Acids, Bases, and Salts Overview

Overview

This lecture covers the core concepts of acids, bases, salts, indicators, their reactions, important processes (like chlor-alkali), and their uses, with key exam-focused examples and explanations.

Acids and Bases: Definitions and Properties

• Acids donate H⁺ ions in water and taste sour; bases donate OH⁻ ions and feel bitter/soapy.
• Acids turn blue litmus paper red; bases turn red litmus paper blue.
• Examples of natural acids: acetic acid (vinegar), lactic acid (curd), citric acid (lemon), tartaric acid (tamarind).
• Sodium hydroxide and ammonia are common bases.

Indicators and Their Types

• Indicators change color/smell in acids or bases to identify their nature.
• Natural indicators: litmus (purple in neutral, red in acid, blue in base), turmeric (yellow in acid, red in base), hydrangea flower.
• Synthetic indicators: phenolphthalein (colorless in acid, pink in base), methyl orange (red in acid, yellow in base).
• Olfactory indicators: onion, vanilla, and clove oil lose smell in base.

Chemical Reactions of Acids and Bases

• Acid + metal → salt + hydrogen gas (test: pop sound).
• Acid + metal carbonate/bicarbonate → salt + water + CO₂ (CO₂ test: lime water turns milky).
• Acid + base → salt + water (neutralization).
• Acid + metal oxide → salt + water; metal oxides act as bases.
• Base + zinc (alkaline base only) → sodium zincate + hydrogen gas.
• Base + non-metal oxide → salt + water; non-metal oxides behave as acids.

Strength and Conductivity of Acids/Bases

• Strong acids/bases completely dissociate in water; weak ones do not.
• Acids and bases conduct electricity in water due to ion formation.
• Acids/bases only show properties when dissolved in water (aqueous solution).

pH and Universal Indicator

• pH scale: 0–14; <7 = acidic, 7 = neutral, >7 = basic.
• Strong acids have lower pH values; strong bases have higher.
• Universal indicator shows colors from red (acidic) to blue (basic); green is neutral.

Importance of pH in Daily Life

• Stomach acid (pH 1–3) aids digestion; antacids (bases) neutralize excess acid.
• Soil pH affects crop growth; acidic soil treated with quick/slaked lime, basic soil with gypsum.
• Toothpaste is basic to neutralize mouth acids; if pH <5.5, tooth enamel corrodes.
• Acid rain (pH <5.6) damages monuments, soil, and aquatic life.
• Bee stings (acidic) are neutralized with baking soda (basic).

Salts: Types, Preparation, and Uses

• Salt is an ionic compound from acid and base reactions (cation + anion).
• Neutral salt: strong acid + strong base (e.g., NaCl).
• Acidic salt: strong acid + weak base; basic salt: weak acid + strong base.
• Common salt (NaCl): used in food, preservation, de-icing, and as raw material for other chemicals.

Preparation and Uses of Key Compounds

• Sodium hydroxide (caustic soda, NaOH): made by chlor-alkali process (electrolysis of brine); by-products are chlorine and hydrogen gases.
• Chlorine (from chlor-alkali): used in bleaching powder, water treatment, plastics.
• Sodium hydroxide: used in soap, paper, textiles.
• Hydrogen: fuel, ammonia production, hydrogenation.
• Bleaching powder (CaOCl₂): made from slaked lime + chlorine; used for bleaching, disinfection, and water treatment.
• Baking soda (NaHCO₃): made via sodium chloride, ammonia, CO₂; used in baking, antacids, fire extinguishers.
• Washing soda (Na₂CO₃·10H₂O): prepared by recrystallization, used for cleaning, softening hard water, glass/soap production.
• Plaster of Paris (CaSO₄·½H₂O): made by heating gypsum; used in construction, casts, dental molds, art.

Key Terms & Definitions

• Acid — Substance that donates H⁺ ions in water.
• Base — Substance that donates OH⁻ ions in water.
• Indicator — Substance that changes color/smell in acids/bases.
• Neutralization — Reaction of acid and base forming salt and water.
• pH scale — Measures acidity/basicity (0–14).
• Salt — Ionic compound from acid-base reaction.
• Chlor-alkali process — Electrolysis of brine to make NaOH, Cl₂, and H₂.
• Bleaching powder — CaOCl₂, used for disinfection/bleaching.
• Baking soda — NaHCO₃, used in baking, cleaning, fire extinguishers.
• Washing soda — Na₂CO₃·10H₂O, cleaning and softening agent.
• Plaster of Paris (POP) — CaSO₄·½H₂O, construction/casting material.
• Water of crystallization — Water molecules incorporated in a salt’s crystal structure.

Action Items / Next Steps

• Practice writing chemical equations for all reactions discussed.

• Memorize examples of natural acids/bases, indicators, and salt types.

• Complete assigned homework: Write chemical reactions for lime with acid, explain why POP is kept airtight, identify compounds X/Z in baking/cake questions.

• Review the chlor-alkali process diagram and uses of by-products.

• Read NCERT exercises and practice past exam questions for this ch

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