Acids, Bases, and Salts: Indicators, pH Scale, and Common Salts Class 10

From the tangy sourness of citric acid in lemons and the bitter, slippery feel of soap to the stinging formic acid of an ant bite and the rapid setting of Plaster of Paris in hospital fracture clinics, acids, bases, and salts are woven into every facet of daily chemistry.

In CBSE Class 10 Science, Chapter 2 (Acids, Bases and Salts) is one of the most content-rich chemistry chapters in the curriculum. Board examinations test visual indicator color changes, the physiological role of the pH scale, and the balanced chemical synthesis of five essential industrial and household salts: Sodium Hydroxide, Bleaching Powder, Baking Soda, Washing Soda, and Plaster of Paris.

What You Will Learn

• Chemical definitions: Arrhenius acids ($H^+$ / $H_3O^+$ ions) vs. bases ($OH^-$ ions)
• The complete Indicator Color Change Chart (Natural, Synthetic, and Olfactory)
• The pH Scale ($0 - 14$) and the importance of pH in everyday life (digestion, tooth decay, soil, stings)
• Salt classification: Neutral, Acidic, and Basic salts
• Industrial preparation, equations, and applications of the 5 Master Salts:
  1. Sodium Hydroxide ($NaOH$) via the Chlor-Alkali Process
  2. Bleaching Powder ($CaOCl_2$)
  3. Baking Soda ($NaHCO_3$)
  4. Washing Soda ($Na_2CO_3 \cdot 10H_2O$)
  5. Plaster of Paris ($CaSO_4 \cdot \frac{1}{2}H_2O$)
• Water of crystallization and heating hydrated copper sulphate

1. Acid-Base Indicators: The Complete Color Matrix

An indicator is a chemical dye that changes color (or odor) depending on the acidic or alkaline nature of the medium:

Important: <u>Why does dry HCl gas not turn dry blue litmus paper red? Because acids show acidic behavior ONLY in the presence of water, where they dissociate to generate hydronium ions ($H_3O^+$ or $H^+$)! In the complete absence of moisture, no ions are released, so litmus does not change color.</u>

2. The pH Scale and Everyday Life

The pH scale (where p stands for potenz, the German word for power) measures hydrogen ion concentration $[H^+]$ on a scale from $0$ to $14$:

• Neutral Solution: $\text{pH} = 7$ (e.g., pure water)
• Acidic Solution: $\text{pH} < 7$ (lower pH $\implies$ higher $[H^+]$ concentration)
• Basic Solution: $\text{pH} > 7$ (higher pH $\implies$ higher $[OH^-]$ concentration)

       Strong Acid         Weak Acid        Neutral        Weak Base        Strong Base
    +---------------+-------------------+-------------+-------------------+---------------+
    0               3                   7             10                  14
    <-- High [H+] ---------------------------------------------------- High [OH-] -->

Critical Roles of pH in Everyday Biology:

1. Human Digestive System: The stomach secretes hydrochloric acid ($HCl$), maintaining a pH of $1.5 - 2.0$ to activate pepsin. Excess acid causes acidity/heartburn, neutralized by mild bases called Antacids (e.g., Milk of Magnesia $Mg(OH)_2$ or Baking Soda $NaHCO_3$).
2. Tooth Decay (Dental Enamel): Tooth enamel is composed of calcium hydroxyapatite (the hardest substance in the human body). <u>Tooth decay begins when the pH inside the mouth drops BELOW 5.5 due to bacterial acid production fermenting leftover sugar! Toothpastes are alkaline to neutralize mouth acid.</u>
3. Soil pH for Agriculture: Most crops thrive best in neutral or slightly acidic soil ($ext{pH } 6.5 - 7.5$). Highly acidic soil is treated by adding slaked lime ($Ca(OH)_2$) or quicklime ($CaO$).
4. Self-Defense in Nature: Honeybee and ant stings inject methanoic acid (formic acid), causing burning pain (relieved by applying mild baking soda paste). Nettle leaf stings inject methanoic acid, cured traditionally by rubbing the alkaline leaf of the wild Dock plant!

