$ The chlorine liberated by the action of dilute acid acts as a powerful oxidizing and bleaching agent.

5. Uses of Bleaching Powder in Daily Life & Industry

1. Textile and Paper Industries:
   • For bleaching cotton and linen fabrics in the textile industry.
   • For bleaching wood pulp in paper factories.
   • For bleaching washed clothes in commercial laundries.
2. Disinfectant for Drinking Water:
   • Used as a disinfectant and germicide to make drinking water free from disease-causing microorganisms.
3. Chemical Industry:
   • Used as a powerful oxidizing agent in many chemical syntheses.
   • Used in the preparation of chloroform ($\text{CHCl}_3$).

6. Summary and Examination Tips

Exam Tip: When writing the preparation of bleaching powder, always write "dry slaked lime" and "chlorine gas". Writing just "lime" or "calcium hydroxide solution" will lose marks!

Common Mistake: Confusing the anode and cathode products of the chlor-alkali process. Mnemonic: Anode attracts Anions ($\text{Cl}^- \to \text{Cl}_2$). Cathode attracts Cations ($\text{H}^+ \to \text{H}_2$).