PRELIMS

Acids, Bases, and Salts

The classification of substances into acids, bases, and salts is foundational to chemical behavior in aqueous solutions.

Acids: Nature and Theories

• Characteristics: Sour in taste; turns blue litmus paper red; contains replaceable hydrogen.
• Theories:
  1. Arrhenius: Gives ions in aqueous solution.
  2. Brönsted-Lowry: Proton ( ) donor.
  3. Lewis: Electron pair acceptor.

Common Acids and Their Sources

• Citric Acid: Source: Lemons, Oranges. Acid: Lactic Acid. Source: Sour milk
• Butyric Acid: Source: Rancid butter. Acid: Tartaric Acid. Source: Grapes
• Acetic Acid: Source: Vinegar. Acid: Maleic Acid. Source: Apples
• Carbonic Acid: Source: Soda water, aerated drinks. Acid: Stearic Acid. Source: Fats
• Oxalic Acid: Source: Tomato, wood sorrel. Acid: Formic Acid. Source: Red ants
• Uric Acid: Source: Urine (mammals). Acid: Ascorbic Acid. Source: Vitamin C source

• Basicity: Number of removable ions.
  • Monobasic: .
  • Dibasic: .
  • Tribasic: .
• Strength Order: .

Industrial and Biological Uses of Acids

1. Hydrochloric Acid (HCl): Gastric juices (digestion), bathroom cleaner, pickling agent (before galvanization), tanning, dyeing.
2. Nitric Acid ( ): Purification of Gold and Silver; manufacturing fertilizers (Ammonium Nitrate); manufacturing explosives (TNT, TNB, Picric Acid); Dynamite (Nitro Glycerine).
3. Sulphuric Acid ( ): Lead storage batteries; manufacturing Alum, fertilizers, and explosives.
4. Boric Acid: Constituent of eye wash; antiseptic.
5. Oxalic Acid: Removing rust spots from clothes.
6. Formic Acid: Leather tanning; preservative.

Bases and Alkalis

• Characteristics: Bitter in taste; turns red litmus blue; soapy to touch.
• Theories:
  1. Arrhenius: Gives ions in aqueous solution.
  2. Brönsted-Lowry: Proton ( ) acceptor.
  3. Lewis: Electron pair donor.
• Alkalis: Water-soluble bases (e.g., ). All alkalis are bases, but not all bases are alkalis.
• Indicators:
  • Methyl Orange: Orange Yellow.
  • Phenolphthalein: Colourless Pink.

pH Scale

Measures acidity/basicity based on hydrogen ion concentration: .

• Acidic: .
• Basic: .
• Neutral: .

Strong vs. Weak Acids/Bases

• Strong Acid/Base: Completely (nearly 100%) ionises in aqueous solution (e.g., ; ).
• Weak Acid/Base: Only partially ionises (e.g., ; ).
• Confused Pair — Strong vs. Concentrated: "Strong/weak" refers to the degree of ionisation, while "concentrated/dilute" refers to the amount of acid/base dissolved per unit volume. A weak acid can still be concentrated (e.g., concentrated acetic acid).

Classification of Salts

• Normal Salt: Formed by complete replacement of replaceable H⁺ of an acid by a base; neutral in nature (e.g., ).
• Acidic Salt: Formed by incomplete neutralisation of a polybasic acid; retains replaceable H⁺ (e.g., ).
• Basic Salt: Formed by incomplete neutralisation of a base having more than one ; retains (e.g., , basic lead acetate).
• Salts of strong acid + weak base → acidic in water (e.g., ); weak acid + strong base → basic (e.g., ); strong acid + strong base → neutral (e.g., ); weak acid + weak base → depends on relative strengths.

Salts and Their Applications

Formed by the reaction of an acid and a base (Neutralisation).

• Sodium Chloride ( ): Food flavoring; saline water for dehydration (0.9% solution); manufacturing .
• Sodium Iodate: Added to salt to prevent Goitre.
• Sodium Carbonate: Washing soda; glass manufacturing.
• Sodium Benzoate: Food preservative (pickles).
• Potassium Nitrate ( ): Fertilizers; Gun powder ( ); Match sticks.
• Calcium Carbonate: Building construction, cement, metal extraction.
• Calcium Sulphate: Plaster of Paris ( ); Gypsum ( ).
• Bleaching Powder: Disinfectant; bleaching agent.
• Alum: Water purification; antiseptic after shaving.

Household Acid-Base Checklist

• Bathroom acid: Milk of Magnesia (Antacids)
• Vitamin C tablets: Toothpaste
• Lemon/Orange/Tomato juice: Soap/Detergent solution
• Vinegar: Solution of Washing Soda
• Fizzy drinks (Colas): Slaked lime / White wash

UPSC Relevance

The practical applications of acids and bases are very high-yield for General Science.

• Everyday Science: Natural sources of acids (Citric, Lactic, Oxalic) and their household uses (Oxalic for rust, Boric for eyes).
• Indicators: Color changes in Litmus, Phenolphthalein, and Methyl Orange are standard Prelims targets.
• Industrial Chemicals: The composition of Gun powder and the role of as the "King of Chemicals" in batteries/industry.