3. The 5 Essential Salts of Class 10 Science

1. Sodium Hydroxide ($NaOH$) — The Chlor-Alkali Process

• Method: Electrolysis of a concentrated aqueous solution of sodium chloride (called brine): $\mathbf{2NaCl\text{ (aq)} + 2H_2O\text{ (l)} \xrightarrow{\text{Electricity}} 2NaOH\text{ (aq)} + Cl_2\text{ (g)} + H_2\text{ (g)}}$
• Products:
  • At Anode ($+$): Chlorine gas ($Cl_2$) (used in water disinfection, PVC, CFCs).
  • At Cathode ($-$): Hydrogen gas ($H_2$) (used as fuel, margarine, ammonia synthesis).
  • Near Cathode: Sodium Hydroxide solution ($NaOH$) (used in soap and paper manufacture).

2. Bleaching Powder ($CaOCl_2$)

• Preparation: Action of chlorine gas on dry slaked lime: $\mathbf{Ca(OH)_2\text{ (s)} + Cl_2\text{ (g)} \longrightarrow CaOCl_2\text{ (s)} + H_2O\text{ (l)}}$
• Uses: Bleaching cotton and linen in textile factories; oxidizing agent; disinfectant for drinking water.

3. Baking Soda ($NaHCO_3$ — Sodium Hydrogen Carbonate)

• Preparation (Solvay Process): $\mathbf{NaCl + H_2O + CO_2 + NH_3 \longrightarrow NH_4Cl\text{ (aq)} + NaHCO_3\text{ (s)}}$
• Thermal Decomposition on Heating (Cooking): $\mathbf{2NaHCO_3 \xrightarrow{\Delta} Na_2CO_3 + H_2O + CO_2 \uparrow}$ The released carbon dioxide gas causes cake and bread dough to rise, making them soft and spongy.
• Baking Powder vs. Baking Soda: Baking powder is a mixture of baking soda ($NaHCO_3$) and an edible mild acid like tartaric acid. The tartaric acid neutralizes the bitter sodium carbonate produced upon heating!

4. Washing Soda ($Na_2CO_3 \cdot 10H_2O$)

• Preparation: Re-crystallization of sodium carbonate with 10 molecules of water of crystallization: $\mathbf{Na_2CO_3 + 10H_2O \longrightarrow Na_2CO_3 \cdot 10H_2O}$
• Uses: Glass, soap, and paper industries; removing permanent hardness of water.

5. Plaster of Paris (CaSO_4 \cdot rac{1}{2}H_2O)

• Preparation: Controlled heating of Gypsum ($CaSO_4 \cdot 2H_2O$) to exactly $373\text{ K}$ ($100^\circ\text{C}$): $\mathbf{CaSO_4 \cdot 2H_2O \xrightarrow{373\text{ K}} CaSO_4 \cdot \frac{1}{2}H_2O + 1\frac{1}{2}H_2O}$
• Setting Action: When mixed with water, POP rehydrates back into a hard crystalline mass of gypsum within 15 minutes: $CaSO_4 \cdot \frac{1}{2}H_2O + 1\frac{1}{2}H_2O \longrightarrow CaSO_4 \cdot 2H_2O\text{ (Hard Gypsum)}$
• Uses: Setting fractured bones in orthopedic casts, dental casts, decorative ceilings, and statues.

4. Summary and Examination Tips

Exam Tip: In questions asking "Why is Plaster of Paris written with half a molecule of water?": Explain that two formula units of $CaSO_4$ share one molecule of water of crystallization: $2CaSO_4 \cdot H_2O$, which is conventionally written as $CaSO_4 \cdot \frac{1}{2}H_2O$!

Common Mistake: Heating gypsum beyond $373\text{ K}$. Overheating produces anhydrous calcium sulphate ($CaSO_4$), also known as "dead burnt plaster", which completely loses its setting ability!