Acids and Bases – Introduction

  • Taste and Effect:
    • Acids taste sour; examples include lemon juice (contains citric acid).
    • Bases taste bitter and feel slippery, like soap.
  • Litmus Test:
    • Acids: Turn blue litmus red.
    • Bases: Turn red litmus blue.
  • Natural Indicators:
    • Litmus (from lichens), turmeric, red cabbage leaves, and flower petals like Hydrangea.
    • Synthetic Indicators: Methyl orange and phenolphthalein.
    • Olfactory Indicators: Substances like onion, vanilla, and clove change odor in acidic or basic environments.

Chemical Reactions of Acids and Bases

  1. Reaction with Metals:
    • Acids react with metals to produce hydrogen gas and a salt.
    • Example:
      Zn+HCl → ZnCl2​+H2​
    • Test for Hydrogen: Brings a burning matchstick near the gas—it burns with a pop sound.
  2. Reaction with Metal Carbonates and Bicarbonates:
    • Produces carbon dioxide gas, salt, and water.
    • Example:
      Na2​CO3​+HCl → NaCl+H2​O+CO2​
    • Test for CO₂: Pass it through lime water—it turns milky due to the formation of calcium carbonate.
  3. Reaction with Bases (Neutralization):
    • Acid + Base → Salt + Water.
    • Example:
      NaOH+HCl → NaCl+H2​O
    • Neutralization reaction is exothermic (releases heat).
  4. Reaction with Metallic Oxides:
    • Metallic oxides (basic in nature) react with acids to form salt and water.
    • Example:
      CuO+HCl → CuCl2​+H2​O
  5. Reaction with Non-Metallic Oxides:
    • Non-metallic oxides (acidic in nature) react with bases to form salt and water.
    • Example:
      CO2​+Ca(OH)2​ → CaCO3​+H2​O

Key Concepts and Properties

  • Common Property of Acids: Release H⁺ ions in water (e.g., HCl → H⁺ + Cl⁻).
  • Common Property of Bases: Release OH⁻ ions in water (e.g., NaOH → Na⁺ + OH⁻).
  • Olfactory Indicators:
    • Onion loses its smell in bases but not in acids.
    • Vanilla essence loses its smell in bases but retains it in acids.
  • Conductivity:
    • Acids and bases conduct electricity in water due to ion formation.

Dilution of Acids and Bases

  • Dilution: Mixing acids/bases with water decreases the concentration of H⁺ or OH⁻ ions.
  • Always add acid to water slowly while stirring (exothermic reaction).

Important Notes for Competitive Exams

  1. Difference Between Acid and Base:
    • Acids: Proton donors, turn blue litmus red.
    • Bases: Proton acceptors, turn red litmus blue.
  2. Neutralization Applications:
    • Antacids: Neutralize stomach acidity (e.g., baking soda).
    • Soil treatment: Lime (calcium hydroxide) is used to reduce acidity.
  3. Indicators and Their Role:
    • Methyl orange: Turns red in acids and yellow in bases.
    • Phenolphthalein: Colorless in acids, pink in bases.
  4. Common Acids and Bases:
    • Acids: HCl, H₂SO₄, CH₃COOH.
    • Bases: NaOH, Ca(OH)₂, NH₄OH.
  5. Safety Precautions:
    • Never taste or touch acids/bases directly.
    • Use protective equipment when handling concentrated acids or bases.
  1. Understanding Acids and Bases:
    • Acids and bases can be identified using indicators, including universal indicators, which show different colors based on the concentration of hydrogen ions (H+).
    • pH Scale: Measures hydrogen ion concentration in solutions, ranging from:
      • 0 to 7 (acidic)
      • 7 (neutral)
      • 7 to 14 (alkaline)
    • The higher the H+H^+H+ concentration, the lower the pH value.
  2. Strength of Acids and Bases:
    • Strong acids produce more H+ ions; weak acids produce fewerH+ ions.
    • Similarly, strong bases yield more OH– ions compared to weak bases.
  3. Everyday Importance of pH:
    • Human Body: Operates within a narrow pH range of 7.0 to 7.8. Extreme pH changes can harm organisms.
    • Acid Rain: pH < 5.6 can harm aquatic life when it lowers river water pH.
    • Soil: Plants need a specific pH for growth. Farmers can adjust soil pH using substances like lime.
    • Digestive System: Stomach acid (HCl) helps digestion. Antacids neutralize excess acid during indigestion.
    • Tooth Decay: Starts when mouth pH < 5.5. Toothpaste (basic) neutralizes acids and prevents decay.
    • Defense Mechanisms: Bee stings contain acid, relieved by bases like baking soda. Nettle stings have methanoic acid.
  4. pH and Chemical Reactions:
    • Acids produceH+, and bases produce OH–.
    • Reactions of acids with:
      • Metals: Form salt and release hydrogen gas.
      • Carbonates/Bicarbonates: Form salt, water, and carbon dioxide.
  5. Salts:
    • Formed by neutralization of acids and bases.
    • Families of salts share the same cation or anion (e.g., sodium salts like Na2​SO4​).
  6. Chemicals Derived from Common Salt:
    • Sodium Hydroxide(NaOH): Produced via electrolysis of brine. Used in soap, paper, and textile industries.
    • Bleaching Powder(CaOCl2): Formed by chlorine gas reacting with slaked lime. Used for disinfection and bleaching.
    • Baking Soda (NaHCO3): Used in cooking, fire extinguishers, and antacids.
    • Washing Soda(Na2CO3⋅10H2O): Recrystallized from baking soda. Used in cleaning, glass, and paper industries.
  7. Water of Crystallization:
    • Some salts (e.g., copper sulfate, gypsum) have fixed water molecules in their structure.
    • Gypsum (CaSO4⋅2H2O): On heating, forms Plaster of Paris (CaSO4⋅½H2O), used for molds and decoration.

Key Questions to Enhance Understanding:

  1. Why do acids like HClHClHCl show acidic behavior in water but not compounds like glucose?
    • Acids ionize in water to release H+, whereas non-electrolytes like glucose do not.
  2. Why do acids conduct electricity?
    • Their ionization in water produces free ions that carry charge.
  3. Why does dry HCl gas not change dry litmus paper color?
    • No water to ionize HCl and product H+.
  4. Why add acid to water, not vice versa?
    • To prevent sudden heat release and splashing.
  5. How does dilution affect H+ or OH– ion concentration?
    • Dilution decreases the concentration of ions, reducing the solution’s strength.
  6. How does excess base affect OH– ion concentration in sodium hydroxide?
    • Increases OH– ion concentration, raising alkalinity.

Additional Tips for Competitive Exams:

  • Memorize the pH values of common substances (e.g., lemon juice ~2, tap water ~7, baking soda ~9).
  • Understand the difference between physical and chemical changes in reactions.
  • Practice equations involving acid-base reactions and their applications in real life.